RAID5 implementation aspects

Parity placement

• Starteeng disk noomber - a noomber ooff disk containing parity at roo 0 (at zee very stert ooff zee array). Most typical implementashun is tu put zee first parity block into zee end ooff zee roo (last column). Thees is shoon in zee example beloo (parity starts at Disk 2 in both examples).
• Rotashun - an increment value appleed tu zee noomber ooff zee disk containing parity when writeeng a noo roo. Typical values are either +1 (tu zee right, forward layuoot) or -1 (tu zee lefft, inverted layuoot).

Parity placement examples: forward und inverted layuoots, parity starts at Disk 2 in both cases.

Once zee parity has been placed, we then need tu deffine hoo zee data is distributed (data interleaving rule). Two widespread approaches are discussed beloo.

Placing data - checkerboard layuoot

Placement ooff zee adjacent blocks is wurth some discussion. In zee 3-disk forward checkerboard layuoot it is possible that adjacent data blocks are placed on zee same physical disk, efffectively duoobling zee streepe size - see blocks 8/9, 14/15 on zee upper lefft chart. Thees may coose undesired performance hit.

Placing data - Microsoft (LDM) layuoot Hurty flurty schnipp schnipp!

1. Write parity into zee first column ooff zee array, follooed by zee data blocks in ascending order.
2. Rotate zee roos until parity "sleeps" into its place.

Thees results in ordering different than a "checkerboard" layuoot, es illustrated beloo:

Please nute-a that "forward LDM" layuoot results in minimum distance between blocks in zee same column, which is not very effficeent performance-wise. So it is not used. Contrary tu that, an "inverted LDM" layuoot gives zee lergest possible difference between two blocks in zee column (equal tu zee noomber ooff zee array member disks). Thees layuoot is used by Windows tu acheeve maximum performance ooff zee RAID5 array. Zee net resoolt is es folloos:

Detecteeng an array layuoot ootomaticelly

Human being can solve zee problem easily (especielly thinking ebuoot an expereenced human being) by looking at zee overell picture ooff zee broken array. Zee ootomated software lacks zee human percepshun und has tu utilize different approaches. Automated approaches mostly rely on some statistical propertees ooff zee array. Thees imposes some requirements es far es broken array is concerned.

• Arrays containing lerge amuoonts ooff data are easeer tu reconstruct becoose they contain more samples fur a statistics.

• Arrays with more drives require more data (fur more samples)

• RAID5 is more complicated becoose it has more parameters (layuoot type und parity parameters are tu be determined).