Slow VM clone on ZFS

[proxmox_st]

For some reason, cloning a VM (on the same storage) takes really long all of a sudden.
It used to take ~30 seconds or so to clone a ~16GB VM.
Now, it suddenly takes >10 minutes. AFAIK nothing about the host was changed.
Also, the cloning itself is still really fast (at least according to the job output):

transferred: 17179869184 bytes remaining: 0 bytes total: 17179869184 bytes progression: 100.00 %

However, after this nothing happens for several minutes and only then is the task finished with "TASK OK".
The task process doesn't appear to run anything, it doesn't use any CPU and barely any RAM during this period.
The host is also fairly idle with ~64GB of 256GB RAM used and ~15% CPU. IO delay is also <5% with occasional jumps to ~10% (while actual cloning takes place).

Any clue what's going on or anything I can try to get to the bottom of this?
Running PVE 5.4-3 w/ ZFS 0.7.13 from the subscriber repo.

 

[wolfgang]

Hi,

a full clone is a copy of the image and will take longer if the storage is under load.
So two runs can have two different results.
And ZFS get slow if you pass the 70% usage mark.

 

[proxmox_st]

Hi,
thanks for your reply.
I don't think that the storage is under much load; the VMs on the host are mostly idle. Is there some benchmark I can run to measure the storage pool performance to rule this out? Also, as I said, the actual copying of the disk seems to be fast, the job status log shows the copy percentages and reaches 100% (see above message) in ~30 seconds, just like before.

Pool usage shouldn't be a problem, the pool is <25% usage.