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Set queue_depth for SAN disks

Posted on 2008-10-17
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How can we actually determine to set the  queue_depth value on the SAN disks. Is there a default value.
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Question by:jdenver47
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dfke earned 500 total points
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Use can use the homogeneous queue depth calculation for this.  

The homogeneous queues must meet one of the following statements:
The queued commands are shared among all paths rather than providing servers with additional resources.
The virtual disks (VDisks) are distributed evenly among the input/output (I/O) groups in the cluster.

You can set the queue depth for each VDisk on the servers using the following calculation:
q = ((n ×7000) / (v×p×c))

where:
q is the queue depth per device path
n is the number of nodes in the cluster
v is the number of VDisks configured in the cluster
p is the number of paths per VDisk per host. A path is a route from a server fibre-channel port to a SAN Volume Controller fibre-channel port that provides the server access to the VDisk.
c is the number of hosts that can concurrently access each VDisk. Very few applications support concurrent access from multiple hosts to a single VDisk. This number typically is 1.

Consider the following example:

An eight-node SAN Volume Controller cluster (n = 8)
4096 VDisks (v = 4096)
One server with access to each VDisk (c = 1)
Each host has four paths to each VDisk (p = 4)
The calculation is ((8×7 000)/(4096×4×1)) = 4.

The queue depth in the operating systems must be set to four concurrent commands per path.

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