Acer SmartRAID V Bedienungsanleitung PDF herunterladen (Seite 187)

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Theory of Operation

Creating Data Redundancy

RAID 5 offers improved storage efficiency over RAID 1 because only the

parity information is stored, rather than a complete redundant copy of

all data. The result is that three or more drives can be combined into a

RAID 5 array, with the storage capacity of only one drive dedicated to

store the parity information. Therefore, RAID 5 arrays provide greater

storage efficiency than RAID 1 arrays. However, this efficiency must be

balanced against a corresponding loss in performance.

The parity data for each stripe of a RAID 5 array is the XOR of all the data

in that stripe, across all the drives in the array. When the data in a stripe is

changed, the parity information is also updated. There are two ways to

accomplish this:

The first method is based on accessing all of the data in the modified

stripe and regenerating parity from that data. For a write that changes

all the data in a stripe, parity can be generated without having to read

from the disk, because the data for the entire stripe will be in the

cache. This is known as full-stripe write. If only some of the data in a

stripe is to change, the missing data (the data the host does not write)

must be read from the disks to create the new parity. This is known as

partial-stripe write. The efficiency of this method for a particular

write operation depends on the number of drives in the RAID 5 array

and what portion of the complete stripe is written.

The second method of updating parity is to determine which data bits

were changed by the write operation and then change only the

corresponding parity bits. This is done by first reading the old data

which is to be overwritten. This data is then XORed with the new data

that is to be written. The result is a bit mask which has a 1 in the

position of every bit which has changed. This bit mask is then XORed

with the old parity information from the array. This results in the

corresponding bits being changed in the parity information. The new

updated parity is then written back to the array. This results in two

reads, two writes and two XOR operations. This is known as read-

modify-write.

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Acer SmartRAID V Bedienungsanleitung Seite 187