Hi Aaron You did a perfect job of explaining a situation I've run into time after time - high latency on the disk subsystem causing a backup in the NSD queues. I was doing what you suggested not to do - "mmfsadm saferdump nsd' and looking at the queues. In my case 'mmfsadm saferdump" would usually work or hang, rather than kill mmfsd. But - the hang usually resulted it a tied up thread in mmfsd, so that's no good either.
I wish I had better news - this is the only way I've found to get visibility to these queues. IBM hasn't seen fit to gives us a way to safely look at these. I personally think it's a bug that we can't safely dump these structures, as they give insight as to what's actually going on inside the NSD server. Yuri, Sven - thoughts? Bob Oesterlin Sr Storage Engineer, Nuance HPC Grid From: <[email protected]> on behalf of "Knister, Aaron S. (GSFC-606.2)[COMPUTER SCIENCE CORP]" <[email protected]> Reply-To: gpfsug main discussion list <[email protected]> Date: Tuesday, August 16, 2016 at 8:46 PM To: gpfsug main discussion list <[email protected]> Subject: [EXTERNAL] [gpfsug-discuss] Monitor NSD server queue? Hi Everyone, We ran into a rather interesting situation over the past week. We had a job that was pounding the ever loving crap out of one of our filesystems (called dnb02) doing about 15GB/s of reads. We had other jobs experience a slowdown on a different filesystem (called dnb41) that uses entirely separate backend storage. What I can't figure out is why this other filesystem was affected. I've checked IB bandwidth and congestion, Fibre channel bandwidth and errors, Ethernet bandwidth congestion, looked at the mmpmon nsd_ds counters (including disk request wait time), and checked out the disk iowait values from collectl. I simply can't account for the slowdown on the other filesystem. The only thing I can think of is the high latency on dnb02's NSDs caused the mmfsd NSD queues to back up. Here's my question-- how can I monitor the state of th NSD queues? I can't find anything in mmdiag. An mmfsadm saferdump NSD shows me the queues and their status. I'm just not sure calling saferdump NSD every 10 seconds to monitor this data is going to end well. I've seen saferdump NSD cause mmfsd to die and that's from a task we only run every 6 hours that calls saferdump NSD. Any thoughts/ideas here would be great. Thanks! -Aaron
_______________________________________________ gpfsug-discuss mailing list gpfsug-discuss at spectrumscale.org http://gpfsug.org/mailman/listinfo/gpfsug-discuss
