Dell UCPM-800 Physical Disk Write Cache Policy, Fault Tolerance, Virtual Disk Read Cache Policies

NOTE: If the virtual drives are created with write-through as selected mode, make sure that the virtual drives are changed to write-back mode.

Conditions under which write-through is employed

Write-through caching is used under conditions in which the battery fails or is not present.

Virtual Disk Read Cache Policies

The read policy of a virtual disk determines how the controller handles reads to that virtual disk. The read policies are:

•Read Ahead — Allows the controller to read sequentially ahead of the requested data and store the additional data in cache memory. This speeds up reads for sequential data, but there is little improvement when accessing random data.

•No Read Ahead — Disables the Read-Ahead capability.

•Adaptive Read Ahead — Begins using Read-Ahead if the two most recent disk accesses occurred in sequential sectors. If the read requests are random, the controller reverts to No Read Ahead mode.

NOTE: The default read cache setting for virtual disks is Adaptive Read Ahead.

Physical Disk Write Cache Policy

Management utilities provide the option to modify disk cache policies for physical disks associated with a virtual disk. The following options are available for the Shared PERC controller:

Option	Description
Enabled	Enables physical disk cache for drives associated with the selected virtual disk.
Disabled	Disables physical disk cache for drives associated with the selected virtual disk.
Default	The default cache policy on the physical disk is used. See the physical disk
	documentation for details on the default setting.

WARNING: Enabling physical disk cache is not recommended for disks with critical data as a power failure may result in the loss of data prior to it being flushed to the persistent media.

NOTE: Any changes to the physical disk cache applies to all virtual disks in an entire disk group. It is not possible to have different cache policies on virtual disks from the same disk group.

Fault Tolerance

The fault tolerance features of Shared PERC 8 are as follows:

•Support for Self Monitoring and Reporting Technology (SMART)

•Support for Patrol Read

•Physical disk failure detection

•Physical disk rebuild using hot spares

•Controller Cache Preservation

•Battery and Non-Volatile Cache backup of controller cache to protect data

•Detection of batteries with low charge after boot up

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