absurd I/O perf with ZFS: hangs on zfs->cv)

[Fabian Keil]

Eitan Adler <lists at eitanadler.com> wrote:

> On 17 January 2013 07:52, Fabian Keil <freebsd-listen at fabiankeil.de> wrote:
> > Eitan Adler <lists at eitanadler.com> wrote:
> > I don't think there are any laptops with "large amounts of RAM"
> > as far as ZFS is concerned.
> 
> Haha okay: 8GB of RAM.
> 
> >> It is taking me 45 minutes to make 5 commits to git.  Something is
> >> wrong here but I have no idea what I should be looking at.  Any ideas?
> >
> > Try sysutils/zfs-stats to get a rough idea of how ZFS is using
> > the available memory.
> 
> Anything in particular I should be looking for?

I mainly look at the "ARC Summary" and the "ARC Efficiency"
sections but I suppose all the information is useful in
some situations.
 
> > If you already followed tuning advice from the Internet without
> > benchmarking it, try reverting it.
> 
> I have done absolutely no tuning.  Is there anything in particular I
> *should* tune?

A common recommendation is to disable atime for all datasets where
it isn't needed as it can cause lots of unnecessary write operations.

With 8 GB of RAM ZFS enables prefetching by default and I assume
for the git use case it's not too useful and could hurt performance
by amplifying read operations. The "Data Prefetch Efficiency" is
shown by zfs-stats and if it doesn't look too impressive you might
want to disable prefetching to see if it helps.

If your repository isn't fresh, you could also try "git gc".
My impression is that the automatic doesn't trigger frequently
enough for larger repositories like /usr/src.

Fabian
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