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 %util
05:29:01 AM dev8-0 36.16 94.01 683.65 0.00 21.51 0.03 0.67 1.11
05:29:01 AM dev8-16 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00
05:29:01 AM dev8-32 0.02 0.00 0.00 0.00 0.00 0.00 1.00 0.00
05:29:01 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.