DataDirect Networks EF2800, User Manual

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R A I D M a n a g e m e n t
DataDirect Networks EF2800 FC RAID Storage System User Guide (V 1.0)
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6.3.5 Changing the Read-Ahead Cache Size
You can change the read-ahead cache size for each partition. This
setting controls how much data the SM reads ahead and stores in its
cache memory during sequential reads.
If you want to change this size, you should be prepared to monitor the
system performance using the array statistics and adjust the size until
you find the optimal size for your application.
The default setting is one chunk for the first access in a sequential read
and one stripe for all subsequent accesses. The size of the chunk is
based on the chunk size used when you created the array (default is 64
KB). The SMs treat volumes and mirrored arrays (RAID 1) internally as
if they have a stripe size of 64 KB, even though they are not striped.
If you specify a read-ahead cache size, that amount of data is read first,
and the same amount is read for all read-ahead accesses.
Read-ahead is triggered by two back-to-back accesses to consecutive
logical block address (LBA) ranges. Read-head can be forward (that is,
increasing LBAs) or reverse (that is, decreasing LBAs).
Setting the read-ahead size to 0 turns off read-ahead cache. This is
useful if the host is triggering read-ahead for what are random accesses.
This can happen if the host breaks up the random I/O into two smaller
reads, triggering read-ahead. You can use the partition statistics read
histogram to determine what size accesses the host is doing.
Setting the read-ahead size to a very large size (say four to five stripes)
can help improve the performance of multiple (three or more)
sequential read streams, in some cases improving performance by three
times. This would be useful for providing multiple video streams, for
example.
To change the read-ahead cache size:
1. From SAM, select
Manage EF2800
>
LUN Management
>
Partition
Menu
>
Read Ahead Cache
.
The Read Ahead Cache page displays with a list of all existing
arrays