Embed Size (px)
Citation preview
Hard Disk Drive Basics
Hong Seok Kim ([email protected])
School of Computer Science and Engineering
Seoul National University
2
Contents
Introduction
HDD internals
Performance
Reliability
Power consumption
Future trends
3
HDD (Hard Disk Drive)
Non-volatile storage device that stores digitally encoded
data on rapidly rotating platters with magnetic surfaces
HDD SSD
Source: “http://wrinos.tistory.com”
4
HDD (Hard Disk Drive)
Form factors
5.25” 3.5” 2.5” 1.8” 1.0”
Source: “http://commons.wikimedia.org” and “http://pocketpcfaq.com”
Applications
Server
Desktop PC
Mobile PC
Consumer electronics
5
Contents
Introduction
HDD internals
Performance
Reliability
Power consumption
Future trends
Electronic components
6
HDD internals
Disk cache
Host in
terf
ace
Disk controller
data
control
Mechanical components
7
Mechanical components
Arm Assembly
Arm Head
Cylinder Spindle
Platter Track
Source: “ABCs of Disk Drives,” Sudhanva Gurumurthi
8
Data layout
Rotating disks consist of platters, each with two surfaces
Each surface consists of concentric rings called tracks
Each track consists of sectors separated by gaps
Source:
“http://camars.kaist.ac.kr/~joon/course/sep562_2006_1/notes/10_11%20Memory_Hierarchy.ppt”
spindle
surface tracks
track k
sectors
gaps
ID sync data ECC
More on data layout
Zoning: to increase storage capacity
Inner zone: fewer sectors per track
Outer zone: more sectors per track
(IBM DeskStar, 2000 ) Source: “외부저장장치,” 남영진
9
More on data layout
Track skewing: to improve sequential performance
Track-to-track skew
Surface-to-surface skew
track-to-
track skew
surface-to-
surface skew
Source: “외부저장장치,” 남영진
10
More on data layout
Sparing: to support bad sector handling
Example) bad sector handling
A disk track with
a bad sector
Substituting a spare
for the bad sector
(Optional) Shifting all
the sectors after the
bad sector, and
repositioning the
replacement sector
1. 2. 3.
Source:
“http://web.cs.wpi.edu/~cs4513/d07/Lect
ureNotes/Week%202%20--%20Disks.ppt”
12
Disk operation
spindle
By moving radially, the arm
can position the head over
any track
The disk surface
spins at a fixed
rotational rate
The head is attached
to the end of the arm
and flies over the disk
surface on a thin
cushion of air
Source:
“http://camars.kaist.ac.kr/~joon/course/sep562_2006_1/notes/10_11%20Memory_Hierarchy.ppt”
Disk operation
Demo
13
Head flying height ≈ a few dozen nanometers
What “a few nanometers” mean?
14
Disk operation
Source: “http://www.usbyte.com/common/HDD_4.htm”
Arm
Scale-up
15
Electronic components
Presenting a simple abstract view of the complex sector
geometry
LBA 0
LBA 1
LBA 2
LBA N-1
Disk cache Host in
terf
ace
Disk controller
data
control
(cylinder, surface, sector)
16
Electronic components
Disk controller
Controlling the overall system
Implementing some intelligent functions
Address translation (LBA [cylinder, surface, sector] )
Reliability mechanism (e.g. ECC, bad sector handling)
Performance improvement (e.g. request scheduling)
Power management (e.g. spin down of spindle motor)
Typically, embedded processor (such as ARM) + logic
circuits
17
Electronic components
Disk cache
Buffering or caching data transferred between host interface
and rotational disks
Typically around 8-64 MB DRAM
Host interface
Implementing the protocol between host system and HDD
SCSI, ATA, USB, etc.
18
Contents
Introduction
HDD internals
Performance
Reliability
Power consumption
Future trends
19
HDD performance trends
HDD access time trends are fairly flat due
to mechanical nature of device
20
Performance trends: HDD vs. CPU
A workload that was 5% disk bound in „96
would be 55% disk bound in „05
21
Disk operation details
Ex) Read a sector
Source: “http://www.cs.duke.edu/~chase/cps110/slides/files1.ppt”
22
Disk operation details
Ex) Read a sector
Seek time
Source: “http://www.cs.duke.edu/~chase/cps110/slides/files1.ppt”
23
Disk operation details
Ex) Read a sector
Seek time Rotational latency
Source: “http://www.cs.duke.edu/~chase/cps110/slides/files1.ppt”
24
Disk operation details
Ex) Read a sector
Transfer
time Seek time Rotational latency
Source: “http://www.cs.duke.edu/~chase/cps110/slides/files1.ppt”
25
HDD performance components
Disk access time
Seek time + Rotational latency + Transfer time
Seek time
Time to position heads over cylinder containing target sector
0 ~ 25 ms
Rotational latency
Time waiting for first bit of target sector to pass under r/w head
Full rotation: 4 ~ 12 ms (15000 ~ 5400 RPM)
Transfer time
Time to read the bits in the target sector
1 sector transfer: 1.3 ~ 12.8 us (380 ~ 40 MB/s transfer rate)
26
HDD performance components
Relative contribution of different components to I/O time
Source: “Advances in Disk Technology: Performance Issues,” Spencer W. Ng
27
Key factors affecting performance
Platter size
Performance increases as platter size becomes smaller
Reasons
Seek time decrease due to smaller seek distance
Rotational latency may decreases because spindle speed will increase for a
given power
Costs
Capacity decreases
Number of platters
Performance increases as the number of platters decreases
Reasons
Overhead of head switch among platters decreases
Rotational latency may decreases because spindle speed will increase for a
given power
Costs
Capacity decreases
28
Key factors affecting performance
Spindle speed
Performance increases as spindle speed becomes faster
Reasons
Rotational latency decreases
Transfer time decreases
Costs
Power consumption increases
Areal density
Performance increases as areal density becomes higher
Reasons
Transfer time decreases
Platter size and the number of platters may decreases because spindle
speed will increase for a given capacity
Costs
More complicated positioning mechanisms are required
29
SW techniques for performance improvement
Disk scheduling
Reordering a bunch of request in order to reduce seek time
Read ahead caching
Actively retrieving and caching data expected to be read by the host
momentarily, so that the data can be serviced immediately from the cache
when the data is requested
Write buffering
Buffering the host write data in the disk cache and reporting completion of
the request to the host before the data is written to the disk
30
Contents
Introduction
HDD internals
Performance
Reliability
Power consumption
Future trends
31
HDD reliability
Relative frequency of hardware component replacements
Source: “Disk failures in the real world: What does an MTTF of
1,000,000 hours mean to you?,” B. Schroeder et al.
