ZFS v13 performance drops with low memory on FreeBSD-7 STABLE

[Ivan Voras]

Kai Gallasch wrote:
> Hi.
> 
> I upgraded a server with 7-STABLE-amd64 and the MFC'd ZFS v13 about 8
> days ago. Since then the machine is running stable and this without
> manually tuning vm.kmem_size, vfs.zfs.arc, etc. in loader.conf - so far
> so good :-)
> 
> In the last few days I noticed some performance issues with zfs, as some
> customers complained about slow mysql database responses.
> 
> MySQL is running in a database jail on a zfs v13 zpool, websites using
> the mysql database are also running on zfs on the same server.
> 
> The server is running about 30 in production jails, has 16GB RAM and 8GB
> swap. Swap usage is about only 1% currently.
> 
> After debugging the mysql settings for a while I found out, that when I
> stopped some processes on the server that were using high amounts of
> RAM, the datbase response times for queries were almost back to normal
> again..
> 
> So for me this looks like when running applications and ZFS compete for
> free RAM, ZFS looses. Is that so?

Yes, that was the point of recent work in stabilizing ZFS - without it
you would probably panic. With it, ZFS's memory is shrunk down. Unless
there are other factors (like swapping; are you swapping on ZFS?), this
is the probable reason for what you're seeing.

It's kind of bad when the file system competes this directly with
applications :(



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