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I saw that it is not a good idea to use btrfs CoW functionality for big files, such as data directories of a PostgreSQL database.

Since I use docker for databases, I now ask myself, if I should disable CoW for the whole /var/lib/docker directory. But I'm not sure, because docker's layered filesystem makes use of this feature, or not?

Or is it possible, to disable CoW for just some specific volumes?

Ethan Leroy
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2 Answers2

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Put the following in /etc/docker/daemon.json:

{
    "storage-driver": "btrfs"
}

and disable copy-on-write on your volumes if they contain databases:

mkdir /var/lib/docker/volumes
chattr +C /var/lib/docker/volumes

Hoping that helps.

Thomas Hugel
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  • It's important, that the directory does not exists yet. `chattr +C` on a directory does only affect new files and directories in it. Enabling `nodatacow` on `/var/lib/docker/volumes` is only affecting new volumes. You should not disable CoW totally. I recommend to use a prepared directory as bind mount for database files and writeable image files. – sausix Jan 18 '21 at 04:36
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I don't think docker directly plugs into BTRFS (or any filesystem) CoW behavior; rather, it can use snapshots and/or reflinks to avoid replicating entire container images.

While disabling CoW will be surely benefical for performance, be aware that:

  • disabling it means no data checksum, so no protection against data loss;
  • snapshotting a volume automatically re-enables CoW;
  • reflink will not work anymore;
  • existing file will remain CoW-enabled (until you delete and recreate them).

While for a database (as PostgreSQL) the above points can be non-issues (ie: data checksum is done at the DB record level and snapshots are provided by the transactional layer), for a VM or container missing these features can be problematic.

Anyway, I really suggest you to read the BTRFS FAQ about nodatacow

shodanshok
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    When using docker on top of a btrfs filesystem, it creates a directory called `/var/lib/docker/btrfs` containing BTRFS subvolumes. So this looks like docker plugs into filesystem specific features. – Ethan Leroy Oct 08 '19 at 13:51
  • Thank you @shodanshok! I'm a bit confused about phrase "snapshotting a volume automatically re-enables CoW", I thought that if I make a snapshot of a volume, then it will do CoW only once to avoid changing the file in the snapshot, am I wrong then? – Slabko Oct 02 '21 at 12:54
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    @Slabko you are right, but consider that frequent snapshots will cause many CoW "cycles" – shodanshok Oct 02 '21 at 15:16