LaCie 131018 - StorView Snapshot - Mac, User Manual

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Optimizing Write Performance
187
In the figure on the previous page, 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.
A.4 Sub-Array
The difficulty in realizing the maximum performance possible introduces the concept of a sub-array.
Suppose an array consisted of two RAID 5 sets,
see Figure A–1 ”Distribution of Data and Parity in a RAID
5 with Five Drives” beginning on page 185. If these are then striped, the resulting array would appear as
shown below. In this case, in order for a Full Stripe Write to be performed, it is still only necessary to
cluster four write commands together, as opposed to the seven necessary as indicated below. The array
of drives appears as two separate sub-arrays, each with it’s own rotating parity.
Figure A–2 Distribution of Data and Parity in a RAID 5 with Eight Drives
Figure A–3 Distribution of Data and Parity in a RAID 5 with Ten Drives and Two Sub-Arrays