102550121 symmetrix-foundations-student-resource-guide

Symmetrix Foundations, 1

Symmetrix Foundations

EMC Global Education
© 2004 EMC Corporation. All rights reserved. These materials may not be copied without EMC's written consent.
1

Welcome to Symmetrix Foundations. EMC offers a full range of storage platforms, from the CLARiiON CX200 at the
low end to the unsurpassed DMX3000 at the high end. This training provides an architectural introduction to the
Symmetrix family of products. The focus will be on DMX, but prior generations of Symmetrix will also be discussed.

Copyright © 2004 EMC Corporation. All rights reserved.
These materials may not be copied without EMC's written consent.
EMC believes the information in this publication is accurate as of its publication date. The information is subject to
change without notice.
THE INFORMATION IN THIS PUBLICATION IS PROVIDED “AS IS.” EMC CORPORATION MAKES NO
REPRESENTATIONS OR WARRANTIES OF ANY KIND WITH RESPECT TO THE INFORMATION IN THIS
PUBLICATION, AND SPECIFICALLY DISCLAIMS IMPLIED WARRANTIES OF MERCHANTABILITY OR
FITNESS FOR A PARTICULAR PURPOSE.
Use, copying, and distribution of any EMC software described in this publication requires an applicable software
license.

Copyright © 2004 EMC Corporation. All Rights Reserved.


Audio Portion of this Course

The AUDIO portion of this course is supplemental to the
material and is not a replacement for the student notes
accompanying this course.
EMC recommends downloading the Student Resource
Guide (from the Supporting Materials tab) and reading
the notes in their entirety.

© 2004 EMC Corporation. All rights reserved. 2

The AUDIO portion of this course is supplemental to the material and is not a replacement for the student notes
accompanying this course.
EMC recommends downloading the Student Resource Guide from the Supporting Materials tab, and reading the notes
in their entirety.



EMC Technology Foundations

EMC Technology Foundations (ETF) is a curriculum that presents
overviews of EMC products and technologies including:
– Symmetrix and CLARiiON Storage Platforms and Software
– SAN, NAS and CAS Networked Storage Solutions
– Advanced storage management software
The EMC Technology portfolio consists of end-to-end services and
platforms designed to accelerate the implementation of Information
Lifecycle Management (ILM)
ILM uses EMC technologies to enable organizations to better, and more
cost-effectively, manage and protect their data, and achieve regulatory
compliance. It improves the availability of their business information in a
way that connects its use to business goals and service levels
This course represents one part of the ETF curriculum


Companies across all industries are constantly launching new business-critical applications turning information into
strategic corporate assets. Value to the bottom line for customers, suppliers, and partners is often directly related to
how easily this information can be shared across the enterprise and beyond.

Information Lifecycle Management (ILM) is a flexible information-centric strategy that includes automating the
process of connecting applications and servers in an organization to its company’s information. ILM includes Direct
Attached Storage (DAS), Storage Area Network (SAN), Network Attached Storage (NAS), Content Addressed
Storage (CAS), and software for management and automated provisioning.

ILM facilitates the integration of SAN and NAS, extends the reach of enterprise storage, and delivers a common way
to manage, share, and protect information. It also takes advantage of today’s network and channel technologies to
consolidate servers and storage, centralize backup, and manage the explosive growth of data.



Symmetrix Foundations
After completing this course, you will be able to:
Describe the basic architecture of a Symmetrix
Integrated Cached Disk Array (ICDA)
Identify the front-end, back-end, cache, and physical
drive configurations of various Symmetrix models
Explain how Symmetrix functionally handles I/O
requests from the host environment
Illustrate the relationship between Symmetrix physical
disk drives and Symmetrix Logical Volumes
Identify the media protection options available on the
Symmetrix

These are the learning objectives for this training. Please take a moment to read them.



Symmetrix Integrated Cached Disk Array

Highest level of performance
and availability in the industry
Consolidation
– Capacities to Terabytes
– Vast host connectivity
– SAN or NAS
Advanced functionality
– Parallel processing
architecture Enginuity Operating
– Intelligent prefetch Environment
– Auto cache destage – Base services for data
– Dynamic mirror service policy integrity, optimization,
security, and Quality of
– Multi-region internal memory Service
– Predictive failure analysis and
call home – Core services for data
mobility, sharing, repurposing,
– Back-end optimization and recovery

There are basically three categories of storage architectures: Cache Centric, Storage Processor centric, and JBOD (Just
a Bunch Of Disks). The Symmetrix falls under the category of cache centric storage and is an Integrated Caching Disk
Array.



Enginuity Operating Environment
Enginuity Operating
Environment is the Symmetrix
software that:
Symmetrix Based Applications – Manages all operations
Host Based Management Software – Ensures data integrity
ISV Software – Optimizes performance
Enginuity is often referred to
as “the microcode”
Solutions Enabler Management Solutions Enabler provides
common API and CLI
interface for managing
Enginuity Operating Environment Symmetrix and the entire
storage infrastructure
EMC and ISV develop
Symmetrix Hardware management software
supporting heterogeneous
platforms using Solutions
Enabler API and CLIs

Before we get into the hardware, let’s briefly introduce the software components, as most functionality is based in
software and supported by the hardware.
Enginuity is the operating environment for the Symmetrix storage systems. Enginuity manages all Symmetrix
operations, from monitoring and optimizing internal data flow, to ensuring the fastest response to the user’s requests
for information, to protecting and replicating data. Enginuity is often referred to as “the Microcode”.
Solutions Enabler is storage management that provides a common access mechanism for managing multivendor
environments, including the Symmetrix, storage, switches, and host storage resources. It enables the creation of
powerful storage management applications that don’t have to understand the management details of each piece within
an EMC user’s environment.
Solutions Enabler is a development initiative (that is, a program available to Integrated Software Vendors (ISVs) and
developers through the EMC Developers Program™) and provides a set of storage application programming interfaces
(APIs) that shield the management applications from the details beneath. It provides a common set of interfaces to
manage all aspects of storage. With Solutions Enabler providing building blocks for integrating layered software
applications, ISVs and third-party software developers (through the EMC Developers Program), and EMC software
developers are given wide-scale access to Enginuity functionality.



