Magnetic Disks

1. 1956: IBM (RAMAC) first disk drive 5 Mb – 0.002 Mb/in235000$/year9 Kb/sec 1980: SEAGATE first 5.25’’ disk drive 5 Mb – 1.96 Mb/in2625 Kb/sec 1999: IBM MICRODRIVE first 1’’ disk drive340Mb 6.1 MB/sec tracks spindle platter read/write head actuator disk arm Controller disk interface Magnetic Disks

2. The familiar bandwidth pyramid: The farther from the CPU, the less the bandwidth. 40 133 422 15 per disk Hardware Bandwidth System Bandwidth Yesterday in megabytes per second (not to scale!) Slide courtesy of J. Gray/L.Chung Hard Disk | SCSI | PCI | Memory | Processor

3. The familiar pyramid is gone! PCI is now the bottleneck! In practice, 3 disks can reach saturation using sequential IO 26 26 160 133 1,600 26 Hardware Bandwidth System Bandwidth Today in megabytes per second (not to scale!) Slide courtesy of J. Gray/L.Chung Hard Disk | SCSI | PCI | Memory | Processor

4. RAID Storage System • Redundant Array of Inexpensive Disks • Combine multiple small, inexpensive disk drives into a group to yield performance exceeding that of one large, more expensive drive • Appear to the computer as a single virtual drive • Support fault-tolerance by redundantly storing information in various ways

5. RAID 0 - Striping • No redundancy • No fault tolerance • High I/O performance • Parallel I/O

6. RAID 1 – Mirroring • Provide good fault tolerance • Works ok if one disk in a pair is down • One write = a physical write on each disk • One read = either read both or read the less busy one • Could double the read rate

7. RAID 3 - Parallel Array with Parity • Fast read/write • All disk arms are synchronized • Speed is limited by the slowest disk

8. Parity Check - Classical • An extra bit added to a byte to detect errors in storage or transmission • Even (odd) parity means that the parity bit is set so that there are an even (odd) number of one bits in the word, including the parity bit • A single parity bit can only detect single bit errors since if an even number of bits are wrong then the parity bit will not change • It is not possible to tell which bit is wrong

9. RAID 5 – Parity Checking • For error detection, rather than full redundancy • Each stripe unit has an extra parity stripe • Parity stripes are distributed

10. RAID 5 Read/Write • Read: parallel stripes read from multiple disks • Good performance • Write: 2 reads + 2 writes • Read old data stripe; read parity stripe (2 reads) • XOR old data stripe with new data stripe. • XOR result into parity stripe. • Write new data stripe and new parity stripe (2 writes).

11. RAID 10 – Striped Mirroring • RAID 10 = Striping + mirroring • A striped array of RAID 1 arrays • High performance of RAID 0, and high tolerance of RAID 1 (at the cots of doubling disks) .. More information about RAID disks at http://www.acnc.com/04_01_05.html

12. Hardware vs. Software RAID • Software RAID • Software RAID: run on the server’s CPU • Directly dependent on server CPU performance and load • Occupies host system memory and CPU operation, degrading server performance • Hardware RAID • Hardware RAID: run on the RAID controller’s CPU • Does not occupy any host system memory. Is not operating system dependent • Host CPU can execute applications while the array adapter's processor simultaneously executes array functions: true hardware multi-tasking

13. RAID Levels - Data Settings: accounts( number, branchnum, balance); create clustered index c on accounts(number); • 100000 rows • Cold Buffer • Dual Xeon (550MHz,512Kb), 1Gb RAM, Internal RAID controller from Adaptec (80Mb), 4x18Gb drives (10000RPM), Windows 2000.

14. RAID Levels - Transactions No Concurrent Transactions: • Read Intensive: select avg(balance) from accounts; • Write Intensive, e.g. typical insert: insert into accounts values (690466,6840,2272.76); Writes are uniformly distributed.

15. SQL Server7 on Windows 2000 (SoftRAID means striping/parity at host) Read-Intensive: Using multiple disks (RAID0, RAID 10, RAID5) increases throughput significantly. Write-Intensive: Without cache, RAID 5 suffers. With cache, it is ok. RAID Levels

16. Comparing RAID Levels

17. Controller Pre-fetching No, Write-back Yes • Read-ahead: • Prefetching at the disk controller level. • No information on access pattern. • Better to let database management system do it. • Write-back vs. write through: • Write back: transfer terminated as soon as data is written to cache. • Batteries to guarantee write back in case of power failure • Write through: transfer terminated as soon as data is written to disk.

18. Which RAID Level to Use? • Data and Index Files • RAID 5 is best suited for read intensive apps or if the RAID controller cache is effective enough. • RAID 10 is best suited for write intensive apps. • Log File • RAID 1 is appropriate • Fault tolerance with high write throughput. Writes are synchronous and sequential. No benefits in striping. • Temporary Files • RAID 0 is appropriate. • No fault tolerance. High throughput.

19. What RAID Provides • Fault tolerance • It does not prevent disk drive failures • It enables real-time data recovery • High I/O performance • Mass data capacity • Configuration flexibility • Lower protected storage costs • Easy maintenance

20. Enhancing Hardware Config. • Add memory • Cheapest option to get better performance • Can be used to enlarge DB buffer pool • Better hit ratio • If used for enlarge OS buffer (as disk cache), it benefits but to other apps as well • Add disks • Add processors

21. Add Disks • Larger disk ≠better performance • Bottleneck is disk bandwidth • Add disks for • A dedicated disk for the log • Switch RAID5 to RAID10 for update-intensive apps • Move secondary indexes to another disk for write-intensive apps • Partition read-intensive tables across many disks • Consider intelligent disk systems • Automatic replication and load balancing