consider dropping defrag of journals on btrfs

[Chris Murphy <lists@xxxxxxxxxxxxxxxxx>]

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?


-- 
Chris Murphy
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