[systemd-devel] Slow startup of systemd-journal on BTRFS

[Chris Murphy]

On Jun 15, 2014, at 4:34 PM, Lennart Poettering <lennart at poettering.net> wrote:

> On Wed, 11.06.14 20:32, Chris Murphy (lists at colorremedies.com) wrote:
> 
>>> 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!)
> 
> Not really the case. 
> 
> http://cgit.freedesktop.org/systemd/systemd/tree/src/journal/journal-file.c#n354
> 
> We allocate 8mb at minimum.
> 
>>> 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.
> 
> Well, it would be good if you'd tell me what to do instead...
> 
> I am invoking fallocate() in advance, because we write those files with
> mmap() and that of course would normally triggered SIGBUS already on the
> most boring of reasons, such as disk full/quota full or so. Hence,
> before we do anything like that, we invoke fallocate() to ensure that
> the space is actually available... As far as I can see, that pretty much
> in line with what fallocate() is supposed to be useful for, the man page
> says this explicitly:
> 
>     "...After a successful call to posix_fallocate(), subsequent writes
>      to bytes in the specified range are guaranteed not to fail because
>      of lack of disk space."
> 
> Happy to be informed that the man page is wrong. 
> 
> I am also happy to change our code, if it really is the wrong thing to
> do. Note however that I generally favour correctness and relying on
> documented behaviour, instead of nebulous optimizations whose effects
> might change with different file systems or kernel versions...
> 
>>> 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...
> 
> What? No need to be dick. Nobody ever pinged me about this. And yeah, I
> think I have a very good reason to use fallocate(). The only reason in
> fact the man page explicitly mentions.
> 
> Lennart

For what it's worth, I did not write what is attributed to me above.  I was quoting Dave Chinner, and I've confirmed the original attribution correctly made it onto the systemd-devel@ list.

I don't know whether some people on this distribution list are even subscribed to systemd-devel@ so those subsequent responses aren't likely being posted to systemd-devel@ but rather to linux-btrfs at .


Chris Murphy