Administrator Guide

Linear volumes
Linear volumes make use of a method of storing user data in sequential, fully allocated physical blocks. Mapping between the
logical data presented to hosts and the physical location where it is stored is fixed, or static.
About volume cache options
You can set options that optimize reads and writes performed for each volume. It is recommended that you use the default
settings.
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. Used incorrectly, write-back caching can hinder system performance.
When modifying a volume you can change its 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 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. For more information, see Changing system cache settings.
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 supercapacitor technology, if the system loses power, data is not lost. For most applications, this
is the preferred setting.
NOTE: The best practice for a fault-tolerant configuration is to use write-back caching.
Cache optimization mode
CAUTION:
Changing the cache optimization setting while I/O is active can cause data corruption or loss. Before
changing this setting, quiesce I/O from all initiators.
You can also change the optimization mode.
● Standard. This controller cache mode of operation is optimized for sequential and random I/O and is the optimization of
choice for most workloads. In this mode, the cache is kept coherent with the partner controller. This mode gives you high
performance and high redundancy. This is the default.
● No-mirror. In this mode of operation, the controller cache performs the same as the standard mode with the exception that
the cache metadata is not mirrored to the partner. While this improves the response time of write I/O, it comes at the cost
of redundancy. If this option is used, the user can expect higher write performance but is exposed to data loss if a controller
fails.
Optimizing read-ahead caching
NOTE:
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.
You can change the amount of data read in advance. Increasing the read-ahead cache size can greatly improve performance for
multiple sequential read streams.
Getting started
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