Configuring RAID for Optimal Performance

Impact of RAID Settings on Performance Configuring RAID for Optimal Performance

4.3 Read Ahead Policy

The Read Ahead Policy determines whether the RAID controller will read just a block of data

that an application has requested, or whether it will read the whole stripe from the hard-drives.

This setting can have big impact on read performance.

4.3.1 No Read Ahead (Normal)

The RAID controller will read only the block of data that the application has requested. This

mode is preferred when read requests are primarily random. Also this mode is recommended

when measuring sequential read throughput with IOmeter* under Windows.

4.3.2 Always Read Ahead

The RAID controller will read the whole stripe containing the requested data block and will keep

it in cache. Each read operation will consume more hard drive resources, but if the read

requests are primarily sequential it can substantially reduce the amount of read requests to the

hard drives and can substantially increase performance.

Note: This setting will only make difference if the typical read request size is smaller than the

stripe width.

4.3.3 Adaptive Read Ahead

The RAID controller automatically adjusts the read policy based on the current pattern of read

requests. It combines the benefits of No Read Ahead and Always Read Ahead modes. This

mode is recommended if the workload has mixed sequential and random patterns, or if the

pattern is unknown.

4.4 I/O Policy

The I/O Policy determines whether the RAID controller will keep data in the cache, which can

reduce the access time if subsequent read requests are made to the same data blocks.

4.4.1 Direct I/O

Direct IO mode is recommended in most cases. Most file systems and many applications have

their own cache and do not require caching data at the RAID controller level.

4.4.2 Cached I/O

In Cached I/O mode the controller caches both read and write requests. If there are subsequent

read requests to the same data blocks, they are read from the RAID cache instead of the hard

drives. This mode may be required if the application or file system does not cache read

requests.

4.5 Strip Size

Strip size determines how data is distributed across hard drives. It also determines how many

drives are accessed to service a single I/O request. Strip size can have big impact on