Chapter 4
Planning
32
Plan Hot Spares (Optional)
On Worksheet A for each adapter, log any hot spare disk modules and indicate
whether each is global or dedicated to a particular array.
A hot spare is a powered-on, stand-by disk that is ready for use should another
disk fail. When a disk fails, the disk array controller’s firmware can automatically
rebuild the data from the failed disk onto the hot spare. Unless a rebuild occurs, a
hot spare does not contain user data. When planning hot spares, keep these
considerations in mind:
•
Hot spares are useful only for logical drives with RAID levels of 1, 5, 10,
or 50.
•
Hot spares cannot rebuild logical drives of RAID 0, because this RAID
level does not provide a means of recovering data.
•
A dedicated hot spare is assigned to a specific array. Only one hot spare
can be dedicated to each individual array.
•
Global hot spares stand ready to rebuild any physical drive for any array
with redundancy controlled by the adapter.
•
A hot spare does not count toward the usable capacity of any array.
•
A hot spare must have capacity equal to or greater than the capacity of the
physical drive it would replace.
•
An adapter can support up to eight hot spares.
Decide the Rebuild Rate
During a rebuild, the contents of a complete physical drive is rewritten. Normal
operations can go on during a rebuild, but performance may be degraded. The
Rebuild Rate controls the rate at which a rebuild is done by specifying what
percentage of IOP resources will be dedicated to rebuilding the data on a failed
physical drive.
A high Rebuild Rate (over 50%) speeds up the rebuild, but slows system
performance. A low Rebuild Rate (under 50%) slows the rebuild process, but
speeds up system performance. The default is 50%.
RAID 0 data cannot be rebuilt because it has no redundancy.
Log the Rebuild Rate on Worksheet A.
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