IO Delay issue

[Terry]

I have a recent 4.4 installation that uses a mirrored ZFS root pool on two USB 3.0 SSD thumb drives. The drives are Corsair Voyager GS 64GB sticks with a claimed read speed of 295MB/s and write speeds of 170MB/s, however I am experiencing continuous IO Delays on this pool of between 15% and 30% on an idle machine with CPU at less than 1%.

Using atop, I can see that the drives are both sitting at near 100% utilisation, but the data rates associated are negligible.

Using iotop, I can see that txg_sync is consuming 99% of IO with one kworker thread consuming around 20% and everything else negligible, so it seems to be a ZFS related issue.

Please can anyone suggest an approach to to gathering further debugging information to work out what might be causing ZFS to generate so much IO traffic?

 

[wolfgang]

Hi,

do not use USB Sticks.
This drive will lose very fast speed and are unusable in about 6 month.

 

[Terry]

I believe that the Voyager sticks are SSD over USB rather than Flash, so should not have that problem. There do seem to be several unresolved ZoL bugs in this area, however, so it might be worth migrating onto SATA drives to test.

What is the best approach to migrating a mirrored rpool nowadays?

 

[wolfgang]

I believe that the Voyager sticks are SSD over USB rather than Flash

May be but not all ssd perform good with ZFS.
This are not unresolved bug this is missing sync capacity of consumer devices.

You can use dd to migrate or zfs send and receive.