On mine when I first access the NFS volume it takes 5-10 seconds for the disks to spin up. Mine will spin down later in the day if little or nothing is going on and I will get another delay.
I have also seen delays if a disk gets bad blocks and corrects them. About 1/2 of time that does have a message but some of the time there are no messages at all about it, and I have had to resort to using Sar to figure out which disk is causing the issue.
So on my machine I see this (sar -d):
05:29:01 AM DEV tps rkB/s wkB/s dkB/s areq-sz aqu-sz await %util05:29:01 AM dev8-0 36.16 94.01 683.65 0.00 21.51 0.03 0.67 1.1105:29:01 AM dev8-16 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.0005:29:01 AM dev8-32 0.02 0.00 0.00 0.00 0.00 0.00 1.00 0.0005:29:01 AM dev8-48 423.65 71239.92 198.64 0.00 168.63 12.73 29.72 86.07
05:29:01 AM dev8-64 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00
05:29:01 AM dev8-80 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00
05:29:01 AM dev8-144 2071.22 71311.58 212.22 0.00 34.53 11.37 5.47 54.81
05:29:01 AM dev8-96 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00
05:29:01 AM dev8-128 1630.99 71389.49 198.18 0.00 43.89 15.72 9.62 57.05
05:29:01 AM dev8-112 2081.05 71426.01 182.48 0.00 34.41 11.32 5.42 55.68
There is a 4 disk raid6 check going on.
You will notice that dev8-48 is busier than the other 3 disks, in this case that is because it is a 3TB disk vs the other 3 being all newer 6tb disks with higher data/revolution.
If you have sar setup with 60 second samples the one disk that pauses should stand out more obvious than this since the 3tb seems to be only marginally faster than the 6tbs.
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In my case the servers /home is on a partition of the two main Raid0 disks that is shared with the OS and so are active most of the time. No errors reported.
I will try setting up sar with a 60 second sample time on the
client, thanks for the idea.