zfs performance degradation

Paul Kraus paul at kraus-haus.org
Wed Sep 23 20:08:41 UTC 2015

On Sep 22, 2015, at 13:38, Dmitrijs <war at dim.lv> wrote:

>  I've encountered strange ZFS behavior - serious performance degradation over few days. Right after setup on fresh ZFS (2 hdd in a mirror) I made a test on a file 30Gb size with dd like
> dd if=test.mkv of=/dev/null bs=64k
> and got 150+Mbs speed.

> I've got brand new 2x HGST HDN724040ALE640, 4Тб, 7200rpm (ada0, ada1) for pool data4.
> Another pool, data2, performs slightly better even on older\cheaper WD Green 5400 HDDs, up to 99Mbs.

> Zpool list:
> nas4free: /mnt# zpool list
> data2  1.81T   578G  1.25T         -    11%    31%  1.00x ONLINE  -
> data4  3.62T  2.85T   797G         -    36%    78%  1.00x ONLINE  -
> Could it happen because of pool being 78% full? So I cannot fill puls full?
> Can anyone please advice how could I fix the situation - or is it normal?

ZFS write performance degrades very steeply when you reach a certain point in terms of zpool capacity. The exact threshold depends on many factors including your specific workload. This is essentially due to the “Copy on Write” (CoW) nature of ZFS. When you write to an existing file ZFS needs to find space for that write operation as it does not overwrite the existing data. As the zpool fills, it becomes harder and harder to find contiguous free space and the write operation ends up fragmenting the data.

But, you are seeing READ performance drop. If the file was written when the ZFS was new (it was one of the first files written) then it is certainly un-fragmented. But, if you ran the READ test shortly after writing the file, then some of it will still be in the ARC (Adaptive Reuse Cache). If there is other activity on the system, then the other activity will also be using the ARC.

If you are rewriting the test file and then reading it, the test file will be fragmented and that will be part of the performance difference.

For my systems (generally VMs using VBox) I have found that 80% is a good threshold because when I get to 85% capacity the performance drops to the point where VM I/O starts timing out.

So the short answer (way too late for that) is that you can, in fact, not use all of the capacity of a zpool unless the data is written once, never modified, and you do not have any snapshots, clones, or the like.

P.S. I assume you are not using DeDupe ? You do not have anywhere enough RAM for that.

Paul Kraus
paul at kraus-haus.org

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