MAXDATA SR1202 M1 Guide de l'utilisateur télécharger pdf (Page 142)

142 Optimizing RAID 5 Write Performance

Sequential Access

If the commands sent from the host are not sequential, the controller will not be able to cluster

them together. So, unless each individual access is sufficient to fill a stripe, a Full Stripe Write will

not occur.

Number of Outstanding Commands

For the controller to successfully cluster commands, there has to be a number of write commands sent

simultaneously. Setting the host to send up to 64 commands should prove adequate. Alternatively,

enabling writeback cache will have a similar effect, as the controller can then cluster sequential

commands even if the host only sends a small number of commands at a time.

Access Size

With very small accesses, it is necessary to have a large number of commands to cluster together to

fill up a full stripe. So, the larger the access size the better. It is best to use an access size that will

fill a chunk. Of course, even if a stripe is not filled up, small sequential writes will still benefit from

command clustering.

Access Alignment

The alignment of a command from a host system is determined by the command’s address. In an

optimal system, a write of one chunk of data would reside exactly within a chunk on one disk. However,

if this is not the case, this write will be split up into two separate writes to two different data drives.

This will have a negative effect on performance. To overcome these problems, the user can, with

more sophisticated operating systems, set the access size and alignment to an optimal value.

As can be seen from the figure below, to get the highest performance from this system, it is necessary

to have a number of stripes being written in parallel. As the array expands, with more and more drives,

the number of commands (and amount of sequential data) necessary to do this increases.

Figure 111. Distribution of Data and Parity in a RAID 5 with Eight Drives

In the figure above, we can see that seven chunks of sequential data are necessary to fill a stripe.

To have multiple commands active for all disk drives, this requires more data than for the case with

five drives. As can be seen, this number will increase as the number of drives increases. If a large

number of drives are used, it may get difficult to achieve maximum performance, as it becomes more

difficult to cluster a large number of commands to achieve a Full Stripe Write.

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MAXDATA SR1202 M1 Guide de l'utilisateur Page 142