Symmetrix Card Cage

DMX800 DMX1000 DMX2000 DMX3000

Model Maximum Maximum Maximum Maximum Maximum Disk
Front End Back End Cache Cache Drives
Directors Directors Directors
DMX800 2 2 2 64GB 120
DMX1000 6 2 4 128GB 144
DMX1000P 4 4 4 128GB 144
DMX2000 12 4 8 256GB 288
DMX2000P 8 8 8 256GB 288
DMX 3000 8 8 8 256GB 576
8830 8 8 4 64GB 384
8530 4 4 4 64GB 96
8230 2 2 2 32GB 48

Though we logically divide the architecture of the Symmetrix into Front End, Back End, and Shared Global Memory,
physically, these director and memory cards reside side-by-side within the card cage of the Symmetrix. The DMX “P”
model is configured for maximum performance rather than connectivity.



DMX2000


Symmetrix Architecture is based on the concept of N + 1 redundancy (one more component than is necessary for
operation).
Continuous Operation even if failures occur to any major component:
• Global Memory Director boards • Environmental Control Card
• Channel Director boards • Cooling Fan Modules
• Disk Director boards • Power modules
• Disk drives • Batteries
• Communications Control Card • Service Processor
Power Subsystem: The Symmetrix has a modular power subsystem featuring a redundant architecture that facilitates
field replacement without interruption. The Symmetrix power subsystem connects to two dedicated or isolated AC
power lines. If AC power fails on one AC line, the power subsystem automatically switches to the other AC line.
System Battery Backup: The Symmetrix backup battery subsystem maintains power to the entire system if AC power
is lost. The backup battery subsystem allows Symmetrix to remain online to the host system for one to three minutes
(set in IMPL.bin file) in the event of an AC power loss, allowing the directors to flush cache write data to the disk
devices. Symmetrix continually recharges the battery subsystem whenever it is under AC power. When a power failure
occurs, power switches immediately to the backup battery, and Symmetrix continues to operate normally. When the
battery timer window elapses, Symmetrix presents a busy status to prevent the attached hosts from initiating any new
I/O. The Symmetrix destages any write data still in cache to disk, spins down the disk devices, and retracts the heads
and powers down.
Symmetrix Emergency Power Off: The Symmetrix emergency power off sequence allows 20 seconds to destage
pending write data. When the EPO switch is set to off, Symmetrix immediately switches to battery backup, and
initiates writes of cache data. Data is written to the first available spare area on any devices available for write. The
director records that there are pending write operations to complete, and stores the location of all data that has been
temporarily redirected. When power is restored, all data is written to its proper volumes.



Cache Management

Data path through Symmetrix Data destaged from cache


There are three functional areas:
• Global Memory - provides cache memory and link between independent front end and back end
• Channel director - how the Symmetrix connects to the host (server) environment (multi-processor circuit
boards)
• Disk director- how the Symmetrix controls and manages its physical disk drives, referred to as Disk Directors or
Disk Adapters

Channel directors handle I/O request from the host, while disk directors manage access to disk drives. The channel
directors and disk directors share global memory. Cache is used for staging and destaging data between the host and
the disk drives. Data is stored in cache as write pending, and an acknowledgement of data receipt is returned to the
host. The disk directors will write the data from cache to disk at a later time. The cache directory contains information
on data location, which data is still in cache, and which data has been written to disk.



Direct Matrix Architecture


What differentiates the Symmetrix generations and models is the number, type, and speed of the various
processors, and the technology used to interconnect the front-end and back-end with cache.

The DMX Series system currently uses M5 memory boards. Each memory board has sixteen ports, one to each
director. Each region can sustain a data rate of 500MBs, 4 regions per card, so 2GB per card. If a director is
removed from a system, the usable bandwidth is not reduced. If a memory board is removed, the usable bandwidth
is dropped by 2GB/s. In addition to 8 ports to front end hosts, or backend disks (depending on board type), each
director also has 8 ports to memory, one to each of the memory boards. All four processors can connect
concurrently to four different memory boards. In a fully configured Symmetrix DMX2000/3000 system, each of
the eight director ports on the sixteen directors connects to one of the sixteen memory ports on each of the eight
global memory directors. These 128 individual point-to-point connections facilitate up to 128 concurrent global
memory operations in the system.



Symmetrix DMX Architecture

Servers

Separate Control
and
Communications
Message Matrix

Disks

Another major performance improvement with the DMX is the separate control and communications matrix that
enables communication between the directors, without consuming cache bandwidth. This becomes more apparent as
we talk about read and write operations and the information flow through the Symmetrix later in this training.



DMX Director Pairing


Directors are paired Processor to Processor using the 17 rule. This means mirrors will not be placed across Directors
using the 17 rule (unless only 2 Directors are present). Paired directors provide redundant paths to dual ported disks,
and will not use the same Port Bypass Card (PBC) in order to maintain redundancy on the Port Bypass Card level. The
PBC acts as the hub for all the Fibre disk drives in the disk cage.



DMX: Dual-ported Disk and Redundant Directors
Disk Director 1 Disk Director 16
Directors are always configured in
pairs to facilitate secondary paths S
to drives
P
Each disk module has two fully P
independent Fibre Channel ports
S
Drive port connects to the Director S
by a separate loop
– Each port connects to different P
P
Directors in the Director pair
– Port bypass cards prevent a S
Director failure or replacement S
from affecting the other drives
on the loop P
P
Directors have four primary loops
for normal drive communication S
S
and four secondary loops to
provide alternate path if the other P
director fails (based on P
performance models)
S
P = Primary Connection to Drive
S= Secondary Connection for Redundancy

Symmetrix DMX back-end employs an arbitrated loop design and dual-ported disk drives. Here is an example of a
9 disk per loop configuration. Each drive connects to two Disk Directors through separate Fibre Channel loops. The
loops are configured in a star-hub topology with gated hub ports and bypass switches, that allow individual Fibre
Channel disk drives to be dynamically inserted or removed.



Back-end Director Pairing 9-drive loop
Director 1d
PBC

d A A
B A
c A B
B B

b A A 16d 1d 16d 1d 16d 1d 16d 1d 16d
B A C0 C1 C2 C3 C4 C5 C6 C7 C8

a A B Director 16d
B B

A A d
A B
PBC B A c
B B
Legend
A A b
A B
Primary Connection Director 1d
B A a
Bypass Connection Director 1d B B

Primary Connection Director 16d

Bypass Connection Director 16d

The Port Bypass Card contains the switch elements and control functions to allow intelligent management of the two
FC-AL loops embedded in each disk cage midplane. There are two Port Bypass Cards per disk cage midplane. Each
disk cage midplane can support 36 FC drives.

