Re: Performance Issues

["Holger Hoffstätte" <holger.hoffstaette@googlemail.com>]

On Fri, 19 Sep 2014 13:18:34 +0100, Rob Spanton wrote:

> I have a particularly uncomplicated setup (a desktop PC with a hard
> disk) and I'm seeing particularly slow performance from btrfs.  A `git
> status` in the linux source tree takes about 46 seconds after dropping
> caches, whereas on other machines using ext4 this takes about 13s.  My
> mail client (evolution) also seems to perform particularly poorly on
> this setup, and my hunch is that it's spending a lot of time waiting on
> the filesystem.

This is - unfortunately - a particular btrfs oddity/characteristic/flaw,
whatever you want to call it. git relies a lot on fast stat() calls,
and those seem to be particularly slow with btrfs esp. on rotational
media. I have the same problem with rsync on a freshly mounted volume;
it gets fast (quite so!) after the first run.

The simplest thing to fix this is a "du -s >/dev/null" to pre-cache all
file inodes.

I'd also love a technical explanation why this happens and how it could
be fixed. Maybe it's just a consequence of how the metadata tree(s)
are laid out on disk.

> I've tried mounting with noatime, and this has had no effect.  Anyone
> got any ideas?  

Don't drop the caches :-)

-h