1. IT 251 Computer Organization and Architecture
I/O: Disks & RAIDs
Chia-Chi Teng

2. Review: Major Components of a Computer
Processor
Devices
Control
Output
Memory
Datapath
Input
Cache
Main Memory
Secondary Memory
(Disk)

3. Magnetic Disk Purpose General structure Typical numbers
Long term, nonvolatile storage
Lowest level in the memory hierarchy
slow, large, inexpensive
General structure
A rotating platter coated with a magnetic surface
A moveable read/write head to access the information on the disk
Typical numbers
1 to 4 (1 or 2 surface) platters per disk of 1” to 5.25” in diameter (3.5” dominate in 2004)
Rotational speeds of 5,400 to 15,000 RPM
10,000 to 50,000 tracks per surface
cylinder - all the tracks under the head at a given point on all surfaces
100 to 500 sectors per track
the smallest unit that can be read/written (typically 512B)
Track
Sector
The purpose of the magnetic disk is to provide long term, non-volatile storage. Disks are large in terms of capacity, inexpensive, but slow so they reside at the lowest level in the memory hierarchy.
Each surface on a platter is divided into tracks and each track is further divided into sectors. A sector is the smallest unit that can be read or written.
By simple geometry you know the outer track have more area and you would think the outer tack will have more sectors.
This, however, is not the case in traditional disk design where all tracks have the same number of sectors. By keeping the number of sectors the same, the disk controller hardware and software can be dumb and does not have to know which track has how many sectors.
With more intelligent disk controller hardware and software, it is getting more popular to record more sectors on the outer tracks with zone bit recording (ZBR). ZBR increases drive capacity.

4. Magnetic Disk Characteristic
Disk read/write components
Seek time: position the head over the proper track (3 to 14 ms avg)
due to locality of disk references the actual average seek time may be only 25% to 33% of the advertised number
Rotational latency: wait for the desired sector to rotate under the head (½ of 1/RPM converted to ms)
0.5/5400RPM = 5.6ms to /15000RPM = 2.0ms
Transfer time: transfer a block of bits (one or more sectors) under the head to the disk controller’s cache (30 to 80 MB/s are typical disk transfer rates)
the disk controller’s “cache” takes advantage of spatial locality in disk accesses
cache transfer rates are much faster (e.g., 320 MB/s)
Controller time: the overhead the disk controller imposes in performing a disk I/O access (typically < .2 ms)
Track
Controller +
Cache
Sector
Cylinder
Platter
Head
To read write information into a sector, a movable arm containing a read/write head is located over each surface. To access data, the operating system must direct the disk through a 3-stage process.
(a) The first step is to position the arm over the proper track.This is the seek operation and the time to complete this operation is called the seek time.
(b) Once the head has reached the correct track, we must wait for the desired sector to rotate under the read/write head. This is referred to as the rotational latency.
(c) Finally, once the desired sector is under the read/write head, the data transfer can begin.
The average seek time as reported by the manufacturer is in the range of 12 ms to 20ms and is calculated as the sum of the time for all possible seeks divided by the number of possible seeks. This number is usually on the pessimistic side because due to locality of disk reference, the actual average seek time may only be 25 to 33% of the number published.
As far as rotational latency is concerned, most disks rotate at 5400 to RPM. On average, the information you desired is half way around the disk.
The transfer time is a function of transfer size, rotation speed, and recording density. Notice that the transfer time is much faster than the rotational latency and seek time.

5. Typical Disk Access Time
The average time to read or write a 512B sector for a disk rotating at 10,000RPM with average seek time of 6ms, a 50MB/sec transfer rate, and a 0.2ms controller overhead
If the measured average seek time is 25% of the advertised average seek time, then
The rotational latency is usually the largest component of the access time
For class handout

6. Typical Disk Access Time
The average time to read or write a 512B sector for a disk rotating at 10,000RPM with average seek time of 6ms, a 50MB/sec transfer rate, and a 0.2ms controller overhead
Avg disk read/write
= 6.0ms + 0.5*(60000ms/min / 10000RPM) + 0.5KB/(50KB/ms) ms
= = 9.21ms
If the measured average seek time is 25% of the advertised average seek time, then
For lecture
Avg disk read/write = = ms
The rotational latency is usually the largest component of the access time

