If you open the defragment section of btrfs-filesystem(8), you will see the following ominous inscription left by the developers:
Warning: Defragmenting with Linux kernel versions < 3.9 or ≥ 3.14-rc2 as well as with Linux stable kernel versions ≥ 3.10.31, ≥ 3.12.12 or ≥ 3.13.4 will break up the ref-links of COW data (for example files copied with
cp --reflink, snapshots or de-duplicated data). This may cause considerable increase of space usage depending on the broken up ref-links.
That sounds terrible. A selling point of btrfs is its ability to create snapshots without copying everything. I mostly create readonly snapshots.
Do the files of readonly snapshots also count as “COW-data” or will parent subvolume deduplication survive without making disk space bloat?
Best Answer
Yes, files in a readonly snapshot count as COW-data and will contribute to disk space bloat caused by defragmenting.
When defragmentation happens, data is copied from the old extents into fewer new extents. The new extents are distinct from the old extents. All other copies of the file (in snapshots, for instance) still point to the old extents. Therefore, you have data bloat.
There's a long thread about defragmenting on the mailing list starting here that has some interesting points.