Avago Technologies MegaRAID SATA 150-4 (523) - Manual (Page 101)

Configuring RAID Arrays and Logical Drives 4-17
Version 2.0 Copyright © 2003–2006 by LSI Logic Corporation. All rights reserved.
c. Select the read policy.
The read ahead parameter enables the read-ahead feature for
the logical drive. You can set this parameter to
No Read Ahead ,
Read-ahead , or Adaptive . No Read Ahead is the default
setting. No Read Ahead specifies that the controller does not
use read-ahead for the current logical drive. Read-ahead
specifies that the controller uses read-ahead for the current
logical drive.
Adaptive specifies that the controller begins using
read-ahead if the two most recent disk accesses occurred in
sequential sectors.
d. Select the write policy.
You can set the write policy to Write Back or Write Thru .
The default setting is Write Thru caching. In Write Back
caching, the controller sends a data transfer completion signal
to the host when the controller cache receives all the data in a
transaction. In
Write Thru caching, the controller sends a
data transfer completion signal to the host after the disk
subsystem receives all the data in a transaction.
Write Thru
caching has a data security advantage over Write Back
caching. Write Back caching has a performance advantage
over Write Thru caching.
Caution. Do not use write back caching for any logical drive in a
NetWare volume.
e. Select the cache policy.
The cache policy applies to reads on a specific logical drive.
The default is
Direct I/O . It does not affect the read ahead
cache. The options are Cached I/O or Direct I/O . Cached
I/O buffers all reads in cache memory and is the default
setting. Direct I/O does not buffer reads in cache memory.
Direct I/O does not override the cache policy settings.
Direct I/O transfers data to cache and the host concurrently.
If the same data block is read again, the host reads it from
cache memory.