Re: Slow startup of systemd-journal on BTRFS

[Dave Chinner <david@fromorbit.com>]

On Thu, Jun 12, 2014 at 11:21:04AM +1000, Dave Chinner wrote:
> On Wed, Jun 11, 2014 at 11:28:54PM +0200, Goffredo Baroncelli wrote:
> > Hi all,
> > 
> > I would like to share a my experience about a slowness of systemd when used on BTRFS.
> > 
> > My boot time was very high (about ~50 seconds); most of time it was due to NetworkManager which took about 30-40 seconds to start (this data came from "systemd-analyze plot").
> > 
> > I make several attempts to address this issue. Also I noticed that sometime this problem disappeared; but I was never able to understand why.
> > 
> > However this link
> > 
> > 	https://bugzilla.redhat.com/show_bug.cgi?id=1006386
> > 
> > suggested me that the problem could be due to a bad interaction between systemd and btrfs. NetworkManager was innocent. 
> 
> systemd has a very stupid journal write pattern. It checks if there
> is space in the file for the write, and if not it fallocates the
> small amount of space it needs (it does *4 byte* fallocate calls!)
> and then does the write to it.  All this does is fragment the crap
> out of the log files because the filesystems cannot optimise the
> allocation patterns.
> 
> Yup, it fragments journal files on XFS, too.
> 
> http://oss.sgi.com/archives/xfs/2014-03/msg00322.html
> 
> IIRC, the systemd developers consider this a filesystem problem and
> so refused to change the systemd code to be nice to the filesystem
> allocators, even though they don't actually need to use fallocate...

BTW, the systemd list is subscriber only, so thay aren't going to
see anything that we comment on from a cross-post to the btrfs list.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com