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Old 11-06-2012, 03:52 PM   #1
dbrazeau
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scsi device queue_depth effect on performance


I have a system with a few LSI controllers. There are 3 RAIDs in the system with different number and types of drives with different performance characteristics. I noticed that reducing the queue_depth from 128 for scsi devices to something really small like 2 or 4 seems to help the overall performance when read or writing to all of the RAIDs simultaneously.

Performance seems fine with only a couple of RAIDs, but when I add more there seems to be some system resource that causes a bottleneck. When I run into this issue for some reasons reducing the queue_depth from 128 to 2, 3, or 4 for all of the drives helps alleviate some if not almost all of the bottle-necking.

First of all this seems rather weird since having a larger queue depths usually helps performance as apposed to hurt them.

Can anyone explain this behavior to me?

Is there some other queue depth I need to increase in the scsi or block layer when having lots of drives and RAIDs?

Last edited by dbrazeau; 11-06-2012 at 03:53 PM.
 
Old 11-06-2012, 04:16 PM   #2
dbrazeau
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Oh, should also probably note these are all RAID 0 arrays.
 
Old 11-08-2012, 02:03 PM   #3
dbrazeau
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So it looks like reducing the queue_depth of the drives results in more I/O requests being merged which leads to an increase to overall performance.
 
  


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