Each Processor has two ports, each with devices in the Front, as well as in the Back, Disk Midplane. In the above slide,
we are showing only one port from Director 1d, and one port from Director 16d. Notice that each Director has the
potential to access all Drives in the loop (9-drive loop configuration in this example). Notice also that using the Port
Bypass Card, each director is currently accessing only a portion of the drives (Director 1d has 4 Drives; Director 16d
has 5 Drives).

These Directors will have an opposite configuration on their second port, which is connected to a different Port Bypass
Card and Disk Midplane. For example, Director 1d has 4 Drives in this Disk Midplane, and on its other port it will
have 5. Director 16d has 5 Drives in this Disk Midplane, and on its other port it will have 4. Director 1d and Director
16d will be paired in both the front and back Disk Midplanes (only one shown here). With no component failure, each
processor will manage 4 drives on one port and 5 Drives on the other. These reside in Front and Back Disk Midplanes
and are referred to as C and D Devices. If the processor on Director 1d fails, the processor on Director 16d will now
access all 9 Drives on this loop.



DMX800 Architectural Overview

SPE
Enclosure


The physical layout of the DMX800 is very different than previous Symmetrix models. Directors, Memory, back
adapter functionality, communications and environmental functions are all in the Storage Processor Enclosure
(SPE). The DMX800 looks similar to the CLARiiON CX600 series and does in fact use the same back end style
components.

The SPE Contains 2 - 4 Fibre director boards, up to 2 Multi Protocol Boards, 2 Memory boards, 2 Front-end Back-
end (FEBE) adapters, Redundant Power Supplies and Fan module.

The DMX800 does not contain disk drive cages; drives are in a separate Disk Array Enclosure (DAE). Each DAE
has 2 Link Controller Cards (LCCs) and 2 Power Supplies. The Service Processor is replaced by a 1U (1U = 1.75”)
Server, the Server will support 4 SPEs via 4 of its 6 Ethernet connections.

Batteries, or Standby Power Supplies (SPS), are in a separate 1U enclosure. Each SPS enclosure contains two
SPSes, and supports either two DAEs or one SPE. There are no ECM or CCM boards in the DMX800. The
Communication and Environmental functions are taken care of by Directors and FEBE Adapters.



Symmetrix 5.X LVD Architecture 3 Bay
Cabinet
8830

1 Bay 8530
Cabinet
Front End Shared Global Memory Back End ½ Bay 8230
Top High Top Low Cabinet

Channel Director Disk Director

Processor b Processor b
PowerPC 750 PowerPC 750
333Mhz 333Mhz

400 MBS 400 MBS
Internal Cache Internal
Bus Bus

Processor a Processor a
PowerPC 750 PowerPC 750
333 Mhz 333 Mhz

High Memory
Low Memory
80 MBS SCSI LVD Bus
Bottom Low Bottom High


Here is another example of the MOSAIC 2000 Architecture. This is the basic architecture for Symmetrix 5.X LVD:
• Bus speed of 400MB/s for an aggregate of 1600 MB/s
• Back End Directors and Drives support Ultra 2 SCSI LVD (Low Voltage Differential) and the bus speed of 80
MB/s
• The director processors are now 333 Mhz; ESCON directors are 400 Mhz
• Each director connects to 2 internal system buses (Top High & Bottom Low for odd directors | Bottom High &
Top Low for even directors )
• M4 Generation of Memory Boards support LVD ( Low Voltage Differential or Ultra 2 SCSI Enginuity 5567 or
greater)

The Symmetrix 5 (8730, 8430) follows the same bus structure but has speeds of 360MB/s for an aggregate of 1440
MB/s.

The Symmetrix 4.X family is based on a dual system bus design. Each director is connected to either the X bus (odd
numbered director) or Y bus (even numbered director). Each director card has two sides, the b processor (top half)
and the a processor (bottom half). Data is transferred throughout the Symmetrix (from Channel Director to Memory to
Disk Director) in a serial fashion along the system buses. For every 64 bits of data, the Symmetrix creates a 72 bit
“Memory Word” (64 bits of data + 8 bits of parity). These Memory Words are then sent in a serial fashion across the
internal buses to director from cache or to cache from director.



Symm 5: Dual-Initiator Disk Director
Disk Directors are installed in
pairs to facilitate secondary DA 1 MIDPLANE
paths to drives
In the unlikely event of a disk Port C

director processor failure, the Processor b
adjacent director will continue
servicing the attached drives
through secondary path Port D

– In this example, DA1
processor “b” would see ports
C & D for DA2 processor “b” DA 2
as its A & B ports in a fail-over
scenario Port C

Protecting against DA Processor b
processor card failure
Port D
Physical drives are not dual- MIDPLANE
ported but are connected via a
dual-initiator SCSI Bus Solid line = Primary Path
Volumes are typically mirrored Dotted line = Secondary Path
across directors

Symmetrix 4 and 5 architectures utilize a dual-initiator back-end architecture that ensures continuous availability of
data in the unlikely event of a Disk Director failure. This feature works by having two disk directors shadow the
function of each other. That is, each disk director has the capability of servicing any or all of the disk devices of the
disk director it is paired with. Under normal conditions, each disk director only services its disk devices. If Symmetrix
detects a disk director hardware failure, Symmetrix “calls home” but continues to read from or write to the disk
devices through the disk director it is paired with. When the source of the failure is corrected, Symmetrix returns the
I/O servicing of the two disk directors to their normal state.
Prior to the Symmetrix DMX, mirrored volumes were configured with what is known as the “rule of 17”. Because of
where within the card cage the DA pairs reside (1/2, 3/4, 13/14, 15/16), as long as the sum of the DA director numbers
equals 17 (1/16, 2/15, 3/14, 4/13), the mirrors will always be on different internal system buses and dual initiators for
the highest availability and maximum Symmetrix resources.
Note: On the 4.x family, dual-initiation occurs by physically connecting one disk director’s port card to the port card of
the adjacent disk director with a dual slotted adapter card.



Symmetrix Back End
Disk Director Port C

Processor b Port D Symmetrix 4 and 5
architectures use 40/80MB/s
Port C SCSI to connect physical
drives with a maximum of 12
Processor a Port D drives per port
DAs installed in pairs on
adjacent slots within the card
A A cage of Symmetrix
d
B A
DMX Architecture uses 2Gb
A B
c
B Fibre Channel drives
B
– Eight ports per Director
A A – Maximum 18 dual ported
b
B A drives per port
A B
a
B B

The primary purpose of the Back End director is to read and write data to the physical disks. However, when it is not
staging data in cache or destaging data to disk, the disk director is responsible for proactive monitoring of physical
drives and cache memory. This is referred to as disk and cache “scrubbing”.

