User guide

49MAXDATA SR1202 M1 – StorView
®
RAID User Guide
While monitoring the array initialization, under the Array name a progress bar appears displaying the
percent complete for the initialization. Also, in the Enclosure front view, the disk drives being initialized
display an animated icon during the initialization.
You can stop or pause the Initialization process if you wish by clicking on the link located to the
right of the progress bar. Stopping the initialization will cause your array to be trusted. Pausing the
initialization will halt the process until the resume option is selected, see ”Fault Tolerance” beginning
on page 77.
Configuring Array Writeback Cache
In a writeback cache operation, data is sent to the controller from the host and before actually sending
the data to the drives, the controller immediately responds to the host confirming the data was received
and written to the disk (even though the data may have not been written to the disk). The host may
then send more data. This can significantly increase performance for host systems that only send a
low number of commands at a time. The controller caches the data, and if more sequential data is
sent from the host, it can cluster the writes together to increase performance further. If sufficient
data is sent to fill a stripe in RAID 5/50 configurations, the controller can perform a Full Stripe Write,
which significantly reduces the write overhead associated with RAID 5/50.
Disabling writeback cache ensures that the data is sent to the drives before status is returned to the
host. With writeback cache enabled, if a short term power failure occurs, the battery back-up unit
provides adequate power to ensure that cache is written to disk when the power is restored.
In duplex operations, the cache is mirrored to both controllers which provides further redundancy
in the event of a single controller failure. Mirrored cache is designed for absolute data integrity. The
cache in each controller contains both primary cached data for the disk groups it owns, and a copy of
the primary data of the other controllers. Mirrored cache ensures that two copies of cache exist on
both controllers before confirming to the operating system that the write operation has completed.
Normally, write-intensive operations will benefit from the higher performance when writeback cache
is enabled on that array. Read-intensive operations, such as a streaming server, may not benefit from
writeback cache.
Initializing the Array
Initializing an array clears all the data from the drives. This ensures the validity of the data stored on the
array. Two features of initialization are background and parallel. Once the array is created, initialization
automatically begins in the background. While initialization is in progress, logical drives can be created
and the disks are made immediately available to the operating system where data can be loaded.
As arrays are created and the initialization begins, you can have up to 64 arrays initialized in parallel
at the same time. You may also choose to stop the initialization, or pause an initialization and then
resume it at a later time. The controls for managing are displayed on the Main screen next to
the “Array_Name” after the initialization has started. If you Stop an initialization, the array will be
automatically Trusted, see note below.
The array can be initialized at a later time in which you could choose the option to Trust. This option
should only be used in environments where the user fully understands the consequences of the
function. The trust option is provided to allow immediate access to an array for testing application
purposes only.
 NOTE
A trusted array does not calculate parity across all drives and therefore there is no known state on the
drives. As data is received from the host parity is calculated as normal, but it occurs on a block basis.
There is no way to guarantee that parity has been calculated across the entire stripe. The parity data
will be inconsistent and so a drive failure within a trusted array will cause data loss. Before you use
a trusted array in a live environment, you must initialize it.