Setting "zfs_arc_max" value in FreeBSD 8.

[Alexander Leidinger]

On Fri, 22 Jan 2010 21:39:48 -0500 jhell <jhell at DataIX.net> wrote:

> 
> On Fri, 22 Jan 2010 11:47, fbsdlist@ wrote:
> >>> Anyone know if it is adjustable on a system with 1024MB of ram ?
> >>> Is this just being auto calculated by some other value ?
> >
> > You may want to make sure that vm.kmem_size is set to a value much
> > larger than vfs.zfs.arc_max. Default value may be too small to allow
> > such a large ARC.
> >
> > On a side note, I'm not sure that ZFS is a good match for system
> > with only 1G of RAM. By trial and error on my box with 8G or memory
> > I've figured out that I need to set arc_max ~1G below physical
> > memory size to avoid lockups under load. YMMV.
> >
> 
> ZFS on this box with 1G has been quite enjoyable actually. With the 
> settings I have posted I have not had any lockup on stable/7 and no
> sudden freezes or waits for transfers. So this entirely thus far has
> been a godsend. I had even put this thing through some of the
> tortures that others have posted to the list and not come up with the
> same results but better. There is obviously a lot of variables in
> this between hardware and configurations used so the results are
> minimal in comparison. With ZFS in place on this machine it performs
> a little bit under specs for the hardware but I wouldn't expect
> anything less for such a file-system.

You may want to switch to fletcher4 checksums. This is the default in
Solaris and 8.0 now. I didn't merge this change to 7-stable as I didn't
took the time to analyze if the change for the default has some unwanted
implications for existig pools.

I have a 9-current box with 1GB RAM and ZFS which shows the slow-down
after some hours of running (and doing things) too. It would be good to
make a list of OS versions and if there are slowdowns or not (anyone
with time out there to have a look at the mails and get this info out
of the mails / people?). Maybe it is related to changes not in ZFS...

Bye,
Alexander.