User's Manual

Controller — Supported Stripe Sizes

When creating a virtual disk, you must specify the stripe size for the virtual disk. Different controllers have

different limitations on the stripe sizes they can support. For information on the supported stripe sizes for

a controller, see the virtual disk specifications section for the controller in Supported Features.

RAID Controller Read, Write, Cache, And Disk Cache

Policy

When creating a virtual disk, you can specify the read, write, and cache policies for the virtual disk. The

following subsection describes these policies.

Related Links

Changing The Virtual Disk Policy

Read Policy

Does my controller support this feature? See Supported Features.

The read policies indicate whether the controller should read sequential sectors of the virtual disk when

seeking data.

• Read Ahead — The controller reads sequential sectors of the virtual disk when seeking data. Read

ahead policy may improve system performance if the data is written to the sequential sectors of the

virtual disk.

• No Read Ahead — Selecting no read ahead policy indicates that the controller should not use read

ahead policy.

• Adaptive Read Ahead — The controller initiates read ahead only if the two most recent read requests

accessed sequential sectors of the disk. If subsequent read requests access random sectors of the

disk, the controller reverts to no read ahead policy. The controller continues to evaluate whether read

requests are accessing sequential sectors of the disk, and initiates read ahead if necessary.

• Read Cache Enabled — The controller reads the cache information to verify if the requested data is

available in the cache before retrieving the data from the disk. Reading the cache information first can

provide faster read performance because the data (if available in the cache) can be retrieved more

quickly from the cache than from the disk.

• Read Cache Disabled — The controller retrieves data directly from the disk and not from the cache.

Write Policy

Does my controller support this feature? See Supported Features.

The write policies specify if the controller sends a write-request completion signal as soon as the data is

in the cache or after it has been written to the disk.

• Write Back — The controller sends a write-request completion signal as soon as the data is in the

controller cache but has not yet been written to disk. Write back caching may provide improved

performance since subsequent read requests can retrieve data quickly from the cache then from the

disk. However, data loss may occur in the event of a system failure which prevents that data from

being written on a disk. Other applications may also experience problems when actions assume that

the data is available on the disk.