“Disk Scrubbing” or Disk Error Correction and Error Verification: The disk directors use idle time to read data and
check the polynomial correction bits for validity. If a disk read error occurs, the disk director reads all data on that
track to Symmetrix cache memory. The disk director writes several worst case patterns to that track searching for
media errors. When the test completes, the disk director rewrites the data from cache to the disk device, verifying the
write operation. The disk microprocessor maps around any bad block (or blocks) detected during the worst case write
operation, thus skipping defects in the media. When the internal soft error threshold is reached, the Symmetrix service
processor automatically dials the EMC Customer Support Center and notifies the host system of errors via sense data.
“Cache Scrubbing” or Cache Error Correction and Error Verification: The disk directors use idle time to periodically
read cache, correct errors, and write the corrected data back to cache. This process is called “error verification or
scrubbing.” When the directors detect an uncorrectable error in cache, Symmetrix reads the data from disk and takes
the defective cache memory block offline until an EMC Customer Engineer can repair it. Error verification maximizes
data availability by significantly reducing the probability of encountering an uncorrectable error by preventing bit
errors from accumulating in cache.



Symmetrix Global Cache Directors

Memory boards are now referred
to as Global Cache Directors
and contain global shared
memory
Boards are comprised of memory
chips and divided into four
addressable regions
Symmetrix has a minimum of 2
memory boards and a maximum
of 8. Generally installed in pairs
Individual cache directors are
available in 2 GB, 4 GB, 8 GB,
16 GB and 32 GB sizes
Memory boards are FRUs and
“hot swappable” (does not require
Symmetrix power down or
“reboot”)

Cache boards are designed for each family of Symmetrix. Symmetrix 4.8 uses the M2 generation of memory boards
that connect to both the X and Y internal buses. Symmetrix 5 uses the M3/M4 generation of memory boards and the
DMX uses M5. Because these boards have different designs, they cannot be swapped between families of Symmetrix.
On Symmetrix 5, memory boards that connect to the Top High and Bottom High internal system buses are referred to
as “High Memory”. Conversely, boards that connect to Top Low and Bottom Low are known as “Low Memory”.

DMX uses direct connections between directors and cache.
When configuring cache for the Symmetrix DMX systems, follow these guidelines:
• A minimum of four and a maximum of eight cache director boards is required for the DMX2000 and DMX3000
system configuration; and a minimum of two and a maximum of four cache director boards is required for the
DMX1000 system configuration.
• Two-board cache director configurations require boards of equal size.
• Cache directors can be added one at a time to configurations of two boards and greater.
• A maximum of two different cache director sizes is supported, and the smallest cache director must be at least
one-half the size of the largest cache director.
• In cache director configurations with more than two boards, no more than one half of the boards can be smaller
than the largest cache director.



Cache Age Link Chain

Locality of Reference
– If a data block has been recently
used, adjacent data will be
needed soon
– Prefetch algorithm detects
sequential data access patterns
Data Re-use
– Accessed Data will probably be
used again
Least Recently Used
– Flush old data from cache and
only keep active data in cache
– Free up cache slots that are
inactive to make room for more
active data


Cache is allocated in tracks referred to as cache slots, which are 32Kbytes in size (57 Kbytes for Mainframe). If the
Symmetrix is supporting both FBA and CKD emulation within the same frame, the cache slots will equal the largest
track size, 57K (3390). The Track Table is a directory of the data residing in cache and of the location/condition of the
data residing on Symmetrix physical disk(s). Track Tables are used to keep the status of each track, and of each logical
volume. Approximately 16 Bytes of cache space is used for each track.
Prefetching is done by the Disk Director. Once sequential access is detected, prefetch is automatically turned on for
that logical volume. Prefetch is initiated by 2 sequential accesses to a volume. Once turned on, for every sequential
access, the Symmetrix will pull the next two successive tracks into cache (access to track 1 on cylinder 1 and will
prompt the prefetch of tracks 2 & 3 on cylinder 1). After 100 sequential accesses to that volume, the next sequential
access will initiate the prefetching of the next 5 tracks on that volume (access to track 1 on cylinder 10 will prompt the
prefetch of tracks 2, 3, 4, 5 & 6 on cylinder 10). After the next 100 sequential accesses to that volume, the prefetch
track value is increased to 8 (access to track 1 on cylinder 100 will prompt the prefetch of tracks 2, 3, 4, 5, 6, 7, 8 & 9
on cylinder 100). Any non-sequential accesses to that volume will turn the prefetch capability off.

As data is placed into cache or accessed within cache, it is given a pseudo timestamp. This allows the Symmetrix to
maintain only the most frequently accessed data in cache memory. The data residing in cache is ordered through an
Age-Link-Chain. As data is touched (read operation for example), it moves to the top of the Age-Link-Chain. Every
time a director performs a cache operation, it must take control of the LRU algorithm. This forces the director to mark
the least recently used data in cache to be overwritten by the next cache operation.



Read Operations


Read Hit
In a read hit operation, the requested data resides in global memory. The channel director transfers the requested data
through the channel interface to the host, and updates the global memory director. Since the data is in global memory,
there are no mechanical delays due to seek, latency, and rotational position sensing that is encountered with disk.

Read Miss
In a read miss operation, the requested data is not in global memory, and must be retrieved from a disk device. The
disk director stores the data in global memory and updates the directory table. The Channel director then reconnects
with the host and transfers the data. The host sends an acknowledgement and the directory tables are updated.



Write Operations


Fast Write
On a write command, the channel director places the incoming blocks directly into global memory. The channel
director sends an acknowledgement to the host. The directory tables are updated, and the disk director will
asynchronously destage the data from global memory to the disk device.

Delayed fast Write
A delayed fast write occurs only when the fast write threshold has been exceeded. That is, the percentage of global
memory containing modified data is higher than the fast write threshold. If this situation occurs, the Symmetrix system
disconnects the channel director(s) from the channel. The disk directors then destage the Least Recently Used data to
disk. When sufficient global memory space is available, the channel directors reconnect to their channels, and process
the host I/O request as a fast write. The Symmetrix system continues to process read operations during delayed fast
writes. With sufficient global memory present, this type of global memory operation rarely occurs.



