Dell PowerVault 221S (SCSI) - Manual (Page 32)

A REFERENCE GUIDE FOR OPTIMIZING DELL™ SCSI SOLUTIONS VER A02
PAGE 32 11/17/2005
back up unit (see Table 5-2), there is a risk of data loss if system power
is lost while unwritten information is still in the cache.
o Write Through
In write-through caching, a data transfer is completed when the disk
subsystem receives all of the data from the host. There is no risk of data
loss in a power-loss scenario (as in write back), but it is slower, since the
controller must wait for the hard drive to return a good status on the write
to the controller before proceeding to the next operation.
If the PERC controller has a battery backup unit (see Table 5-2) or if a
host system is powered by an alternate power source (e.g. UPS etc), the
controller’s cache retains data during a power loss. Thus PERCs with
battery back-up not only provide the performance benefit of write-back
caching but also help ensure integrity of data during system power loss.
For data retention time please refer to Table 5-2.
Data Protection Strategy
• Battery back up: see “Write Cache policy considerations” section above.
• Global hot spare
o A global hot spare can be used for any array that is on the same
controller as the hot spare.
o More cost-effective than dedicated hot spares, but does not protect
multiple arrays as completely as dedicated hot spares.
o A global hot spare is selected randomly and may be in a different
enclosure than the failed drive it is replacing. Can impact performance if
rebuilt array spans enclosures/channels.
• Dedicated hot spare
o A dedicated hot spare is assigned to one or more arrays.
o Dedicated hot spares that reside in the same storage enclosure typically
have better performance than global hot spares.
o Dell recommends using dedicated hot spares for critical data.
• Consistency checks
o Consistency checks verify the correctness of redundant data in an array.
o As during a rebuild, array performance will be degraded while check is
running.
o Some PERC controllers allow pausing of consistency check and resume
it later or to resume the consistency check after the system reboots.
o Recommended as part of regular system maintenance for critical data.