Questions re swap-on-zfs

[Volodymyr Kostyrko]

04.10.2013 12:50, 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.

Yes.

> 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

Nope.

> 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?

There's no need for SWAP to go through any caches or ZIL.

> 4. Any other ideas to speed-up swap performance on tank0/swap?

In fact I even tried this:

  * primarycache=metadata (that's would speed up finding blocks without 
caching them;
  * logbias=throughput (don't use ZIL);
  * sync=disabled (write data fast and right now);
  * checksum=off (if it's gone it's gone, period).

The machine still hangs from time to time so I think you wouldn't be 
lucky too.

PS: compression=lz4 makes this host survive longer but this can be just 
my imagination...

-- 
Sphinx of black quartz, judge my vow.