ZFS on FreeBSD 11.3 slower than 10.4

[Walter Cramer]

On Sat, 23 May 2020, Miroslav Lachman wrote:

> I upgraded my old desktop computer few month ago from old 10.4 based PC-BSD 
> to stock FreeBSD 11.3. It uses single 2TB HDD 7200rpm.
> My problem is that upgraded version is really slow and some desktop 
> applications are very lagging (playing multimedia is interrupted for a 
> fraction of seconds) when there is heavy filesystem activity.
>
> I am using zfsnap2 for taking snapshots periodically and when there is enough 
> snapshots zfs destroy is called. In this time the user experience is 
> terrible. Starting new application like browser or even something much 
> smaller takes minutes. The old version based on FreeBSD 10.4 behaves much 
> better. I used the old version for years and never have problems with 
> interrupted multimedia playback.
>
> Are there some sysctls to tune to get better desktop interactivity in heavy 
> filesystem operations like zfs destroy, pkg check or other "find" periodic 
> scripts?
>
> Kind regards
> Miroslav Lachman
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Does `top` or `vmstat` show that Free Memory is quite low when you are 
experiencing the problem?  We had ARC / Free Memory performance problems 
with 11.X, and found that setting the sysctl "vfs.zfs.arc_max" much lower 
was necessary.