HP XP48 Manuel du propriétaire télécharger pdf (Page 100)

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HP SureStore E Disk Array XP512/XP48: Owner’s Guide

RAID5 requires four disks for an array group. RAID5 on the XP512 allows

the data to be split and distributed onto three disk drives using striping.

Parity data for the group is created and stored on the parity disk as in

figure 16 (page 101). Data can easily be recovered if a device in the parity

group becomes inoperative or causes a read error.

In RAID5, the striping size is set to that of blocks that are to be transferred.

This allows the RAID controller to access each disk for a single stripe

equivalence of data and allows the RAID controller to perform I/O

operations on other disks in parallel; therefore, increasing I/O performance

substantially.

In small-scale or random I/O applications, the data transfer rate remains the

same as conventional RAID systems. In large or sequential I/O

applications, RAID5 permits the blocks in the same parity group to be

processed in parallel, resulting in an increase in the data transfer rate. For

small writes to single blocks, RAID5 requires extra reads from the data and

parity disks before a small block can be written to the disk. Since the parity

data is distributed on all disks in the group, it still allows parallel I/O

processing of multiple blocks.

If the parity disk were fixed at a single disk device, the parity disk would be

busied during a single write that is executed to update the parity data. This

would make it impossible to perform parallel I/O processing because the

parity disk would always be busy with the first block of data. This is a

problem with RAID3 that RAID5 has alleviated.

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HP XP48 Manuel du propriétaire Page 100