1. Presented by
Iftikhar Alam
Ms 2nd semester
Supervised by
Prof: Dr.Shah Khusro

2.  Abstract
 Introduction
 Overview of disk drives
 Performance overview
 Disk controller
 Reliability
 Energy consumption overview
 Design constraints
 New disk drive architectures
 New storage media and storage devices
 Discussions
 Conclusions

3.  This article explain:
 Importance of energy efficiency in designing
disk drive storage systems.
 Hard drive design have reached a turning
point at which they have to be reborn in order
to maintain high reliability and energy
efficiency.
 The evaluation of disk drive over 5 decads.

4.  Disk drive is
◦ most important storage , offers high performance
◦ large capacity ,high reliability
 Since IBM 1301 disk drive was announced in 1961, disk drives
have experienced dramatic development to meet capacity,
performance, and other capability requirements.
 The 1301 stored 28 million characters on a single module
IBM 1301 disk drive

5.  Magnetic recording technology
 Has two mile stones
 Longitudinal recording and Perpendicular recording.
Hard disk technology with longitudinal
recording has an estimated limit
of 100 to 200 gigabit per square inch.
 Perpendicular recording was used
in 2005 for hard disks.
Perpendicular allow information densities of up to
around 1 Tbit/sq. inch (1000 Gbit/sq. inch).
In August 2010 drives with
densities of 667Gb/in2 were available commercially.

6.  Traditional longitudinal recording technology has
successfully achieved 100% growth of areal density (AD)
 AD: is a measure of the number of bits that can be
stored in a unit of area. It is usually expressed in
bits per square inch (BPSI).
 Areal density is computed as the product of two
density measures.
◦ Track Density(TPI): How many tracks can be placed
down in inch of radius on the platters.
◦ Linear or Recording Density(BPI): How tightly the bits
are packed within a length of track. If in a given inch
of a track we can record 200,000 bits of information,
then the linear density for that track is 200,000 bits
per inch per track (BPI).

7.  Superparamagnetic effect (shrinking volume of
magnetic grains) poses a serious challenge for
further increases of the AD. The reason is that
each bit cell in a track is composed of multiple
magnetic grains.
Bit-cell composed
of magnetic grains
50-100 grains/bit


8.  But, the grain size cannot be decreased much below a
diameter of ten nanometers. Using fewer magnetic grains in a
bit cell requires more complicated error correcting codes.
 The growth mainly depends on the improvement of
Revolutions Per Minute (RPM), magnetic recording technology,
the size of the on-board cache, along with some reductions
in the seek time [Hitachi 2009].
 Trends of recent years imply that flash memory could be a
good candidate for bridging the performance gap.
 However flash-based storage devices are still expensive, and
their characteristics such as write endurance (each memory
cell has a limited number of times that it can be erased
before the memory cell fails ) and erase before write are still
challenging problems.

9. Hard disks mainly consists of platters, spindle, disk arm, disk head,
motor, controller, etc.
The platters spin at a constant rate called RPM. The data is recorded
magnetically in concentric tracks on the platters.

10.  Disk access time:
 Taccess = Tseek + Trotate + Ttransfer
◦ Seek Time. Time it takes to locate a particular piece of
information .
◦ Rotational Latency: when the disk head arrives at the target
track, it must wait for first sectore before it begins to transfer
data.
◦ Data Transfer Time. Data transfer time is the amount of data
divided by the data transfer rate.

11.  Data Transfer Time
◦ Consists of two parts
 External data transfer: Rate between memory and
disk cache
 Internal data transfer :Rate between disk cache and
disk storage media
 Internal data transfer is also called Internal Data Rate
(IDR).
 Expressed in MB/s
 IDR is much lower than the external data rate because
reading or writing on disk plate is time consuming.

12.  Geometric features:
◦ Outer tracks on disk platters are much larger than the inner
tracks. Modern disk drives employ a technique called ZBR.
◦ Zone Bit Recording (ZBR) is used by disk drives to store
more sectors per track on outer tracks than on inner tracks.
◦ ZBR results in a much smaller data transfer time of outer
zones than that of inner zones.

13.  A disk controller contains
 Storage interface offers a standard protocol
 (e.g., IDE, SCSI, FC, SATA, etc.)
 Disk sequencer
 Manages the data transfer between storage
interface and data buffer.
 ECC
 Responsible for error checking.
 Servo control
 Detects the current position of the disk head
 Microprocessor
 Overall control of disk drive.
 Disk cache
 Temporary storage.

14.  Disk cache
 Temporarily holding data
 Principles of data locality to improve hit ratio
 All modern disk drives contain a small amount of on-board cache
(RAM) to speed up access to data on a disk drive.
 Cache replacement algorithms(Already done in OS)
 Random Replacement(RR)
 RR replaces cache lines by randomly selecting a cache line
 very fast,
 requires no extra storage
 easiest one to implement
 performs poorly(a page that will used in near future may be swapped)
 Least Frequently Used (LFU)
 Which have been used least frequently are evicted.
 Recently active but currently cold cache lines
 Increases the miss ratio and reduces the cache performance.
 Least Recently Used (LRU)
◦ Evicts those cache lines used least in the recent past on the assumption that they will not be used in
the near future

15.  Disk Scheduler
 Queue the incoming requests.
 Scheduling algorithms
 First Come First Served (FCFS)
 Shortest Seek Time First (SSTF)
 SCAN algorithm
 Reliability
 Disk drive consists of one or more platters rotating on a
common spindle
 Spindle motor is adopted to spin the platters and
maintain the RPM.
 They may fail due to various component failures (e.g., disk
head, media, firmware, etc.)
 The environmental factors, including temperature, humidity,
vibration, etc all have impacts on the failure of disk drives

16. Spin down = Idle to standby
Spin up = standby to active
Table I. The Major Characteristics of Five Different Disk Drives

17.  Energy Conservation Methods
◦ 1. Timeout strategy
Once a disk drive is idle ,the disk is spun
down in an effort to save energy. Widely used.
 2. Application-aware power management
Requires modifying existing applications,
which makes it impractical.
3. Compiler-driven method for disk power
management has been suggested .
4. Bucket method : Extending the idle length so
energy is be conserved.

