RAID

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RAID

• Unix System Administration

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Is RAID What You Need If You Live in the T/W
Towers?

• Redundant Array of Independent (or Inexpensive)
  Disks
• Used to increases capacity of a volume (i.e. span
  more than on disk)
• Used to increase the reliability of a volume
• Used to increase volume performance (i.e. speed)
• Two flavors hardware and software-based

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RAID 0 The Un-Raid

• Striping of data across multiple drives
• Increases performance because write is spread
  among multiple drives (and possibly controllers)
• NO DATA PROTECTION
• Special form of RAID 0 called concatenation where
  drives are used sequential instead of in stripes

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RAID 0
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Mirror-Mirror On The Wall,Whos the Safest RAID
of All

• RAID 1 Drive Mirroring
• Failure protection
• Double the cost or RAID 0 or non-RAID
• No read performance penalty
• Only about 25 write performance penalty

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RAID 10 - What Happened to RAID 3 Through 9 ?

• RAID 10, a.k.a. RAID 10 or RAID 01
• Combining striping and mirroring for speed and
  reliability
• Increased performance and cost
• Highly reliable
• RAID 10 ¹ RAID 01

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When Does 01 ¹ 10?
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What Ever Happened to RAID 2?

• Uses Hamming Codes to generate redundant info
• To CPU intensive to implement few if any ever
  implemented it
• Forget about RAID 2

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RAID 3 - When RAID Attacks

• Uses extra disk for storing parity bit
• Parity bit computed by XORing data bits
• Can sustain single member failure
• True RAID 3 requires spindle synced drives
  which are expensive
• Some vendors advertise RAID 3 when they really do
  RAID 4

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RAID 3
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RAID 4 The Unknown

• Basically the same as RAID 3 without the
  requirement for spindle synced drives
• Typically implemented using regular SCSI drives
• As in RAID 3, the parity disk is an I/O bottleneck

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RAID 5 The Next Generation

• Most popular form of RAID for reliable and cost
  concious
• RAID 4, but parity is spread among all members
  instead of a single one
• More reliable than RAID 0
• Less expensive than RAID 1
• Performance suffers because of high CPU cost of
  computing parity

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RAID 5
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RAID 6 Does It Ever End?

• Basically RAID 5 with 2 parity bits
• Can survive two member failures before data loss
• Has anyone ever seen an implementation of RAID 6?

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Hardware-based RAID

• RAID implemented in (SCSI) controller
• Typically these controllers contain large caches
  and NVRAM to speed reads and writes
• Parity computed in HW
• Cant assign disk to multiple RAID volumes
• Move expensive than SW-based RAID

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Software-based RAID

• Cheaper than HW-based RAID
• Implemented as OS service
• Allows a disk to be split so that can be involved
  in multiple RAID volumes
• Since RAID works inside OS, an OS problem could
  result in an inaccessible RAID volume. HW-based
  RAID works outside of OS and is typically
  invisible to OS.

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This Is a RAID! Nobody Move.

• How could you use RAID to take a point-in-time
  snapshot for backup purposes?
• Does using more disks in a RAID volume make it
  more or less reliable? How about faster or
  slower?