Virtual Disk Read Cache Policies; Fault Tolerance; The Smart Feature - Dell PowerEdge VRTX User Manual

Conditions Under Which Write-Back Is Employed
Write-Back caching is used under all conditions in which the battery is present and in good condition.
Conditions Under Which Forced Write-Back With No Battery Is Employed
CAUTION: It is recommended that you use a power backup system when forcing Write-Back to ensure there is no
loss of data if the system loses power.
Write-Back mode is available when you select Write Back Force. When Write Back Force mode is selected, the virtual
disk is in Write-Back mode even if the battery is not present.
Write-Through
In Write-Through caching, the controller sends a data transfer completion signal to the host system when the disk
subsystem has received all the data in a transaction.
NOTE: Some data patterns and configurations perform better with a Write-Through cache policy.

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.

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
The following sections describe some methods to achieve fault tolerance.

The SMART Feature

The SMART feature monitors certain physical aspects of all motors, heads, and physical disk electronics to help detect
predictable physical disk failures. SMART-compliant physical disks have attributes for which data can be monitored to
identify changes in values and determine whether the values are within threshold limits. Many mechanical and
electrical failures display some degradation in performance before failure.
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