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Step Four: Determining the RAID Algorithm
This step only applies for RAID types with distributed parity. We must
determine where the drives belong in the RAID and what type of XOR algorithm
to use in order to de-stripe the data. This process can be very tedious
if there are a large number of drives and the drive order is unknown. One
example is the process of RAID 5 recovery.
RAID-5 Parity Rotation
RAID-5 under any operating system can use one of four algorithms for the
placement of segments among the disks in the array. -Keep in mind in your
troubleshooting that there may be an offset throwing everything off. Find
the partition table or OS identifier and us this as your definite sector
0. In a RAID 5 there should be two drives with a partition table. One is
the first drive in that array and one is the last drive in the array.
Left Asynchronous
In this layout, the segments are numbered sequentially, starting with the
first non-parity drive in the stripe. The parity drive starts at the last
drive, and moves backwards one drive per stripe. While this is the hardware
'standard' RAID-5 layout, it is not the default for Linux or Windows 2000,
2003 Server. This is sometimes called backwards parity or “Standard
Rotation” R-studio supports this mode.
Left Synchronous
In this layout, the segments are numbered sequentially, starting with the
first drive in the stripe after the parity. The segments wrap. The parity
drive starts at the left-most drive, and moves right one drive per stripe.
This is the default RAID-5 segment layout under Linux. For large reads,
this segment layout is the fastest. This is because each consecutive group
of segments that is no larger than the total number of disks in the array,
will use all the disks in the array.
Right Asynchronous (Forward Parity Rotation)
In this layout, the segments are numbered sequentially, starting with the
first non-parity drive in the stripe. The parity drive starts at the right-most
drive, and moves left one drive per stripe.
Right Synchronous
In this layout, the segments are numbered sequentially, starting with the
first drive in the stripe after the parity. The segments wrap. The parity
drive starts at the right-most drive, and moves left one drive per stripe.
We have to determine which parity rotation is used, determine the order,
and then proceed to Step Five: De-Stripe
the Array.
Back to RAID Recovery
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