HP Surestore Disk Array 12h - And FC60, Advanced User'S Manual, Page: 249 / 466

Managing Disk Array Capacity
249
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•
If you choose to limit the number of global hot spares, make sure you are able to
respond quickly to replace a failed disk. If an operator is always available to replace a
disk, you may not need the added protection offered by multiple global hot spares.
Setting Stripe Segment Size
Another factor you may have to consider is the stripe segment size you use for a LUN. The
stripe segment size determines how much data is written to a disk before moving to the
next disk in the LUN to continue writing. For example, if the stripe segment size is set to 4
K
bytes, the disk array will write 4
K
bytes of data to disk 1, then 4
K
bytes of data to disk 2,
then 4
K
bytes of data to disk 3, and so on. For more information, see "Data Striping" on
page 49 .
When creating a LUN, the default value used for the stripe segment size is the value
currently set for the cache page size (4
K
bytes or 16
K
bytes). The default cache page size is
4
K
bytes.
When setting stripe segment size, consider the following:
•
Stripe segment size can affect disk array performance. The smaller the stripe segment
size, the more efficient the distribution of data read or written across the stripes in the
LUN. However, if the stripe segment is too small for a single I/O operation, the operation
requires access to two disk. Called a stripe crossing, this action reduces performance.
The optimum stripe segment size is the smallest size that will rarely force I/Os to a
second disk.
•
The stripe segment size can be set to any multiple of the cache page size setting. For
example, if the cache page size is set to 4
K
bytes, the stripe segment size can be set to 4
K
bytes, 8
K
bytes, 16
K
bytes, 32
K
bytes, etc.