Questions re swap-on-zfs

Johan Hendriks joh.hendriks at gmail.com
Fri Oct 4 10:42:27 UTC 2013


Beeblebrox wrote:
> I have 2 swap devices on my system. #1-tank0/swap on ZFS, #2-/dev/ada0p1 on
> SSD 1GB. I have some questions for swap-on-zfs:
>
> 1. I had read in the past that swap-on-zfs can cause instability if memory
> gets used to full capacity. Is this still the case, or has that issue been
> corrected? The reason I created swap-#2 is precisely for this purpose; a
> small backup resource.
>
> 2. If the problem still persists, I would like to tweak the setup so that
> swap-#2 is only used as last resort. I tried setting the priority (pri) in
> fstab, but it does not seem to work - how can this be done?
> /dev/ada0p1   none   swap   sw,pri=0   0  0
> tank0/swap    none   zfs    sw,pri=9   0  0
>
> 3. Is it necessary for the data on swap to go through the Intent Log? I have
> a separate ZIL device for tank0 pool. What risk is there if I disable the
> log for the swap dataset? Is this done by modifying the "primarycache"
> parameter for dataset?
>
> 4. Any other ideas to speed-up swap performance on tank0/swap?
>
> Regards.
>
I have one machine that has swap on ZFS.
This is the only machine that hangs completely when a swap starvation 
occurs.
It happens regualy because it is an old machine with only 2GB of RAM. 
That machine is running a version of FreeBSD 9.1 stable from april i guess.
It is not an heavy duty server just an offsite backup system, so I can 
live with it.
All other machines tho heavier on RAM have no problems even when a swap 
starvation occurs.

I will not use swap on ZFS anymore.
regards
Johan Hendriks


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