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November 25th, 2009 14:00
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?
SKT2
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December 10th, 2009 05:00
Brion2
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December 10th, 2009 08:00
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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December 11th, 2009 04:00
is there a possible performance impact by enabling the path_latency_monitor?
Did u mean high watermark is the "latency max" displayed from `powermt display latency`
Brion2
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December 11th, 2009 07:00
Hello. No performance impact has been documented to date. Yes, I am describing max latency.