[systemd-devel] journal fragmentation on Btrfs

[Chris Murphy]

Here's an example rotated log (Btrfs, NVMe, no compression, default
ssd mount option). As you can see it takes up more space on disk than
it contains data, so there's a lot of slack space for some reason,
despite /etc/systemd/journald.conf being unmodified and thus
Compress=Yes.

file: system at 2547b430ecdc441c9cf82569eeb22065-0000000000000001-00054c3c31bec567.journal
extents 41 disk size 143511552 logical size 100663296 ratio 0.70

$ sudo btrfs fi defrag -c
system at 2547b430ecdc441c9cf82569eeb22065-0000000000000001-00054c3c31bec567.journal

And now:

file: system at 2547b430ecdc441c9cf82569eeb22065-0000000000000001-00054c3c31bec567.journal
extents 768 disk size 21504000 logical size 100663296 ratio 4.68

That's nearly 1/7th smaller. The existing defrag without compression
is probably just increasing write amplification on SSDs. If it's badly
fragmented just leave it alone.

This also works on nocow journals with +C set, although I'm not sure
whether this is intended behavior (I thought nocow implies no
compression); so I've asked about that on the Btrfs list.

Chris Murphy