Users Guide

Write-back
In Write-back caching, the controller sends a data transfer completion signal to the host when the controller cache has received all the data
in a transaction. The controller then writes the cached data to the storage device in the background.
The risk of using write-back cache is that the cached data can be lost if there is a power failure during or when the data is written to the
storage device. This risk is mitigated by using a battery-backed non-volatile cache on the Shared PERC 8 card. See Shared PERC 8 card
cache preservation for further information.
NOTE: The default cache setting for virtual disks in single and dual controller systems is write-back caching.
Conditions under which write-back is employed
Write-back caching is the default caching mode for single and dual Shared PERC 8 card congurations. Write-back caching can be
congured under all conditions in which the battery is present and in good condition.
If a virtual disk is already created on the VRTX system with the rmware version that does not support write-back caching, then the
default will remain as write-through. The virtual disk must be manually changed to write-back before it goes to write-back mode, even if
the system has been updated to a write-back supported rmware version.
Conditions under which forced write-back with no battery is
employed
CAUTION
: It is recommended that you use a power backup system when forced write-back is enabled to ensure there is no loss
of data if the system loses power.
Forced write-back mode may be enabled to allow the virtual disk to operate in write-back mode even if the battery is not present or not
functioning properly.
Write-through
In write-through caching, the controller sends a data transfer completion signal to the host system when the disk subsystem has received
all the data in a transaction.
NOTE
: Most congurations and workloads experience some reduction in performance when congured with Write-Through
cache, as compared to Write-Back cache.
Conditions under which write-through is employed
Write-through caching is used under conditions in which the battery fails or is not present. Write-Through is also enabled when the
controller has pinned cache.
Virtual disk read cache policies
The read policy of a virtual disk determines how the controller handles reads to that virtual disk. The read policies are:
• Read Ahead — Allows the controller to read sequentially ahead of the requested data and store the additional data in cache memory.
This speeds up reads for sequential data, but there is little improvement when accessing random data.
16
Shared PERC 8 card features