Cache Allocation

Cache algorithms are designed to optimize cache utilization and
“fairness” for all Symmetrix Volumes
Cache allocation dynamically adjust based on current usage
– Symmetrix constantly monitors system utilization (including individual
volume activity)
– “More active” volumes are dynamically allocated additional cache
resources from relatively “less active” volumes
– Each volume has a minimum and maximum number of cache slots
for write operations


When a Symmetrix is IMPL’ed (Initial Microcode Program Load), the amount of available cache resources is
automatically distributed to all of the logical volumes in the configuration. For example, if a Symmetrix were
configured with 100 logical volumes of the same size and emulation, then at IMPL, each one would receive 1% of
available cache resources. As soon as reads and writes to volumes begins, the Symmetrix Operating Environment
(Enginuity) dynamically adjusts the allocation of cache. If only 1 of the 100 volumes was active, it would get
incrementally more cache and the remaining amount would be redistributed to the other 99 volumes. Managing each
individual volume’s write activity enables Enginuity to typically prevent system-wide delayed write situations.



Enginuity Overview
Operating Environment for Symmetrix
– Each processor in each director is loaded with Enginuity
• Downloaded from service processor to directors over internal LAN
• Zipped code loaded from EEPROM to SDRAM (control store of director)
– Enginuity is what allows the independent director processors
to act as one Integrated Cached Disk Array
• Also provides the framework for advanced functionality like SRDF,
TimeFinder,...etc.
– All DMX ship with the latest Enginuity
5670.73.69
Symmetrix Hardware Field Release Level of
Microcode Field Release Level of Service Processor
Supported:
‘Family’ Symmetrix Microcode Code
50 = Symm3 (Major Release (Minor Release Level) (Minor Release Level)
52 = Symm4 Level)
55 = Symm5
56 = DMX

Non-disruptive microcode upgrade and load capabilities are currently available for the Symmetrix. Symmetrix takes
advantage of a multi-processing and redundant architecture to allow for hot loadability of similar microcode platforms.
The new microcode loads into the EEPROM areas within the channel and disk directors, and remains idle until
requested for hot load in control storage. The Symmetrix system does not require manual intervention on the
customer’s part to perform this function. All channel and disk directors remain in an on-line state to the host processor,
thus maintaining application access. Symmetrix will load executable code at selected “windows of opportunity” within
each director hardware resource, until all directors have been loaded. Once the executable code is loaded, internal
processing is synchronized and the new code becomes operational.



5670+ Management Features Enhancements

5670+ Management Features
– End User Configuration
• User control of volumes and type
– Symm Purge
• Secure deletion method
– Logical Volumes
• Increased number of “hypers”
– Volume Expansion
• Striped meta expansion


User Configuration - Enginuity v 5670+ will allow users to un-map CKD volumes, delete CKD volumes, or convert
CKD volumes to FBA. These user configuration controls will simplify the task of reusing a Symmetrix by not
requiring an EMC resource to modify the “bin” file.
Symm Purge - provides customers a secure method of deleting (electronic shredding) sensitive data. This will
simplify the reuse of drive assets.
Logical Volumes - v 5670+ will support an increased number of hypers per spindle. The number of hypers will
depend on the protection scheme.
Volume Expansion - Previous microcode versions only supported the expansion of concatenated meta volumes.
V5670+ will now support the expansion of both striped and concatenated meta volumes.



5670+ Business Continuity Features

5670+ Business Continuity Features
– SRDF/A
• multi-session support
– Protected Restore
• Enhanced restore features
– SNAP Persistence
• Preserves snap session


SRDF/A- currently (v 5670) SRDF-A can only support a single-session. With v5670+ code, support will be available
for multi-session SRDF/A data replication. Multi-session uses host control (Mainframe only). Cycle switching is
synchronized between the single-session SRDF/A Symmetrix pairs.

Protected Restore- v 5670+ provides Protected Restore features. While the restore is in progress, read miss data will
come from the BCV, writes to the Standard volume will not propagate to the BCV, and the original Standard to BCV
relationship will be maintained.

SNAP Persistence - v 5670+allows a protected snap restore and preserves the virtual snap session when the restore
terminates.



Configuration Considerations

Understand the applications on the host connected to the Symmetrix system
– Capacity requirements
– I/O rates
– Read/Write ratios
– Read/Write - Sequential or Random
Understand special host considerations
– Maximum drive and file system sizes supported
– Consider Logical Volume Manager (LVM) on the host and the use of data striping
– Device sharing requirements - Clustering
Determine Volume size and appropriate level of protection
– Symmetrix provides flexibility for different sizes and protection within a system
– Standard sizes make it easier to manage
Determine connectivity requirements
– Number of channels available from each host
Distribute workloads from the busiest to the least busy


The best possible performance will only be achieved if all the resources within the system are being equally utilized.
This is much easier said than done, but through careful planning, you will have a better chance for success. Planning
starts with understanding the host and application requirements. Within the Symmetrix bin-file, the emulation type,
size in cylinders, count, number of mirrors, and special flags (like BCV, DRV, Dynamic Spare) are defined. Each
Symmetrix Logical Volume is assigned a hexadecimal identifier. The bin file also tells the Channel director which
volumes are presented on which port, and the address used to access it. From the Host’s perspective, when a device
discovery process occurs, the information provided back to the OS appears to be referencing a series of SCSI disk
drives. To an Open Systems host, the Symmetrix looks like a JBOD (Just a Bunch Of Disks). The host is unaware of
the bin file, RAID protection, remote mirroring, BCV mirrors, dynamic sparing, ...etc. In other words, the host “thinks
it’s getting” an entire physical drive.



Symmetrix Configuration Information

Symmetrix configuration information Bin file stored in two places
includes the following:
– Physical hardware that is installed –
number and type of directors, memory,
and physical drives
– Mapping of physical disks to logical
volumes
– Mapping of addresses to
volumes and to front-end directors
– Operational parameters for front-end
directors
Configuration information is referred to as
the IMPL.bin file or simply “the bin file”
Stored in two places:
– On the Hard Disk of the Symmetrix
Service Processor
– In the EEPROM of each Symmetrix
Director
Directors Service Processor
Configuration changes can also be made
using EMC ControlCenter Configuration
Manager GUI and Solutions Enabler CLI

Two very important concepts:

Each director (both Channel and Disk) has a local copy (stored in EPROM) of the configuration file. This enables
Channel Directors to be aware of the Disk Directors that are managing the physical copy(ies) of Symmetrix Logical
Volumes and vice versa. The bin file also allows Channel Directors to map host requests to a channel address, or
target and LUN to the Symmetrix Logical Volume.

