> Old news is that systemd-journald journals end up pretty > heavily fragmented on Btrfs due to COW.
This has been discussed before in detail indeeed here, but also here: http://www.sabi.co.uk/blog/15-one.html?150203#150203 > While journald uses chattr +C on journal files now, COW still > happens if the subvolume the journal is in gets snapshot. e.g. > a week old system.journal has 19000+ extents. [ ... ] It > appears to me (see below URLs pointing to example journals) > that journald fallocated in 8MiB increments but then ends up > doing 4KiB writes; [ ... ] So there are three layers of silliness here: * Writing large files slowly to a COW filesystem and snapshotting it frequently. * A filesystem that does delayed allocation instead of allocate-ahead, and does not have psychic code. * Working around that by using no-COW and preallocation with a fixed size regardless of snapshot frequency. The primary problem here is that there is no way to have slow small writes and frequent snapshots without generating small extents: if a file is written at a rate of 1MiB/hour and gets snapshot every hour the extent size will not be larger than 1MiB *obviously*. Filesystem-level snapshots are not designed to snapshot slowly growing files, but to snapshots changing collections of files. There are harsh tradeoffs involved. Application-level shapshots (also known as log rotations :->) are needed for special cases and finer grained policies. The secondary problem is that a fixed preallocate of 8MiB is good only if in betweeen snapshots the file grows by a little less than 8MiB or by substantially more. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
