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

[Phillip Susi]

Lennart Poettering writes:

> You are focussing only on the one-time iops generated during archival,
> and are ignoring the extra latency during access that fragmented files
> cost. Show me that the iops reduction during the one-time operation
> matters and the extra latency during access doesn't matter and we can
> look into making changes. But without anything resembling any form of
> profiling we are just blind people in the fog...

I'm curious why you seem to think that latency accessing old logs is so
important.  I would think that old logs tend to be accessed very
rarely.  On such a rare occasion, a few extra mS doesn't seem very
important to me.  Even if it's on a 5400 rpm drive, typical latency is
what?  8 mS?  Even with a fragment every 8 MB, that's only going to add
up to an extra 128 mS to read and parse a 128 MB log file.  Even with no
fragments it's going to take over 1 second to read that file, so we're
only talking about a ~11% slow down here, on an operation that is rare
and you're going to be spending far more time actually looking at the
log than it took to read off the disk.