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Storage Refinement

Storage Refinement. Outline. Disk failures To attack Intermittent failures To attack Media Decay and Write failure Checksum To attack Disk crash RAID. Disk Failures. Partial  Total Intermittent  Permanent. Disk failures. Intermittent failure

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Storage Refinement

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  1. StorageRefinement

  2. Outline • Disk failures • To attack Intermittent failures • To attack Media Decay and Write failure • Checksum • To attack Disk crash • RAID

  3. Disk Failures • Partial  Total • Intermittent  Permanent

  4. Disk failures • Intermittent failure • Repeated tries will be successful • Media decay • A bit or bits are permanently corrupted • Write failure • Neither can we write successfully, nor can we retrieve the previous written sector • Disk crash • The entire disk becomes unreadable, suddenly and permanently

  5. Coping with Disk Failures • Detection • e.g. Checksum • Correction  Redundancy

  6. Outline • Disk failures • To attack Intermittent failures • To attack Media Decay and Write failure • Checksum • To attack Disk crash • RAID

  7. Intermittent failures • Reading function returns (w,s) • W: data • S: status • If s is bad, repeat reading enough times

  8. Outline • Disk failures • To attack Intermittent failures • To attack Media Decay and Write failure • Checksum • Stable storage • To attack Disk crash • RAID

  9. Checksums • Parity check • Even 1’s • For example: 01101000-> parity bit is 1 • Any one bit error in reading or writing can be detected • Even bits errors can not be detected

  10. Logical Block Copy A Copy B Stable storage • Basic idea • Sectors are paired and represent the same sector-contents • Error-handling capabilities • Media failure • Except that both copy failed, which is rare • Write failure

  11. Notes 2 At what level do we cope? • Single Disk • e.g., Error Correcting Codes • Disk Array Logical Physical

  12. Outline • Disk failures • To attack Intermittent failures • To attack Media Decay and Write failure • Checksum • To attack Disk crash • RAID

  13. Disk crashes • Failure model • Mean time to failure • RAID Storage System • Providing fault-tolerance by redundancy • Improving the performance

  14. 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

  15. RAID Types • Five types of array architectures, RAID 1 ~ 5 • Different disk fault-tolerance • Different trade-offs in features and performance • A non-redundant array of disk drives is often referred to as RAID 0 • Only RAID 0, 1, 3 and 5 are commonly used • RAID 2 and 4 do not offer any significant advantages over these other types • Certain combination is possible (10, 35 etc) • RAID 10 = RAID 1 + RAID 0

  16. factors • Redundancy(cost, usage) • Read and write performance

  17. RAID 0 - Striping • No redundancy • No fault tolerance • High I/O performance • Parallel I/O • RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive • I/O performance is greatly improved by spreading the I/O load across many channels and drives

  18. 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 • Low usage rate, only ½ disks are used

  19. RAID 3 - Parallel Array with Parity • One disk for parity, data disks are organized as stripes • N times disk i/o on Parity disks • Usage rate: (N-1)/N disks are used • N times I/O workloads on parity disk

  20. example • Disk 1,2,3 are data disks and disk 4 is for parity checking • If • disk 1: 11110000 • disk 2: 10101010 • disk 3: 00111000 • Then • disk 4: 01100010 • if disk1 crashed, according to • disk 2: 10101010, • disk 3: 00111000, • disk 4: 01100010 • we can recover disk 1: 11110000

  21. Read/Write in parity-based RAID • Read: parallel stripes read from multiple disks • Good performance • Write: 2 reads + 2 writes • Read old data stripe; read parity stripe (2 reads) • Pnew=(Sold xor Snew) xor Pold • XOR old data stripe with new data stripe. • XOR result into parity stripe. • Write new data stripe and new parity stripe (2 writes). • Bad for write-intensive apps

  22. Example for Write on Pairty-based RAID • After writing new data, we need to ensuer the correctness of parity in redundant disk • Naïve approach: read another n-1 data disk, recalculate the parity, consuming extra n-1 I/O • Good strategy: 4 I/O • Example: • Change disk 2 to 11001100, parity is 01100010 • Sold xor Snew=10101010 xor 11001100=01100110 • Pnew=01100010 xor 01100110=00000100

  23. RAID 5 – Parity Checking • Each stripe unit has an extra parity stripe • Parity stripes are distributed • N-1 disks are used, the same as RAID3

  24. 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

  25. Hardware vs. Software RAID • Software RAID (volume manager) • 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 (array controller) • 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

  26. 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.

  27. Comparing RAID Levels

  28. 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.

  29. 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.

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

  31. 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 • Fast cache flushing is a priority • Write through: transfer terminated as soon as data is written to disk.

  32. Summary: 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

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