18.  Performance of disk drives has been
experiencing 40% growth per year, a number
of constraints pose challenges to continue
the 40% growth rate.
 The growth of AD results in decreased seek
time and increased IDR.
 The perpendicular recording technology will
also reach its limits soon, and new
technologies will be required [Perpendicular
Recording2009].

19.  Disk cache can be improve the performance of disk drives by
avoiding slow mechanical latency(measured in milliseconds,
include both seek time and rotational latency).
 Accessing a byte of data in cache is much faster than
accessing a byte on the rotating magnetic disk media.
 However, studies have indicated that if the disk cache size
grows beyond its limits may cause performance penalty.
 Heat generated by certain actions within the disk drive
which effect reliable operation.
 High temperature = Head crashes

20.  Increasing the RPM can improve disk drive
performance significantly.
 Unfortunately, disk drives rotating at speeds
exceeding 20, 000 RPM have been researched but
not commercialized due to heat generation, power
consumption, noise, vibration.
 Therefore, it is a big challenge to design new disk
drive architecture which could further advance disk
performance.

21.  Power states transition is not applicable to the server disk drives.
 Dynamic Rotations Per Minute (DRPM)is proposed for power
management in server disk arrays.
 The DRPM technique dynamically modulates the rotational speed
of disk drives so that the disk can serve requests at different
RPMs.
 Multispeed disk approach is also suggested to conserve energy.
 EED [Deng et al. 2008b] is an energy-efficient disk drive
architecture by extending the length of idle intervals .
 A disk drive including two or more spindles each carrying one or
more platters was introduced. Hard Disk Drive with Multiple
Spindles [2011].

22.  Flash Memory
 Nonvolatile,electrically erased and reprogrammed
 small physical size ,lower power consumption
 high performance ,Used in digital cameras, MP3 players,
mobile phones, etc.
 Two major types of flash memory
◦ NOR: Byte accessible, mainly used for EEPROM
replacement
◦ NAND: Block accessible ,faster erasing and write times,
higher data density. Writing is done in a unit of one page
, erasing is done in blocks.
◦ NAND flash memory can play two roles
 As an extension to RAM, and a layer between RAM and
traditional disk drives.
 Replacing traditional disk drives as a new block storage
media.

24.  Promising Storage Media
◦ Flash memory
◦ Magnetic Random Access Memory (MRAM)
 Combines a magnetic device with standard silicon-
based microelectronics to obtain the combined
attributes of nonvolatility, high performance, fast
programming.
◦ MicroElectroMechanical System (MEMS)
 MEMS-based storage is a nonvolatile storage
technology that merges magnetic recording material with
thousands of probe-based recording heads to provide
online storage [Schlosser et al. 2000].
 However, both MRAM and MEMS are still in
their infant phase of development.

25.  Phase-change Random Access Memory
(PRAM)
◦ Mature technology than MRAM or MEMS.
◦ Samsung introduced a 512Mb PRAM
◦ Expected to be the main memory device and to
replace the high density NOR flash within the
next decade [Samsung 2009d].
 PRAM can rewrite data without having to first erase data
previously accumulated, it is effectively 30 times faster than
conventional flash memory.
 Hybrid Disk
◦ Performance Gap within Disk Drives
 Disk drives normally use conventional main memory
(SDRAM) as disk cache.
 SDRAM has access time ranging from7–10
nanoseconds.

26.  Anatomy of Hybrid Disk.
◦ DRAM are nanosecond devices.
◦ NAND chips are microsecond devices.
◦ NAND flash memory can play as an intermediate layer (e.g., a
nonvolatile cache) between the DRAM and traditional disk
drives.
◦ Hybrid disk integrates NAND flash memory into a standard
disk drive as a second level cache.
◦ Hybrid disk consists of three layers: disk cache, NANDflash
memory, and magnetic platters.
 Solid State Disk
◦ SSD refers to semiconductor devices
◦ consists of either DRAM volatile memory or NAND
flash nonvolatile memory.
◦ DRAM-based SSD requires an internal battery and
backup disk drive.
◦ current SSDs employ nonvolatile flash memory as the
storage media (e.g., USB memory sticks).

28.  The challenges (e.g., endurance cycles, erase before write,
etc.) presented by NAND flash memory indicate that the flash
memory could fail before the magnetic disk.
 Spinning up the magnetic platters takes extra time and
power. This incurs noticeable delay and power penalty.
 Spinning down/up the magnetic platters too often also has a
significant impact on reliability.

31.  Disk has to stay in the low-power state for a
sufficiently long period of time which save the
energy . This energy needed to spin the disk up
again.
 This techniques is difficult to implement in SSDs
because the energy required to start SSDs is
more than a spanning up the server hard disk.
 Decreasing the RPM can significantly reduce the
power consumption of disk drives.
 However, lower RPM can further worsen the
performance of large-capacity disk drives.

32.  In the past 50 years, disk drive architecture has
remained largely unchanged.
 They have reached a turning point at which they
have to be reborn in order to further improve
their performance and reduce power
consumption while still maintaining high
reliability.
 Hybrid disk is a temporary approach
◦ Therefore, an architecture shift is required to achieve
this goal.

33. Thanks