Ubuntu 19.10 Server KVM ZFS performance issue

Software & Operating Systems Linux
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21

I think it must have to do something with my VM setup because when I just do this it behaves like I expect it to behave.

rsync -avP /mnt/usb/dca.mp4 /mnt/data/
sending incremental file list
dca.mp4
 51,200,000,000 100%  199.81MB/s    0:04:04 (xfr#1, to-chk=0/1)

sent 51,212,500,081 bytes  received 35 bytes  209,458,078.18 bytes/sec
total size is 51,200,000,000  speedup is 1.00
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22

Nice find! So it looks like the filesystem is performing okay. Now that the ashift is set correctly, what happens if you use a raw disk image for your VMs?

I’m far from a storage expert, but a qcow2 image is copy-on-write. Since ZFS already is a copy-on-write filesystem, I’m assuming using qcow2 on ZFS will bring with it some performance loss due to overhead? I’m not sure of this though.

23

have you tried a zvol instead of using a img file?

24

Some good references for what you are trying to do. Make sure to match the dataset and qcow2 recordsizes.

https://jrs-s.net/2018/03/13/zvol-vs-qcow2-with-kvm/

26

I need new specs. I completely glossed over the first link. Ouch.

27

@cloudstone I will give a RAW disk image a try just to see if it makes a difference for me.
@nx2l I am not using ZVOL just for one property alone. In order to take a snapshot of a 4TB ZVOL I actually need 4TB of available space on that pool. If I have just a file in that dataset I can take snapshots at any capacity.
@Log Thanks for sharing, it was an interesting read.

28

With ZFS there isn’t really a performance advantage to preallocating space. It’s a copy on write filesystem.

29

even if you dont have compression on… i dont think it does that, but i cant be 100% sure. so i wont fight you about it.

30

Yea that doesn’t make sense, snapshots don’t take up hardly any space at all to take. Changes after a snapshot do.

1 Like