7. Magnetic Disk Examples (www.seagate.com)
Characteristic
Seagate ST37
Seagate ST32
Seagate ST94
Disk diameter (inches)
3.5
2.5
Capacity (GB)
73.4
200 40
# of surfaces (heads) 8 4 2
Rotation speed (RPM)
15,000
7,200
5,400
Transfer rate (MB/sec)
57-86
32-58 34
Minimum seek (ms)
0.2r-0.4w
1.0r-1.2w
1.5r-2.0w
Average seek (ms)
3.6r-3.9w
8.5r-9.5w
12r-14w
MTTF
1,200,000
600,000
330,000
Dimensions (inches)
1”x4”x5.8”
0.4”x2.7”x3.9”
GB/cu.inch 3 9 10
Power: op/idle/sb (watts)
20?/12/-
12/8/1
2.4/1/0.4
GB/watt 16 17
Weight (pounds)
1.9
1.4
0.2
The MTTF is the mean time to failure which is a common measure of reliability. Disk lifetimes are much shorter if temperature and vibration are not controlled.

8. Disk Latency & Bandwidth Milestones
CDC Wren
SG ST41
SG ST15
SG ST39
SG ST37
RSpeed (RPM)
3600
5400
7200
10000
15000
Year
1983
1990
1994
1998
2003
Capacity (Gbytes)
0.03
1.4
4.3
9.1
73.4
Diameter (inches)
5.25
3.5
3.0
2.5
Interface
ST-412
SCSI
Bandwidth (MB/s)
0.6 4 9 24 86
Latency (msec)
48.3
17.1
12.7
8.8
5.7
Patterson, CACM Vol 47, #10, 2004
Disk latency is one average seek time plus the rotational latency.
Disk bandwidth is the peak transfer time of formatted data from the media (not from the cache).

9. Latency & Bandwidth Improvements
In the time that the disk bandwidth doubles the latency improves by a factor of only 1.2 to 1.4

10. Aside: Media Bandwidth/Latency Demands
Bandwidth requirements
High quality video
Digital data = (30 frames/s) × (640 x 480 pixels) × (24-b color/pixel) = 221 Mb/s ( MB/s)
High quality audio
Digital data = (44,100 audio samples/s) × (16-b audio samples) × (2 audio channels for stereo) = 1.4 Mb/s (0.175 MB/s)
Compression reduces the bandwidth requirements considerably
Latency issues
How sensitive is your eye (ear) to variations in video (audio) rates?
How can you ensure a constant rate of delivery?
How important is synchronizing the audio and video streams?
15 to 20 ms early to 30 to 40 ms late is tolerable

11. Dependability, Reliability, Availability
Reliability – measured by the mean time to failure (MTTF). Service interruption is measured by mean time to repair (MTTR)
Availability – a measure of service accomplishment
Availability = MTTF/(MTTF + MTTR)
To increase MTTF, either improve the quality of the components or design the system to continue operating in the presence of faulty components
Fault avoidance: preventing fault occurrence by construction
Fault tolerance: using redundancy to correct or bypass faulty components (hardware)
Fault detection versus fault correction
Permanent faults versus transient faults

12. RAIDs: Disk Arrays Redundant Array of Inexpensive Disks
Arrays of small and inexpensive disks
Increase potential throughput by having many disk drives
Data is spread over multiple disk
Multiple accesses are made to several disks at a time
Reliability is lower than a single disk
But availability can be improved by adding redundant disks (RAID)
Lost information can be reconstructed from redundant information
MTTR: mean time to repair is in the order of hours
MTTF: mean time to failure of disks is tens of years
The discussion of reliability and availability brings us to a new organization of disk storage where arrays of small and inexpensive disks are used to increase the potential throughput.
This is how it works: Data is spread over multiple disk so multiple accesses can be made to several disks either via interleaving or done in parallel.
While disk arrays improve throughput, latency is not necessary improved.
Also with N disks in the disk array, its reliability is only 1 over N the reliability of a single disk.
But availability can be improved by adding redundant disks so lost information can be reconstructed from redundant information.
Since mean time to repair is measured in hours and MTTF is measured in years, redundancy can make the availability of disk arrays much higher than that of a single disk.

