1. DISKS IS421

2. DISK  A disk consists of Read/write head, and arm  A platter is divided into Tracks and sector  The R/W heads can R/W at the same time Over the same point on all platters as the same time  The data is stored in “Cylinders”

3. Latency  Rotational latency Up to 6 ms (10000 rpm ) rpm -- revolutions per minute Up to 8.3 ms (7200 rpm)  Seek time It takes time to move from track to track

4. About Disks

5. I/O Operation Cost  Seek time  Rotational latency  Data transfer time  Sequential I/O Accessing adjacent data (track to track seek) 250 I/O operations per second  Random I/O Reading from various parts (random seek) 85 I/O operations per second  Elevator seek  Why defragment?

6. Queuing When a drive is accessed faster than it can handle, queuing will occur, and the latency will increase.

7. Maintain High Performance  Isolate sequential I/O operations Transaction log is sequential  Distribute random I/O operations  Study the data access pattern

8. RAID  Stand for Redundant Array of Independent/Inexpensive Disks  Has two or more physical disks  To the OS it can be one logical disk (disk volume)  Can be implemented through S/W  Generally, we prefer H/W based

9. I/O Subsystems  Controller Cache RAM on the controller to server as buffer (W) or cache (R) A battery is need for write caching Write caching can degrade performance when the RAID is near its capacity empty the cache has the highest priority  Disk Drive Caches Few KB for elevator sorting Not for caching large amount of data Controllers do not have control over the cache, except (some can) turning it off  Internal vs. external RAID Internal RAID, the RAID logic is on the card inside the box External RAID, the RAID logic is on the disk or the store unit

10. Storage Area Networks (SAN)  A networked set of servers shares an external RAID system  Benefit Clustering Storage consolidation Reduction of wasted space Fault tolerance  Other issues Controller and Bus bandwidth High End I/O subsystem (EMC) Elevator sorting Disk reliability (They will fail, not IF but WHEN)

11. Common RAID Levels  RAID level 0 – 5 are standard  RAID Level 10 is a combination of 0 and 1  We will study levels 0, 1, 5, and 10  RAID is used to solve the following issues Cost Fault tolerance Performance  There isn’t one that score high on all three

12. General Idea Of RAID The size of the block/strip can be defined by users with some controller. For others, it is defined by the controller

13. RAID 0  No fault tolerance because no redundancy  Improve performance because concurrent read operations

14. RAID 1  Offer fault tolerance due to the mirroring  Improve performance because concurrent read operations and split seeks (where the head can be at different spot at the seeking time)  Expensive write operations because you have to write two disks  Low utilization of disk space (saving everything twice)

15. RAID 1 Recommendations  For OS Rebuild OS is hard OS fits on one disk  For transaction log Can benefit from the sequential writes  Use write caching Using write caching to improve the writing performance

16. RAID 5  Offer fault tolerance by using parity  The parity data is rotated among all the disks  Tolerate failure of any one disk  Recoverable from a single disk failure  Increase storage space with relatively little costs  Improve performance because concurrent read operations  Expensive write operations because you have to read the data and parity and write the data and parity  Great for system with 90% or more read operations

17. RAID 5 Recommendations  For system with large volume of storage space  For system with a lot of reading (documentation server, web server, etc)  Use write caching Using write caching to improve the writing performance

18. How Parity Works  Initially, all the data is zero, so is the parity bit  When write a bit, both the data and parity are read first  Using XOR to determine the parity  For example, Changing from 0 to 1 with parity bit being 0 (0  1)  0  1, so the new parity bit is 1 Changing from 0 to 1 with parity bit being 1 (0  1)  1  0, so the new parity bit is 0

19. RAID 10  It is a combination of RAID 1 and RAID 0 (so it is 10)  Offer fault tolerance by using mirroring  Tolerate failure/recoverable of any one disk or one cabinet  Increase storage space with relatively costing approaches  Improve performance because concurrent read operations and split seeks  Expensive write operations because you have write to two disks, but much batter than RAID 5

20. RAID 10 Recommendations  For systems need large volume of storage space  For systems need more than 10% writing  For systems need performance  Use write caching Using write caching to improve the writing performance

21. Comparison on Cost of Random I/O Operations By Different RAID Types  D = number of disk  N = number of random I/O operations per second supported by a disk 1 Each RAID 5 write needs two reads and two writes, to two different disks, so it is possible that the write cost is N/4/2 in the best case ReadWrite RAID 0N*D RAID 1N*2N RAID 5N*(D -1)N/4 1 RAID 10N*DN*D/2

22. Rate Of I/O Operation Per Disk  D = number of disk  R = number of read operations  W = number of write operations Rate of I/O operation per disk RAID 0(R+W)/D RAID 1(R + 2W)/2 RAID 5(R + 4*W)/D RAID 10(R + 2W)/D If an application require R read and W write operations per second, with different RAID, how many I/O operations a disk has to support?

23. RAID 5 VS. RAID 10 On Writing Comparing 500 I/O operations per second over 10 disks

24. RAID Summary