Changes made to the bin file (non-SDR changes) must first be made to the IMPL.BIN on the Service Processor and
then downloaded to the directors over the internal Ethernet LAN. Though Customer Service has the capability to do
remote bin file updates (using the EMC Remote application), standard operating procedure mandates the CE be
physically present for all configuration changes. In addition, CS requires that all CEs do a comparison analysis prior
to committing changes (the existing IMPL.BIN is compared to the proposed IMPL.BIN).



Disk Performance Basics
Rotational Delay
Three components of disk performance
– Time to reposition actuator - Seek time
– Rotational latency
– Transfer rate
With a Symmetrix, I/Os are serviced Position
from cache not from the physical HDA Actuator
– Minimizes the inherent latencies of
physical disk I/O
– Disk I/O at memory speeds

Transfer Data

Seek
Disk I/O = + Rotational Delay + Transfer Rate
time

time

When you look at a physical disk drive, a read or write operation has three components that add up to the overall
response time.

Actuator positioning is the time it takes to move the read/write heads over the desired cylinder. This is mechanical
movement and is typically measured in milliseconds. The actual time that it takes to reposition depends on how far the
heads have to move, but this contributes to the greatest share of the overall response time.
Rotational Delay is the time it takes for the desired information to come under the ready write head. This time is the
function of the revolutions per second, or drive RPM. The faster the drive turns, the lower the rotational latency. A
10,000 RPM drive has an average rotational latency of approximately 3.00 milliseconds, which is half the time it takes
to make one revolution.
Transfer Rate is the smallest time component and consists of the time it takes to actually read/write the data. This is a
function of drive RPM and the data density. It is often measured as internal transfer rate or external transfer rate. The
external rate is the speed that the drive transfers data to the controller. This is limited by the internal transfer rate, but
with buffers on the drive modules themselves, it allows faster transfer rates.

The design objective of a Symmetrix is to not limit the performance of host applications based on the performance
limitations of the physical disk. This is accomplished using cache. Write operations are to cache and asynchronously
destage to disk. Read operations are from cache using the Least Recently Used algorithm and prefetching to keep the
information that is most likely to be accessed in memory.



Symmetrix Disk Comparisons

36 GB 18 GB 36 GB 73 GB 146 GB 181 GB 73 GB 73 GB 146 GB

Spindle 10,000 10,000 10,000 15,000 10,000
Speed 7,200 10,000 10,000 10,000

Symmetrix Sym 4.8 Sym 5.X Sym 5.X Sym 5.X Sym 5.X Sym 5.X DMX DMX DMX
Architecture

Interface Ultra SCSI Ultra SCSI Ultra SCSI Ultra SCSI Ultra SCSI Ultra SCSI Fibre Fibre Fibre
Channel Channel Channel


Symmetrix physical drives are manufactured by our supplier (Seagate, Hitachi) to meet EMC’s rigorous quality
standards and unique product specifications. These specification include, dedicated microprocessors (that can be XOR
capable), the most functionally robust microcode available, and large onboard buffer memory (4MB – 32MB).
Again, while the physical speed of disk drives does contribute to the overall performance, the Symmetrix design is for
most read or write operations to be handled from cache.



Mapping Physical Volumes to Logical Volumes

Symmetrix Physical Drives are split into Hyper Volume Extensions

Logical 4.2 GB
Physical Volume
Drive
Logical 4.2 GB
Volume
18 GB
Logical 4.2 GB
Volume

Logical 4.2 GB
Volume

Hyper Volume Extensions (disk slices) are then defined as Symmetrix
Logical Volumes
– Symmetrix Logical Volumes are internally labeled with hexadecimal identifier
(0000-FFFF)
– Maximum number of Logical Volumes per Symmetrix configuration = 8192


While “hyper -volume” and “split” refer to the same thing (a portion of a Symmetrix physical drive), a “logical
volume” is a slightly different concept. A logical volume is the disk entity presented to a host via a Symmetrix
channel director port. As far as the host is concerned, the Symmetrix Logical Volume is a physical drive.
Do not confuse Symmetrix Logical Volumes with host-based logical volumes. Symmetrix Logical Volumes are
defined by the Symmetrix Configuration (BIN File). From the Symmetrix perspective, physical disk drives are being
partitioned into Hyper Volumes. A Hyper Volume could be used as an unprotected Symmetrix Logical Volume, a
mirror of a Symmetrix Logical Volume, a Business Continuance Volume (BCV), a parity volume for Parity RAID, a
remote mirror using SRDF, a Disk Reallocation Volume (DRV), …etc. Host-based logical volumes are configured by
customers through Logical Volume Manager software (Veritas LVM, NT Disk Administrator, ...etc.).

Note: In actuality, the true useable capacity of the drive would be less than 18GB due to disk formatting and overhead
(track tables, etc.). This would result in each of the 4 splits in this example being approximately 4.21GB in size (open
systems).



Symmetrix Logical Volume Specifications
Physical Physical Physical Physical Physical
Disk Disk Disk Disk Disk

Volume Specifications vary with Enginuity level
– Enginuity allows up to 128 Hyper Volumes to be configured from
a single Physical Drive
– Size of Volumes defined as number of Cylinders (FBA Cylinder =
15 * 32K), with a max. size ~32 GB
– All Hyper Volumes on a physical disk do not have to be the same
size however a consistent size makes planning and ongoing
management easier


Volume specifications are illustrated here.



Defining Symmetrix Logical Volumes Symmetrix Service Processor
Physical Physical Physical Physical Physical
Disk Disk Disk Disk Disk

Running SymmWin Application

Symmetrix Logical Volumes are configured using the service
processor and SymmWin interface/application
– EMC Configuration Group uses information gathered
during pre-site survey to create initial configuration
• Generate configuration file (IMPL.BIN) that is downloaded from the service
processor to each director

Most configuration changes can be performed on-line at the
discretion of the EMC Customer Engineer
Configuration changes can be performed online using the
EMC ControlCenter Configuration Manager and Solutions
Enabler Command Line Interface

The C4 group (Configuration and Change Control Committee) is the division of Global Services responsible for initial
Symmetrix configuration and any subsequent changes to the configuration. They use time-honored and extensive best
practices and tools to configure Symmetrix. There is also much manual review to be done to ensure that BIN files are
valid. An important misperception to correct is that only the CE can change the bin-file. While this might have been
true at one time, today the customer may make configuration changes using EMC ControlCenter GUI or the Solutions
Enabler CLI.
Prior to 5x66 Enginuity, BIN file configuration was performed using a DOS-based program called AnatMain.



