Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices
December2008–RevisionA01 Page12
4.6 Cache Settings
Read-ahead cache can be configured in the MDSM and through the CLI. The
MDSM has only the defaults to work with, while the CLI can fully configure the
read-ahead cache. Additionally, the global cache block size for read and write
cache can be adjusted through the CLI.
Please refer to the Dell™ PowerVault™ Modular Disk Storage Manager CLI
Guide on the Dell™ technical support site (http://support.dell.com/manuals) for a
complete list of supported commands including the following cache specific
commands.
4.6.1 Setting the Virtual Disk-Specific Write Cache and Write Cache Mirroring
Configured through the CLI – These commands are available at the virtual disk
level.
Write Cache – Disabling write cache puts the controllers into a write-through
mode, adding additional latency while data is flushed to the disk. Except for
specific read-only environments, it is recommended that this setting stay
enabled. Write cache is automatically disabled in the case of cache battery
failure or a cache battery learn cycle.
Write Cache Mirroring – Write Cache Mirroring provides an additional level of
redundancy and fault tolerance in the MD3000 and MD3000i. As a side effect, it
reduces available physical memory and intra-controller bandwidth to perform this
operation. In select, non data-critical cases, it can be beneficial to adjust this
parameter. For normal use, Dell™ always recommends Enabling Cache
Mirroring. Cache Mirroring is automatically disabled in the event of controller
failure or when write caching is disabled.
WARNING: Data loss can occur if a RAID controller module fails while write-
caching without cache mirroring is enabled on a virtual disk.
4.6.2 Setting the Virtual Disk-Specific Read Cache Pre-fetch
Configured through the CLI – This command is available at the virtual disk level.
Read Cache Pre-fetch – The Read Cache setting can be toggled on a virtual disk
level. Disabling read pre-fetch is primarily useful in primarily small transfer size
random read environments, where pre-fetch of random data would not provide
sufficient value. However, the normal observed overhead of read pre-fetch is
negligible. For most environments, Dell™ always recommends Enabling Read
Pre-fetch.
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