3Ware 720-0127-02 Manual page 25

Table 4: StorSave Profile Definitions
Write Journaling
Disable Cache on
Degrade
FUA (Force Unit Access)
controller or a program (such as a database program) can ensure that data
is actually written to the disk drive media, and is not stored in cache.
When a write command is followed with a FUA command, then the disk
drive will only issue "command complete" to the controller once the data
is written to media. When performance is considered more important than
protection, it may be desirable to ignore FUA commands.
FUA commands are a way that the RAID controller or a program (such as
a database program) can ensure that data is actually written to the disk
drive media, and is not stored in cache. When a write command is
followed with a FUA command, then when the disk drive will only issue
'command complete' to the controller once the data is written to media.
The Protection and Balanced profiles honors FUA commands; the
Performance profile ignores them.
If you use a battery backup unit (BBU), FUA is ignored, because the BBU
preserves the contents of the controller cache memory for a limited period
of time (up to 72 hours), in the event of a power failure.
Write Journaling
preserves a copy of data that has not yet been written to the disk media.
Following a power failure or in the event of accidental drive removal and
reinsertion, the firmware can recover the unit without data loss. All
pending writes sitting in the controller cache are replayed after power is
restored or the drive is reinserted and are flushed from the controller to
the drive.
Using write journaling helps protect your data, however it can have an
impact on performance.
The Protection profile enables write journaling; the Performance Profile
disables it. The Balanced profile disables it only if no BBU is present.
If write journaling is disabled and a BBU is present, then it is as if the
BBU was disabled for that unit.
Write cache disabled on degrade
use of write cache can be disabled until the unit is rebuilt. Once the unit is
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Protection (Default) Balanced
Enabled Disabled, if no BBU
present. [Enabled, if
BBU is present.]
Enabled Disabled
. FUA commands are a way that the RAID
. Write journaling tracks the writing of data to disk and
StorSave Profiles
Performance
Disabled. [If BBU is
present, this
essentially disables
the BBU for this unit.]
Disabled
. In the event that a unit degrades, the
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