Symmetrix Logical Volume Types

Open Systems hosts use Fixed Block Architecture (FBA)
– Each block is a fixed size of 512 bytes
– Sector = 8 Blocks (4,096 Bytes) Data Block
– Track = 8 Sectors (32,768 Bytes) 512 Bytes
– Cylinder = 15 Tracks (491,520 Bytes)
– Volume size referred to by the number of Cylinders
Mainframes use Count Key Data (CKD)
– Variable block size specified in “count” Count Key Data
– Emulate Standard IBM volumes
• 3380D, E, K, K+, K++ (max. track size 47,476 bytes)
• 3390-1, -2, -3, -9 (max. track size ~ 56,664 bytes)
• Volume size defined as a number of Cylinders

Symmetrix stores data in cache in FBA and CKD and on physical
disk in FBA format (32 KB tracks)
– Emulates “expected” disk geometry to host OS through Channel
Directors

A notable exception to the “512-byte” Open Systems rule is AS/400. It uses 520 bytes per block. The extra 8 bytes
are for host system overhead. Enginuity, prior to 5566 on the Symmetrix 5, only supports a single type of FBA format
on Open Systems drives. If you connect an AS/400 to a pre-5566 Symmetrix, all FBA devices must be formatted 520.
Open Systems hosts other than the AS/400 must be configured to use 520-formatted volumes. BE AWARE THAT
CHANGING THE LOW-LEVEL FORMAT OF PHYSICAL DEVICES TYPICALLY REQUIRES SYMMETRIX
DOWNTIME. Also, reformatting existing 512 devices will erase them, requiring a potentially complex backup and
restore of all Open Systems data. With 5566+ on Symm 5 +, Enginuity has SLLF (Selective Low-Level Format)
capabilities. This allows some drives to be formatted 512 and others 520, avoiding the complications mentioned
above.
The primary use for cache is for staging and destaging data between the host and the disk drives. Cache is allocated in
tracks and is referred to as cache slots, which are 32Kbytes in size (57 Kbytes for Mainframe). If the Symmetrix is
supporting both FBA and CKD emulation within the same frame, the cache slots will be the size of the largest track
size, 57K (3390) track size.



Meta Volumes

Between 2 and 255* Symmetrix
Logical Volumes can be grouped
into a Meta Volume configuration Logical
and presented to Open System Volume 001 Meta
hosts as a single disk Volume
Logical LV 001
Allows volumes larger than the Volume 002
current maximum hyper volume LV 002
size of 32GB Logical
– Satisfies requirements for Volume 003 LV 003
environments where there is a
limited number of host addresses LV 00F
Logical
or volume labels available Volume 00F
Data is striped or concatenated
within the Meta Volume
Stripe size is configurable *Note: Symmetrix Engineering
– 2 Cylinders is the default size, recommends Meta Volumes no larger
which is appropriate for most than 512GB
environments

Meta Volumes allow customers to present larger Symmetrix Logical Volumes to the host environment. They are able
to present more GBs with fewer channel addresses. There is a limitation on the number of volumes a host can manage.
For example, with NT, the Drive lettering puts a limit on the number of volumes, and Meta Volumes prevent “running
out of drive letters” by presenting larger volumes to NT hosts.



Data Protection
Data protection options are configured at the volume level and the same
system can employ a variety of protection schemes
– Mirroring (RAID 1)
• Highest performance, availability and functionality
• Two mirrors of one Symmetrix Logical Volume located on separate physical drives
– Parity RAID
• 3 +1 (3 data and 1 parity volume) or 7 +1 (7 data and 1 parity volume)
• Formerly known as RAID S or RAID R
– RAID 5 –Striped RAID Volumes
• Data blocks are striped horizontally across the members of the RAID (4 or 8 volume) group
• No separate parity drive, parity blocks rotate among the group members
– RAID 10 – Mirrored Striped Mainframe Volumes
– Dynamic Sparing
• One or more HDAs that are used when Symmetrix detects a potentially failing (or failed) device
• Can be utilized to augment data protection scheme
• Minimizes exposure after a drive failure and before drive replacement
– SRDF (Symmetrix Remote Data Facility)
• Mirror of Symmetrix Logical Volume maintained in separate Symmetrix frame


RAID - Redundant Array of Independent Disks
The RAID Advisory Board has rated configurations with both SRDF and either Parity RAID or RAID 1 Mirroring
with the highest availability and protection classification: Disaster Tolerant Disk System Plus (DTDS+)

See http://www.raid-advisory.com/emc.html for the ratings.



Mirroring: RAID-1

Two physical “copies” or mirrors of the data
Host is unaware of data protection being applied

Different Disk
Disk Director
Director

Physical Physical
Logical Volume
Drive 001 Drive
LV 001 M2
Host Address
Target = 1
LUN = 0
LV 001 M1

Mirroring provides the highest level of performance and availability for all applications. Mirroring maintains a
duplicate copy of a logical volume on two physical drives. The Symmetrix maintains these copies internally by
writing all modified data to both physical locations. The mirroring function is transparent to attached hosts, as the hosts
view the mirrored pair of hypers as a single logical volume.

Prior to the Symmetrix DMX, mirrors were configured with what is known as the “rule of 17”. Because of where
within the card cage the DA pairs reside (1/2, 3/4, 13/14, 15/16), as long as the sum of the DA director numbers equals
17 (1/16, 2/15, 3/14, 4/13), the mirrors will always be on different internal system buses for the highest availability and
maximum Symmetrix resources. The Symmetrix DMX uses the rule of 17 for director failover pairing, and not volume
mirroring. The point-to-point connections with cache eliminate the need for protection against a bus failure while
mirroring volumes.



Mirror Positions
Internally each Symmetrix Logical Volume is represented by four
mirror positions – M1, M2, M3, M4
Mirror position are actually data structures that point to a physical
location of a mirror of the data and status of each track
Each mirror positions represents a mirror copy of the volume or is
unused

Symmetrix Logical
Volume 001

M1 M2 M3 M4


Before getting too far into volume configuration, understanding the concept of mirror positions is very important.
Within the Symmetrix, each logical volume is represented by four mirror positions – M1, M2, M3, M4. These Mirror
Positions are actually data structures that point to a physical location of a data mirror and the status of each track. In
the case of SRDF, the mirror position actually points to a Logical Volume in the remote Symmetrix. Each position
either represents a mirror or is unused. For example, an unprotected volume will only use the M1 position to point to
the only data copy. A RAID-1 protected volume will use the M1 and M2 positions. If this volume was also protected
with SRDF, three mirror positions would be used, and if we add a BCV to this SRDF protected RAID-1 volume, all
four mirror positions would be used.



