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

[Eitan Adler]

On 18 January 2013 05:20, Fabian Keil <freebsd-listen at fabiankeil.de> wrote:
> A common recommendation is to disable atime for all datasets where
> it isn't needed as it can cause lots of unnecessary write operations.

Good call.  I thought I had already disabled atime updating but it turns out
that some datasets had this property still on.

> 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.

I will look at this number - thanks.

> 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.

This isn't git specific. Doing anything remotely I/O related takes forever:

- starting a new shell take over a minute
- opening a file in vim which wasn't opened recently take a few minutes
- etc.

That said, I will be looking closely at the zfs-stats and see if I
notice any patterns.


-- 
Eitan Adler