slow ZFS on FreeBSD 8.1

Peter Jeremy peterjeremy at
Sat Jan 1 03:02:39 UTC 2011

On 2010-Dec-31 15:47:47 -0800, Jeremy Chadwick <freebsd at> wrote:
>Is your ZFS root filesystem associated with a pool that's mirrored or
>using raidzX?

Currently, mirrored.  I'm considering raidz1 at home.  Note that my
work system is a single pool, whereas I'll use a separate pool for
root at home.

>  What about mismatched /boot content (ZFS vs. UFS)?

Can you give me an example of what you mean here.

> What about booting into single-user mode?

I haven't run into any problems here, though I agree that starting
ZFS in single-user mode is a lot messier than starting UFS.

>error/mistake).  Is it worth the risk?  Most administrators don't have
>the tolerance for stuff like that in the middle of a system upgrade or
>what not; they should be able to follow exactly what's in the handbook,
>to a tee.

I've been using FreeBSD for long enough that I'm confident to upgrade
or similar without blindly following a process.  But I agree that
FreeBSD should be usable without needing to be a guru.

>There's a link to at the bottom of the above Wiki page
>that outlines "the madness" that's required to configure the setup, all
>of which has to be done by hand.  I don't know many administrators who
>are going to tolerate this when deploying numerous machines, especially
>when compounded by the complexities mentioned above.

Root on ZFS is still very bespoke.  I agree there's no way you could
roll it out across lots of machines at present but I'm happy to hand-
craft installs on a few machines.  Hopefully, son-of-sysinstall will
support ZFS installs (one prerequisite is someone being willing to do
the work).

>The mmap(2) and sendfile(2) complexities will bite an junior or
>mid-level SA in the butt too -- they won't know why software starts
>failing or behaving oddly (FreeBSD ftpd is a good example).  It just so
>happens that Apache, out-of-the-box, comes with mmap and sendfile use

mmap(2) is a design problem with ZFS - it's present on Solaris as
well.  IMHO, it's the biggest flaw in ZFS.  The sendfile(2) issues
haven't bitten me so I haven't studied them as much but I'm aware
that some fixes were committed recently.

Oh and one root-on-ZFS gotcha that I missed is the lack of gzip
support.  I spent about ½day tracking that down - not helped by the
lack of any documentation or a useful error message (though there is a
comment in the code when you eventually track it down).

Peter Jeremy
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