About Volume Cache Options; Using Write-Back Or Write-Through Caching; Optimizing Read-Ahead Caching - HP P2000 G3 Reference Manual

IMPORTANT:
replication. Attempting to do so could interfere with replication operation.
Volume mapping settings are stored in disk metadata. If enough of the disks used by a volume are moved
into a different enclosure, the volume's vdisk can be reconstructed and the mapping data is preserved.

About volume cache options

You can set options that optimize reads and writes performed for each volume.

Using write-back or write-through caching

CAUTION: Only disable write-back caching if you fully understand how the host operating system,
application, and adapter move data. If used incorrectly, you might hinder system performance.
You can change a volume's write-back cache setting. Write-back is a cache-writing strategy in which the
controller receives the data to be written to disks, stores it in the memory buffer, and immediately sends the
host operating system a signal that the write operation is complete, without waiting until the data is actually
written to the disk. Write-back cache mirrors all of the data from one controller module cache to the other.
Write-back cache improves the performance of write operations and the throughput of the controller.
When write-back cache is disabled, write-through becomes the cache-writing strategy. Using write-through
cache, the controller writes the data to the disks before signaling the host operating system that the process
is complete. Write-through cache has lower write operation and throughput performance than write-back,
but it is the safer strategy, with minimum risk of data loss on power failure. However, write-through cache
does not mirror the write data because the data is written to the disk before posting command completion
and mirroring is not required. You can set conditions that cause the controller to change from write-back
caching to write-through caching.
In both caching strategies, active-active failover of the controllers is enabled.
You can enable and disable the write-back cache for each volume. By default, volume write-back cache is
enabled. Because controller cache is backed by super-capacitor technology, if the system loses power,
data is not lost. For most applications, this is the correct setting. But because back-end bandwidth is used to
mirror cache and because this mirroring uses back-end bandwidth, if you are writing large chunks of
sequential data (as would be done in video editing, telemetry acquisition, or data logging), write-through
cache has much better performance. Therefore, you might want to experiment with disabling the write-back
cache. You might see large performance gains (as much as 70 percent) if you are writing data under the
following circumstances:
• Sequential writes
• Large I/Os in relation to the chunk size
• Deep queue depth
If you are doing random access to this volume, leave the write-back cache enabled.
The best practice for a fault-tolerant configuration is to use write-back caching.

Optimizing read-ahead caching

CAUTION: Only change read-ahead cache settings if you fully understand how the host operating
system, application, and adapter move data so that you can adjust the settings accordingly.
You can optimize a volume for sequential reads or streaming data by changing its read-ahead cache
settings. Read ahead is triggered by two back-to-back accesses to consecutive LBA ranges, whether
forward (increasing LBAs) or reverse (decreasing LBAs).
In an FC/iSCSI combo system, do not connect hosts or map volumes to host ports used for
HP StorageWorks P2000 G3 MSA System SMU Reference Guide 25