Re: Debian Buster: Is it safe to use on autodefrag on a Btrfs filesystem that is used for (Restic) backup only with no Btrfs snapshots or subvolumes?

[Judah Richardson <judahrichardson@gmail.com>]

This. From the Btfrs Gotchas page:

Files with a lot of random writes can become heavily fragmented (10000+ extents) causing thrashing on HDDs and excessive multi-second spikes of CPU load on systems with an SSD or large amount a RAM.

• On servers and workstations this affects databases and virtual machine images.
  • The nodatacow mount option may be of use here, with associated gotchas.
• On desktops this primarily affects application databases (including Firefox and Chromium profiles, GNOME Zeitgeist, Ubuntu Desktop Couch, Banshee, and Evolution's datastore.)
  • Workarounds include manually defragmenting your home directory using btrfs fi defragment. Auto-defragment (mount option autodefrag) should solve this problem in 3.0.
• Symptoms include btrfs-transacti and btrfs-endio-wri taking up a lot of CPU time (in spikes, possibly triggered by syncs). You can use filefrag to locate heavily fragmented files (may not work correctly with compression).

On Wed, Jul 31, 2019 at 5:50 AM Mart van de Wege <mvdwege@gmail.com> wrote:

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> Is it safe to use autodefrag for my use case?
>
> It sounds like it might be "safe" (the text doesn't actually say it's
> unsafe, but just that it has downsides).
>
> I do wonder why you'd want to do that, tho.  Fragmentation is typically
> something that clueless Windows users worry about

No. Fragmentation is an issue with all copy-on-write filesystems
(including ZFS, which avoids periodic defrag by keeping an enormous
amount of information in memory and doing defrag on the fly on that).

Mart

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