Dell PowerVault 221S Optimizing Dell SCSI Solutions - Page 32

Write Through

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A REFERENCE GUIDE FOR OPTIMIZING DELL™ SCSI SOLUTIONS VER A02 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. PAGE 32 11/17/2005

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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.