Changing A Volume's Write-Back Cache Setting - HP 2000fc Reference Manual

Changing a Volume's Write-Back Cache Setting

As an Advanced Manage user, 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 disk, 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 drive. 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 disk 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 as described in "Changing Auto-Write-Through Triggers and
Behaviors" on page 92.
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.
Caution – Only disable write-back cache if you fully understand how the host
operating system, application, and adapter move data. You might hinder your
storage system's performance if used incorrectly.
Chapter 3 Managing Storage 91