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I'm runninga a btrfs raid1 on 5 8 TB disks, where primarily urbackup stores backups to. Urbackup uses snapshots extensively. compsize produces the following output:

Processed 106161732 files, 57335496 regular extents (190449694 refs), 63766108 inline.
Type       Perc     Disk Usage   Uncompressed Referenced
TOTAL       78%       13T          16T          58T
none       100%       11T          11T          47T
zlib        29%      1.4T         5.0T          10T

For a while I ran btrfs with compression and afterwards disabled it. As compsize now shows, compression can reduce the disk usage considerably (see line zlib). So I wonder how to compress the existing data without destroying the snapshot structure of the existing data. Due to performance reasons I would also prefer using zstd compression instead of zlib. System is Debian 5.7.10 x86_64 with btrfs-progs 4.20.1 if that matters. Options I analysed:

  • btrfs fi defrag -czstd would destroy the snapshot structure.
  • Mounting with force-compress=zstd and btrfs fi balance will not compress rebalanced extents, it will just copy them as they are.

Is there anything I can do to compress the existing data without destroying the snapshot structure?

Ideas:

  • Can I somehow copy data to another compressed btrfs partition with keeping the snapshot structure intact?
  • What about defrag and offline deduplication with dduper? Will it recreate the snapshot structure?

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