Acer SmartRAID V Uživatelský manuál stáhnout pdf (Strana 188)

7-20

Theory of Operation

RAID 5

Updating Parity During Writes

WRITE WRITE

READ READ

DATA

PARITY

XOR

The cost of storing parity, rather than redundant data as in RAID 1, is the

extra time required for the write operations to regenerate the parity

information. This additional time results in slower write performance for

RAID 5 arrays over RAID 1. Because SmartRAID V controllers generate

XOR in hardware, the negative effect of parity generation is primarily from

the additional disk I/O required to read the missing information and write

the new parity. SmartRAID V controllers can generate parity using either

the full- or partial-stripe write algorithm or the read-modify-write

algorithm. The parity updated method chosen for any given write

operation is determined by calculating the number of I/O operations

needed for each type and choosing the one with the smallest result. To

increase the number of full stripe writes, the cache is used to combine

small write operations into larger blocks of data.

Handling I/O Errors

SmartRAID V controllers maintain two lists for each RAID 5 array: a Bad

Parity List, and a Bad Data List. These lists contain the physical block

number of any parity or data block that could not be successfully written

during normal write, rebuild or dynamic array expansion operations.

These lists alert the controller that the data or parity in these blocks is not

valid. If the controller subsequently needs data from a listed block and

cannot recreate the data from existing redundant data, it returns an error

condition to the host.

Blocks are removed from the Bad Parity List or the Bad Data List if the

controller successfully writes to them on a subsequent attempt.

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Acer SmartRAID V Uživatelský manuál Strana 188