3Ware 9550SXU-12 User Manual page 171

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Some RAID levels must be initialized for best performance. (For specifics,
see "Initialization of Different RAID Types" on page 160.) When these units
are created in the BIOS (through 3BM), you can choose to do a foreground
initialization, which will take place before the operating system has loaded, or
a background initialization, which allows you to put the unit in service
immediately, but will slow down the unit performance until it completes. You
can elect to cancel foreground initialization, put the units into service, and
have initialization run in the background, instead.
Foreground Versus Background Initialization
Initialization makes parity information valid. Foreground initialization does
this by simply writing zeroes to all the drives so that they all have the same
values, overwriting any existing data in the process. In contrast, background
initialization uses an algorithm to resynch the parity information on the drives
and does not rewrite existing data.
A foreground initialization can be run from the BIOS using 3BM. It clears all
existing data from the drives. Foreground initialization can take up to several
hours, depending upon the size of the unit. After foreground initialization
completes, you can start the operating system and units will perform at peak
efficiency.
Drives that support the Write Same feature (SCT) can write to multiple drive sectors
at once, improving initialization time. To take advantage of this feature, all the drives
in the unit must support Write Same.
If immediate access to the unit is important, select background initialization.
Background initialization will then start automatically within about 10
minutes. Background initialization (resynching) does not write zeroes to the
drives or harm your data. You can partition, format, and use the unit safely
while it is initializing. The unit is fully fault-tolerant while the initialization
takes place. That is, if the unit degrades before the initialization is complete,
the data will remain intact.
When initializing is done after booting to the operating system, the process of
initializing takes longer than it does if initialization is done by writing zeroes
to the unit in the BIOS. Consequently, it will be a longer period of time until
the performance of the unit is fully optimal.
Although you can use the unit while it is being initialized in the background,
initialization does slow I/O performance until completed. You can adjust how
much initialization will slow performance by setting the rate at which it
occurs. (See "Setting Background Task Rate" on page 172.) You can also
postpone initialization until a scheduled time. (See "Scheduling Background
Tasks" on page 173).
Background Tasks
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