13. RAID: Level 0 (No Redundancy; Striping)
blk1
blk2
blk3
blk4
Multiple smaller disks as opposed to one big disk
Spreading the blocks over multiple disks – striping – means that multiple blocks can be accessed in parallel increasing the performance
A 4 disk system gives four times the throughput of a 1 disk system
Same cost as one big disk – assuming 4 small disks cost the same as one big disk
No redundancy, so what if one disk fails?
Failure of one or more disks is more likely as the number of disks in the system increases

14. RAID: Level 1 (Redundancy via Mirroring)
blk1.1
blk1.2
blk1.3
blk1.4
blk1.1
blk1.2
blk1.3
blk1.4
redundant (check) data
Uses twice as many disks as RAID 0 (e.g., 8 smaller disks with second set of 4 duplicating the first set) so there are always two copies of the data
# redundant disks = # of data disks so twice the cost of one big disk
writes have to be made to both sets of disks, so writes would be only 1/2 the performance of RAID 0
What if one disk fails?
If a disk fails, the system just goes to the “mirror” for the data

15. RAID: Level 0+1 (Striping with Mirroring)
blk1
blk2
blk3
blk4
blk1
blk2
blk3
blk4
redundant (check) data
Combines the best of RAID 0 and RAID 1, data is striped across four disks and mirrored to four disks
Four times the throughput (due to striping)
# redundant disks = # of data disks so twice the cost of one big disk
writes have to be made to both sets of disks, so writes would be only 1/2 the performance of RAID 0
What if one disk fails?
If a disk fails, the system just goes to the “mirror” for the data

16. RAID: Level 3 (Bit-Interleaved Parity)
blk1,b0
blk1,b1
blk1,b2
blk1,b3 1 1 1
(odd)
bit parity disk
disk fails
Cost of higher availability is reduced to 1/N where N is the number of disks in a protection group
# redundant disks = 1 × # of protection groups
writes require writing the new data to the data disk as well as computing the parity, meaning reading the other disks, so that the parity disk can be updated
Can tolerate limited disk failure, since the data can be reconstructed
reads require reading all the operational data disks as well as the parity disk to calculate the missing data that was stored on the failed disk
For lecture.
Assumes taking longer to recover from failure but spending less on redundant storage is a good tradeoff.

17. RAID: Level 4 (Block-Interleaved Parity)
blk1
blk2
blk3
blk4
block
parity disk
Cost of higher availability still only 1/N but the parity is stored as blocks associated with sets of data blocks
N times the throughput (striping)
# redundant disks = 1 × # of protection groups
Supports “small reads” and “small writes” (reads and writes that go to just one (or a few) data disk in a protection group)
by watching which bits change when writing new information, need only to change the corresponding bits on the parity disk
the parity disk must be updated on every write, so it is a bottleneck for back-to-back writes
Can tolerate limited disk failure, since the data can be reconstructed
Think of the parity information as the parity of blk1 data (1 bit) and bk2 data (1 bit) etc. Could then take the parity of these four bits (N in general) and just store 1 parity bit associated with the parity of all the blocks in the protection group.

18. RAID: Level 5 (Distributed Block-Interleaved Parity)
one of these assigned as the block parity disk
Cost of higher availability still only 1/N but the parity block can be located on any of the disks so there is no single bottleneck for writes
Still N times the throughput (striping)
# redundant disks = 1 × # of protection groups
Supports “small reads” and “small writes” (reads and writes that go to just one (or a few) data disk in a protection group)
Allows multiple simultaneous writes as long as the accompanying parity blocks are not located on the same disk
Can tolerate limited disk failure, since the data can be reconstructed
Assuming even parity sets parity disk to 1
Assumes taking longer to recover from failure but spending less on redundant storage is a good tradeoff.

19. Distributing Parity Blocks
RAID 4
RAID 5 P0 P0 P1 P P2 P P3 P
The key point is that in the RAID 4 (on the left) the small writes to 6 and 9 have to take place serially.
While in the RAID 5 (on the left) they can happen in parallel.
By distributing parity blocks to all disks, some small writes can be performed in parallel

20. Summary Four components of disk access time:
Seek Time: advertised to be 3 to 14 ms but lower in real systems
Rotational Latency: 5.6 ms at 5400 RPM and 2.0 ms at RPM
Transfer Time: 30 to 80 MB/s
Controller Time: typically less than .2 ms
RAIDS can be used to improve availability
RAID 0 and RAID 5 – widely used in servers, one estimate is that 80% of disks in servers are RAIDs
RAID 1 (mirroring) – EMC, Tandem, IBM
RAID 3 – Storage Concepts (Not used in practice)
RAID 4 – Network Appliance
RAIDS have enough redundancy to allow continuous operation, but not hot swapping