[systemd-devel] consider dropping defrag of journals on btrfs

[Lennart Poettering]

On Di, 26.01.21 21:00, Chris Murphy (lists at colorremedies.com) wrote:

> On Tue, Jan 5, 2021 at 10:04 AM Chris Murphy <lists at colorremedies.com> wrote:
> >
> > f27a386430cc7a27ebd06899d93310fb3bd4cee7
> >     journald: whenever we rotate a file, btrfs defrag it
> >
> > Since systemd-journald sets nodatacow on /var/log/journal the journals
> > don't really fragment much. I typically see 2-4 extents for the life
> > of the journal, depending on how many times it's grown, in what looks
> > like 8MiB increments. The defragment isn't really going to make any
> > improvement on that, at least not worth submitting it for additional
> > writes on SSD. While laptop and desktop SSD/NVMe can handle such a
> > small amount of extra writes with no meaningful impact to wear, it
> > probably does have an impact on much more low end flash like USB
> > sticks, eMMC, and SD Cards. So I figure, let's just drop the
> > defragmentation step entirely.
> >
> > Further, since they are nodatacow, they can't be submitted for
> > compression. There was a quasi-bug in Btrfs, now fixed, where
> > nodatacow files submitted for decompression were compressed. So we no
> > longer get that unintended benefit. This strengthens the case to just
> > drop the defragment step upon rotation, no other changes.
> >
> > What do you think?
>
> A better idea.
>
> Default behavior: journals are nodatacow and are not defragmented.
>
> If '/etc/tmpfiles.d/journal-nocow.conf ` exists, do the reverse.
> Journals are datacow, and files are defragmented (and compressed, if
> it's enabled).

Performance is terrible if cow is used on journal files while we write
them.

It would be great if we could turn datacow back on once the files are
archived, and then take benefit of compression/checksumming and
stuff. not sure if there's any sane API for that in btrfs besides
rewriting the whole file, though. Anyone knows?

Just dropping FS_NOCOW_FL on the existing file doesn#t work iirc, it
can only be changed while a file is empty last time i looked iirc.

Lennart

--
Lennart Poettering, Berlin