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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?



Hi All,

First time on the mailing list.

System:

OS: Debian Buster with KDE
CPU: Intel Core i3-2100
RAM: 8 GB
OS SSD: 1 TB Crucial MX500 SSD, where /home folder is located. Formatted as ext4
Other HDD: 2 x 2 TB Toshiba L200 HDD, both used completely for Btrfs RAID1 (files and metadata) with a single subvolume, no Btrfs snapshots

Goal:

To store rotating snapshots of some folders in /home/MyUsername/ (located on SSD) on the Btrfs filesystem located on the 2 HDDs. I've chosen Restic for this since rsnapshot's developer switched to Borg and Borg doesn't do snapshots.

Problem:

I want to minimize fragmentation on the Btrfs file system, but Debian's Wiki recommends against using -autodefrag. The reason for this seems to be found in the Warnings section:
  • Mounting with -o autodefrag will often duplicate reflinked or snapshotted files when a balance is run. (TODO: is this still current for linux-4.19.x?)
  • Any "btrfs filesystem defrag" operation can potentially duplicate reflinked or snapshotted blocks. Files with shared extents lose their shared reflinks, which are then duplicated with n-copies. The effect of the lack of "snapshot aware defrag" is that volumes that make heavy use of reflinks or snapshots will unexpectedly run out of free space. Avoid this by minimizing the use of snapshots, and instead use deduplicating backup software to store backups efficiently (eg: borgbackup).
Question:

Given that Restic:
  • doesn't use reflinks
  • uses its own snapshot system (not Btrfs')
  • performs deduplication
Is it safe to use autodefrag for my use case?

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