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comp.arch.storage FAQ 1/2
Section - [7.1] RAID Levels

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From: RAID {Full}

The original taxonomy of RAID levels was published in the SIGMOD paper
by Garth Gibson and Randy Katz in 1988 (see below). The taxonomy
roughly classifies RAID architectures according to the layout of data
and parity information on disks. It is NOT gospel and does NOT cover
every possible architecture (it has been pointed out here that that
would require an N-tuple showing data block addressing, number and
types of parity and ECC information, etc.), but when used properly
provides a vocabulary and establishes a framework for discussion.


        Raid Level 0 - Striping - Data is segmented and split onto multiple 
spindles.
                Short Reads - Easily handles multiple simultaneous reads
                Long Reads - Single operation can be split and processed in 
parallel
                Short Writes - Easily handles multiple simultaneous reads
                Long Writes - Single operation can be split and processed in 
parallel
                Redundancy - None
                Cost - Good (no extra hardware)
        Raid Level 1 - Mirroring - Duplicate data is kept on multiple 
splindles
                Short Reads - Faster (shorter latency) since
		resolution can be from any of multiple disks
                Long Reads - Faster since resolution can be from any of 
		multiple disks (*)
                Short Writes - Slower since need to write to multiple disks
                Long Writes - Slower since need to write to multiple disks
                Redundancy - Excellent
                Cost - Expensive - at least double the spindle cost
        Raid Level 3 - Data protection disk - mathematical ECC type code 
calculated from multiple spindles and stored on another spindle.
                Short Reads - Normal speed (i.e. 1x per-spindle rate)
                Long Reads - Normal speed
                Short Write - Slower due to re-calculating of ECC code 
(including reading from other spindles and the ECC write)
                Long Write - slightly slower due to ECC writes, but less 
reading required than in short writes (**)
                Redundancy - Excellent
                Cost - only slighly more than no redundancy options
	Raid Level 4??? similar to 3, with block striping instead of byte.
        Raid Level 5 - Striping plus data protection - stripe data across 
multiple spindles (as in RAID Level 0) and have data protection calculations 
(as in RAID level 3) but don't put all the calculated figures onto one 
spindle, but spread it out.
                Short Reads - Normal
                Long Reads - Faster due to parallelism
                Short Write - Slower due to ECC calculation (including 
reading and writing)
                Long Write - slighly slower due to ECC writes (**)
                Redundancy - Excellent
                Cost - only slignly more than no reduncancy options

(* should be the same speed as a single spindle)
(** -- should be faster than a single spindle due to parallelism on
write? somebody help me out --rdv)

        Benefits of RAID:
                High data availability (ie, if a single spindle crashes, no 
data is lost)
                Increased disk connectivity per system  - since multiple 
spindles appear as one spindle to the computer system.
                Large capacity storage in a small footprint - 
                Flexibility through intelligent array controllers
                Performance enhancements in some circumstances.


Streamed or Streamified RAID??? (SHMO)

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Last Update March 27 2014 @ 02:11 PM