Dell poweredge VRTX Kullanıcı Kılavuzu - Sayfa 13

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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:

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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.

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

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Support for Self Monitoring and Reporting Technology (SMART)

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Support for Patrol Read

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Physical disk failure detection

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Physical disk rebuild using hot spares

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Controller Cache Preservation

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

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Detection of batteries with low charge after boot up

The following sections describe some methods to achieve fault tolerance.

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.