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36 Planning: RAID Controller Modules
Write–Back Cache
In write-back cache, write operations result in a completion signal being sent
to the host operating system as soon as the cache receives the data to be
written. The target physical disk receives the data at a more appropriate time
in order to increase controller performance. In dual-active controller
configurations with write-back caching enabled, the write data is always
mirrored to the cache of the second controller before completion status is
issued to the host initiator. Write-back cache is enabled by default unless
cache mirroring is disabled.
Write–Through Cache
In write-through cache, data is written to the physical disk before completion
status is returned to the host operating system. Write-through cache is
considered more robust than write-back cache, since a power failure is less
likely to cause loss of data. The RAID controller automatically switches to
write-through if cache mirroring is disabled or if the battery is missing or has a
fault condition.
SFP Transceivers, Fiber Optic, and SAS Cables
Figure 3-2 shows fiber optic cable and SFP transceivers.
NOTE: Your SFP transceivers and cables may look different from the ones shown
below. The differences do not affect the performance of the SFP transceivers.
FC host connections may operate at 8 Gbps or at a lower data rate. Ports for 8
Gbps FC host connections require SFP transceivers designed for this data rate.
SFP transceivers that support other data rates are incompatible.
WARNING: Do not disassemble or remove any part of a small form-factor
pluggable (SFP) transceiver due to the possibility of being exposed to laser
radiation.
book.book Page 36 Wednesday, June 8, 2011 5:01 PM