hardware for home use large storage

[Jeremy Chadwick]

On Mon, Feb 15, 2010 at 10:49:47AM +0200, Dan Naumov wrote:
> > I had a feeling someone would bring up L2ARC/cache devices.  This gives
> > me the opportunity to ask something that's been on my mind for quite
> > some time now:
> >
> > Aside from the capacity different (e.g. 40GB vs. 1GB), is there a
> > benefit to using a dedicated RAM disk (e.g. md(4)) to a pool for
> > L2ARC/cache?  The ZFS documentation explicitly states that cache
> > device content is considered volatile.
> 
> Using a ramdisk as an L2ARC vdev doesn't make any sense at all. If you
> have RAM to spare, it should be used by regular ARC.

...except that it's already been proven on FreeBSD that the ARC getting
out of control can cause kernel panics[1], horrible performance until
ZFS has had its active/inactive lists flushed[2], and brings into
question how proper tuning is to be established and what the effects are
on the rest of the system[3].  There are still reports of people
disabling ZIL "for stability reasons" as well.

My thought process basically involves "getting rid" of the ARC and using
L2ARC entirely, given that it provides more control/containment which
cannot be achieved on FreeBSD (see above).  In English: I'd trust a
whole series of md(4) disks (with sizes that I choose) over something
"variable/dynamic" which cannot be controlled or managed effectively.

The "Internals" section of Brendan Gregg's blog[4] outlines where the
L2ARC sits in the scheme of things, or if the ARC could essentially
be disabled by setting the minimum size to something very small (a few
megabytes) and instead using L2ARC which is manageable.

[1]: http://lists.freebsd.org/pipermail/freebsd-questions/2010-January/211009.html
[2]: http://lists.freebsd.org/pipermail/freebsd-stable/2010-January/053949.html
[3]: http://lists.freebsd.org/pipermail/freebsd-stable/2010-February/055073.html
[4]: http://blogs.sun.com/brendan/entry/test

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |