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path_latency_threshold: Why only in Seconds? Anyone using it effectively?
I was excited to learn about this powerpath feature only to see that's it not very useful. I'm hoping someone is finding it useful.
If a system is normally running at 10ms or less for all I/O, it would be nice to know when it goes to 15 or more and, try to resolve it proactively.
The path_latency_threshold seems to work only 1 sec increments. Setting it would only log a notification in the system log when the I/O goes to 1000ms or more. This does not seem very useful in reality.
We get calls on applications that go from 4ms to 6ms, let alone going to 1 sec. It would be nice if this worked with milliseconds instead of seconds.
Anyone finding this parameter useful in their env?
Hi. Please keep in mind that SCSI timeouts are in seconds (with the SCSI driver having a 60 second timeout). PowerPath sits above the SCSI driver. So, an I/O that was retried at the SCSI driver could have a rather large latency value.
You do have a way of using this to discover the latency in this setup.
1. powermt set path_latency_monitor=on
2. powermt set path_latency_threshold = 0
This is called discovery mode and will cause a threshold cross every time a new high watermark is discovered.
This high watermark is also visible from the "powermt display latency".
I hope this helps.
SKT2
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Posted December 10th, 2009 05:00