TRIM, iSCSI and %busy waves

Eugene M. Zheganin eugene at
Thu Apr 5 14:08:29 UTC 2018


I have a production iSCSI system (on zfs of course) with 15 ssd disks 
and it's often suffering from TRIMs.

Well, I know what TRIM is for, and I know it's a good thing, but 
sometimes (actually often) I'm seeing my disks in gstat are overwhelmed 
by the TRIM waves, this looks like a "wave" of 20K 100%busy delete 
operations starting on first pool disk, then reaching second, then 
third,... - at the time it reaches the 15th disk the first one if freed 
from TRIM operations, and in 20-40 seconds this wave begins again.

I'm also having a couple of iSCSI issues that I'm dealing through bounty 
with, so may be this is related somehow. Or may be not. Due to some 
issues in iSCSI stack my system sometimes reboots, and then these 
"waves" are stopped for some time.

So, my question is - can I fine-tune TRIM operations ? So they don't 
consume the whole disk at 100%. I see several sysctl oids, but they 
aren't well-documented.

P.S. This is 11.x, disks are Toshibas, and they are attached via LSI HBA.



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