Adaptec 39160 - SCSI Card Storage Controller U160 160 MBps Guía de usuario descargar pdf (Pagina 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.

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Adaptec 39160 - SCSI Card Storage Controller U160 160 MBps Guía de usuario Pagina 32