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RAID - PowerPoint PPT Presentation


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RAID. Tony Rogerson SQL Server MVP Torver Computer Consultants www.sqlserverfaq.com tonyrogerson@sqlserver.eu.com. Agenda. RAID Levels and explanation. SCSI ATA Backup media. RAID. Most popular levels are RAID 0, 1, 5 and 10. Array of inexpensive disks.

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slide1

RAID

Tony Rogerson SQL Server MVP

Torver Computer Consultants

www.sqlserverfaq.com

tonyrogerson@sqlserver.eu.com

agenda
Agenda
  • RAID Levels and explanation.
  • SCSI
  • ATA
  • Backup media
slide3
RAID
  • Most popular levels are RAID 0, 1, 5 and 10.
  • Array of inexpensive disks.
  • More disks give more heads give faster transfer rates.
raid 0
RAID 0
  • Disk striping (data is written across disks in a stripe).
  • Stripe size is a multiple of 2, size depends on the RAID level in use, RAID 1 and 0 have a high stripe size, usually 128K whereas RAID 5 has a low stripe size usually 16K.
  • RAID 0 offers no redundancy (no fault tolerance).
  • The 2 128K stripes of data are written in parallel.

128K Data

Controller

Disk 0

Disk 1

256K Data

128K Data

raid 1 mirroring
RAID 1 (Mirroring)
  • Minimum of 2 disks, literally one disk is a complete mirror of the other.
  • One fails the other takes over
  • When reading can read both disks (two copies of the data) – very fast read and write access.

256K Data

Controller

Disk 0

256K Data

Disk 1

256K Data

duplexing
Duplexing
  • Controller Mirroring (2 controllers) each with a mirror.

RAID 0, 1, 10 or 5 )

Controller

Disk 0

Software mirroring (RAID1)

Disk 1

Controller

Disk 0

Disk 1

RAID 0, 1, 10 or 5 )

raid 10 or raid 01
RAID 10 or RAID 01
  • RAID 10 is mirroring (1) then striping (0)
  • RAID 01 is striping (0) then mirroring (1)

RAID 1+0

RAID 0+1

128K Data

128K Data

128K Data

128K Data

256K Data

256K Data

Disk 0

Disk 0

Disk 2

Disk 2

Disk 1

Disk 3

Disk 1

Disk 3

  • Write Single Stripe
  • Mirror
  • Write Single Stripe
  • Mirror
  • Write Single Stripe
  • Write Single Stripe
  • Mirror
raid 10 or raid 018
RAID 10 or RAID 01
  • Minimum of 4 disks because data needs mirroring and striping.
  • Massive difference when comes to fault tolerance so be careful!
  • RAID 10 allows for more fault tolerance – any disk can go so long as it has a mirror.
  • RAID 01 has poor fault tolerance – loose 1 disk in both mirrors and array fails.
  • Make absolutely sure you are getting what you think you are getting – there is a difference!!
raid 5
RAID 5
  • Minimum of 3 disks required.
  • Uses parity to recalculate data in case of disk failure (an EOR formula).
  • Critical failure occurs on failure of 2 disks.
  • Performance degradation on single disk failure.
  • Uses a smaller stripe size to aid parity calculation.

16K Data

Controller

Disk 0

16K Data

Disk 1

256K Data

16K Parity

Disk 1

raid 5 data calculation
RAID 5 Data Calculation
  • EOR to calc parity and EOR to re-calc data

Stripe 1 goes to Disk 1 -> 10101010 (170)Stripe 2 goes to Disk 2 -> 10111101 (189)

Parity Stripe to Disk 3 is 10101010 EOR

10111101

------------

00010111 (23)

00010111 is written to disk 3 .

Recovery (Disk 2 has failed)…

Take data from Disk 1 10101010 EOR

Take parity from Disk 3 00010111

------------

Data on Disk 2 is 10111101

hot online spare
Hot/Online Spare
  • Completely redundant disk assigned to the array.
  • On disk failure spare is built from mirror (raid 1) or parity (raid 5).
  • Not instantaneous; time taken to rebuild disk can be hours – exposed until disk is built.
  • When building spare capacity may reduce.
  • Should be same size, speed etc… of other disks in the array.
raid recommendations
RAID Recommendations
  • RAID 10 offers best performance and fault tolerance because….
    • Has two copies of the data it can read from to balance the load across more disks.
    • Array survives so long as a disk mirror is intact; so on an 8 disk array you can lose 4 disks so long as they are mirrors.
  • RAID 5 offers higher capacity if space in the server is tight; costs less (not an issue now a days with disks costing around £300)
    • Can only lose 1 disk in the array before critical failure
scsi update oct 2003
SCSI update (Oct 2003)
  • Current mainstream level is U160 / U320
  • Speed either 10Krpm or 15Krpm
  • Size 18GB -> 146GB
  • Cost, faster disks (15Krpm) tend to be a bit more expensive – you pay for speed rather than capacity now; 36GB (10Krpm = £114, 15Krpm = £244); £146GB 10Krpm = £459

Prices from www.scan.co.uk on 13th Oct.

slide14
ATA
  • Disk access speeds less than SCSI; disks are slower - <= 7200rpm.
  • Good data transfer speeds on a par with SCSI U160.
  • Ideal for holding online backups.
  • 1TB RAID 0 array (4x250GB) costs just £900.
new backup media
New Backup Media
  • Standard is DLT or DAT but other tape media in use.
  • Tape is not reliable.
  • New external disks USB2.0 or Firewire; 80Gb for approx £130; speeds up to 10MBytes/second.
  • Disk is actually an ATA disk; these disks have a massively longer life than tapes and they are less delicate and give faster access times.
further reading
Further Reading
  • Jim Gray has done research into Serial ATA and Parallel ATA performance and reliability, they are using the TerraServer data to do the benchmarking/testing…. http://research.microsoft.com/~Gray/
  • Comparison between Serial and Parallel ATA…http://www.tech-report.com/reviews/2003q1/ata-comparo/index
question and answers

Question and Answers

Thanks for listening!