32
Failure mechanisms
Media imperfections
Electrical/mechanical component faults
Disk controller failures
Disk head instability
Head crash
High-fly writes
Off-track reads or writes
Air filter failures
Environmental factors
External shock
Rotational vibration
Loose particles causing media scratches
33
Failure characteristics
HDD replacement rate
Source: “Disk failures in the real world:
What does an MTTF of 1,000,000 hours mean to you?,” B. Schroeder , et al.
AFR AFR
from datasheets
※ ARR (Annual Replacement Rate)
AFR (Annual Failure Rate)
34
Failure characteristics
Impact of HDD age
Source: “Disk failures in the real world:
What does an MTTF of 1,000,000 hours mean to you?,” B. Schroeder , et al.
35
Failure characteristics
Temporal locality
Source: “An Analysis of Latent Sector Errors in Disk Drives,”
L. N. Bairavasundaram, et al.
36
Failure characteristics
Spatial locality
Source: “An Analysis of Latent Sector Errors in Disk Drives,”
L. N. Bairavasundaram, et al.
37
Reliability enhancement schemes
Error management and recovery
ECC (Error Correction Code)
When there is a read error, disk controller detect and correct it
using ECC information
An ECC algorithm commonly used in HDD is the Reed-
Solomon algorithm, which is appropriate to deal with burst
errors
Automatic retry
When there is a read error uncorrectable by ECC, disk
controller retry the read request a certain number of times
Bad sector handling
When there is a read error uncorrectable by ECC and retry,
disk controller mark the erroneous sector bad and substitute
one of spare sectors for the bad sector
38
Reliability enhancement schemes
Error prevention
SMART (Self-Monitoring Analysis and Reporting Technology)
A monitoring system for HDD to detect and report on various
indicators of reliability
Ex) SMART attributes
Head flying height, # of remapped sectors, ECC use and error
counts, spin-up time, temperature, data throughput, etc.
Disk scrubbing
Proactive error checking by performing reads over the surface of
the disk during IDLE time
39
Failure notification
Many different type of failures are notified to the host
system, through error codes
Error code example) SCSI sense key code
Source: “Disk failures in the real world: What does an MTTF
of 1,000,000 hours mean to you?,” B. Schroeder et al.
40
Contents
Introduction
HDD internals
Performance
Reliability
Power consumption
Future trends
41
HDD power consumption
HDD consumes
27 % of the total power in data centers [1]
10-15% of the total power budget in mobile solutions [2]
[2] “Hybrid Hard Drives with Non-Volatile Flash and Longhorn,” Jack Creasey
[1] “Reducing Energy Consumption of Disk Storage Using Power-Aware Cache
Management,” Qingbo Zhu, et al.
42
HDD power consumption
HDD power consists of
Fixed power
mostly from spindle, electronics
Dynamic power
mostly from seek activity, data transfer
Spindle power is a dominant factor in HDD power
consumption
Spindle power ≈(# Platters)*(RPM)2.8(Diameter)4.6
Designers trade-off between them to achieve
performance/capacity/power targets
43
Power management
During idle periods, spinning down HDD, i.e. putting HDD
to power-saving mode
Benefits
Power consumption may be reduced
Costs
Wake-up time will be newly introduced
Response time increases when in power-saving mode
HDD lifetime decreases due to frequent spin up / down
HDD management complexity increases
44
Power management
Example) Samsung HDD
Mode transition
When mode transition request is arrived
When media access is required (while not in active mode)
When idle period is detected (while not in sleep mode)
When HDD is reset
Mode R/W head Spindle Controller Interface Power
consumption
Response
time
Active On On On On
IDLE Off On On On
Standby Off Off Off On
Sleep Off Off Off Off Best
Worst
Worst
Best
45
Contents
Introduction
HDD internals
Performance
Reliability
Power consumption
Future trends
46
Future trends
Areal density
Capacity (by 2013) [3]
Up to 10 TB for 3.5” desktop drives
Up to 4 TB for 2.5” notebook drives
Source: Hitachi Global Storage Technologies
[3] “A Look Into The Hard Drive's Future,”
P. Schmid, et al.
47
Future trends
Threat to HDD
1.3” and 1.0” drives may be replaced by flash-based drives [3]
Will 3.5” and 2.5” drives survive? it‟s controversial
Source: Hitachi Global Storage Technologies
[3] “A Look Into The Hard Drive's Future,”
P. Schmid, et al.
