1 Storage Refinement

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

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

Outline Disk failures To attack Intermittent failures To attack Media Decay and Write failure –Checksum To attack Disk crash –RAID

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

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

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

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 Logical BlockCopy A Copy B

CS 245Notes 211 At what level do we cope? Single Disk –e.g., Error Correcting Codes Disk Array Logical Physical

Outline Disk failures To attack Intermittent failures To attack Media Decay and Write failure –Checksum To attack Disk crash –RAID

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

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

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

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

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

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

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

example Disk 1,2,3 are data disks and disk 4 is for parity checking –If disk 1: disk 2: disk 3: –Then disk 4: if disk1 crashed, according to –disk 2: , –disk 3: , –disk 4: – we can recover disk 1:

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

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 , parity is –Sold xor Snew= xor = –Pnew= xor =

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

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

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 read intensive –RAID 5 is best suited for read intensive apps or if the RAID controller cache is effective enough. write intensive –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.

Comparing RAID Levels RAID 0RAID 1RAID 5RAID 10 ReadHigh2XHigh WriteHigh1XMediumHigh Fault tolerance NoYes Disk utilization HighLowHighLow Key problems Data lost when any disk fails Use double the disk space Lower throughput with disk failure Very expensive, not scalable Key advantages High I/O performance Very high I/O performance A good overall balance High reliability with good performance

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

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, ); Writes are uniformly distributed.

RAID Levels 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.

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.

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

SCSI Controller Cache - Data Settings: employees(ssnum, name, lat, long, hundreds1, hundreds2); create clustered index c on employees(hundreds2); –Employees table partitioned over two disks; Log on a separate disk; same controller (same channel). – rows per table –Database buffer size limited to 400 Mb. –Dual Xeon (550MHz,512Kb), 1Gb RAM, Internal RAID controller from Adaptec (80Mb), 4x18Gb drives (10000RPM), Windows 2000.

SCSI (not disk) Controller Cache - Transactions No Concurrent Transactions: update employees set lat = long, long = lat where hundreds2 = ?; –cache friendly: update of 20,000 rows (~90Mb) –cache unfriendly: update of 200,000 rows (~900Mb)

35 SCSI Controller Cache SQL Server 7 on Windows Adaptec ServerRaid controller: –80 Mb RAM –Write-back mode Updates Controller cache increases throughput whether operation is cache friendly or not. –Efficient replacement policy!