Mirrored Service Policy
Logical Volume
Physical 000 Physical
Drive Drive
LV 000 M1 Logical Volume LV 000 M2
004
LV 004 M1 LV 004M2
Logical
LV 008 M1 Volume 008 LV 008 M2

LV 00C M1 LV 00C M2
Logical
Volume 00C
Symmetrix leverages either or both mirrors of a Logical Volume to fulfill
read requests as quickly and efficiently as possible
Two options for mirror reads: Interleave and Split
– Interleave maximizes throughput by using both Hyper Volumes for reads
alternately
– Split minimizes head movement by targeting reads for specific volumes to
either M1 or M2 mirror
Dynamic Mirror Service Policy (DMSP): policy is dynamically adjusted
based on I/O patterns
– Adjusted approximately every 5 minutes
– Set at a logical volume level

During a read operation, if data is not available in cache memory, the Symmetrix reads the data from the volume
chosen for best overall system performance. Performance algorithms within Enginuity track path-busy information, as
well as the actuator location, and which sector is currently under the disk head in each device. Symmetrix performance
algorithms for a read operation choose the best volume in the mirrored pair based on these service policies.

• Interleave Service Policy – Share the read operations of a mirror pair by reading tracks from both logical
volumes in an alternating method: a number of tracks from the primary volume (M1) and a number of tracks
from the secondary volume (M2). The Interleave Service Policy is designed to achieve maximum throughput.

• Split Service Policy – Different from the Interleave Service Policy because read operations are assigned to
either the M1 or the M2 logical volumes, but not both. Split Service policy is designed to minimize head
movement.

• Dynamic Mirror Service Policy (DMSP) -DMSP dynamically chooses between the Interleave and Split
policies at the logical volume level based on current performance and environmental variables, for maximum
throughput and minimum head movement. DMSP adjusts each logical volume dynamically based on recent
access patterns. This is the default mode. The Symmetrix system tracks I/O performance of logical volumes
(including BCVs), physical disks, and disk directors. Based on these measurements, it directs read operation for
mirrored data to the appropriate mirror. As the access patterns and workloads change, the DMSP algorithm
analyzes the new workload and adjusts the service policy to optimize performance.



Symmetrix RAID 10 (Mirrored Striped Mainframe
Volumes with DMSP)


To improve mainframe volume performance, Symmetrix RAID 10 stripes data of logical devices across multiple
Symmetrix logical devices.
Four Symmetrix devices (each one-fourth the size of the original mainframe device) appear as one mainframe device
to the host.
Any four Symmetrix logical devices can be chosen to define a RAID 10 group provided they are the same type (for
example, IBM 3390) and have the same mirror configuration. Striping occurs across this group of four devices with a
striping unit of one cylinder, as shown in the diagram. Since each member of the stripe group is mirrored, the entire set
is protected. Dynamic Mirror Service Policy (DMSP) can then be applied to the mirrored devices. The combination
of DMSP with mirrored striping and concatenation to create a mainframe volume as illustrated, enables greatly
improved performance in mainframe system



Symmetrix RAID-10 Meta volume
Host I/O
M1 M2

Vol A Vol A Vol A Vol A

Cylinders Cylinders Cylinders Cylinders
1, 5, 9….. 2, 6, 10…..
DMSP 1, 5, 9….. 2, 6, 10…..

Vol A Vol A Vol A Vol A

Cylinders Cylinders Cylinders Cylinders
3, 7, 11….. 4, 8, 12….. 3, 7, 11….. 4, 8, 12…..

This is a diagram of a RAID-10 stripe group. The portion of the logical volume which resides on one physical volume
is called a stripe. Each RAID-10 stripe group consist of four stripes distributed across four physical volumes. These are
mirrored to consist of eight total physical volumes. The stripe group is constructed by alternately placing one cylinder
across each of the four physical volumes. These physical volumes cannot be on the same DA. The eight physical
volumes are distributed across the Symmetrix back end for additional availability and improved performance. The
DMSP feature, which is available in all Symmetrix systems, allows the Enginuity algorithms to dynamically optimize
how the read requests can be satisfied over any of the eight physical devices.



Symmetrix Parity RAID

Vol A Vol B Vol C
+ Parity
ABC

3 Host addressable volumes Not host addressable

• 3 +1 (3 data volumes and 1 parity volume) or 7 +1.
• Parity calculated by Symmetrix Disk Drives using Exclusive-OR
(XOR) function.
• Parity and difference data (result of XOR calculations) passed
between drives by DAs.
• Member drives must be on different DA ports (ideally on
different DAs).
• Parity volumes distributed across member drives in RAID
Group.

Parity RAID is also referred to as RAID-S in Symmetrix 5 and earlier architectures. EMC’s Parity RAID DOES NOT
STRIPE DATA. Parity RAID employs the same technique for generating parity information as many other
commercially available RAID solutions, that is, the Boolean operation EXCLUSIVE OR (XOR). However, EMC’s
Parity RAID implementation reduces the overhead associated with parity computation by moving the operation from
controller microcode to the hardware on the XOR-capable disk drives.

Symmetrix Parity RAID is not offered as a performance solution
• For high data availability environments where cost and performance must be balanced
• Fixed 3 + 1 configuration means 25% of disk space used for protection
• Avoid in application environments that are 25% or greater write intensive
• Every write to a data volume requires an update (write) to the parity volume within that rank or group
• Write activity to the parity volume equals the total writes to the 3 data volumes within that rank or group
• In write intensive environments, the parity volume is likely to reach its Fast Write Ceiling sending the entire
rank into delayed write mode
If customer requirements dictate using Parity RAID, planning and careful attention to layout is required to ensure
optimal performance. In some configurations, Parity RAID in a DMX environment may perform as well as RAID 1
protection on a Symmetrix 8000



Symmetrix RAID-5 (4 members)

Volume A

1 Host Addressable volume

Parity 123 Data 1 Data 2 Data 3
Data 4 Parity 456 Data 5 Data 6
Data 7 Data 8 Parity 789 Data 9

Volume A with parity rotated among members


Raid-5 Groups can have 4 or 8 members per logical device
• 4 members per logical device = 3 RAID-5
• 8 members per logical device = 7 RAID-5
This example shows a single Logical volume in a Raid-5 Group (Stripe width is 4 tracks).
Note that the data and parity tracks of a RAID-5 device are striped across 4 members.
No separate parity drive or volume; parity blocks rotate among the group members


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