User`s guide

Data Caching
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before this data is written to disk. During this interval there is risk of data loss in the
following situations:
• If only one controller is present and this controller fails.
• If power to the controller is lost and its internal battery fails or is discharged.
Write Cache Disabled (Write-through or Conservative Cache Mode)
If write cache is disabled (write-through data caching is enabled), write data is
transferred to the disk drives before completion status is issued to the host initiator. In
this mode, system drives configured with the write cache enabled policy are treated as
though they were configured with write cache disabled, and the cache is flushed.
Disabling write cache (enabling write-through or conservative mode) provides a higher
level of data protection after a critical storage system component has failed. When the
condition disabling write cache is resolved, the system drives are converted to their
original settings.
Conditions that disable write cache are as follows:
• The Enable Conservative Cache controller parameter is enabled in TPM for a
dual-active controller configuration, and a controller failure has occurred.
• A power supply has failed (not simply that a power supply is not present).
In this case the SES puts the RAID into conservative cache mode. This condition
also triggers the audible alarm.
• An out-of-limit temperature condition exists.
In this case the SES puts the RAID into conservative cache mode.
This condition also
triggers the audible alarm.
• The controller receives an indication of an AC failure.
To protect against single-controller failure, certain releases of the storage system support
dual controllers. To protect against power loss, an internal battery in the controller
module maintains the data for up to 72 hours.