Chapter 8. Configuring Units
118
3ware Serial ATA RAID Controller User Guide
•
FUA (Force Unit Access)
. 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 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.
The Protection and Balanced profiles honor FUA commands if no BBU is
present; the Performance profile ignores them regardless of whether a
BBU is present.
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
. Write journaling tracks the writing of data to disk and
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.
Table 9: StorSave Profile Definitions
Protection (Default)
Balanced
Performance
Definition
Maximum data
protection, but slower
performance.
More data protection than
Performance
but less
data protection than
Protection
.
Maximum performance
for the unit, but less data
protection.
FUA (Force Unit
Access)
Honor FUA (If no BBU is
present)
Ignore FUA (If BBU is
present)
Honor FUA (If no BBU is
present)
Ignore FUA (If BBU is
present)
Ignore FUA
Write Journaling
Enabled
Disabled, if no BBU
present. (Enabled, if BBU
is present.)
Disabled (If BBU is
present, this essentially
disables the BBU for this
unit.)
Disable Cache on
Degrade
Enabled
Disabled Disabled
