You can see this in effect on the qBit_root.img VM image in the example further down. Only when all the data from the first extent is re-written into a new extent, it gets freed up. If you have a 1MiB file that consists of one extent, and then write another 512KiB to it, you end up with two extents 1MiB+512KiB. This is because extents are immutable (cannot be changed). On files that are frequently written too, such as databases or virtual machine files, a smaller target extent size is probably better. On files that are mostly read from, such as media files, it might be worth using 4GiB target extent. Reasonable values highly depends on your use-case, but could be in the range of a few hundred KiB up to maximum of 4GiB. Free space fragmentation and other factors affects this.

It is only an advisory number and Btrfs may not be able to reach this target, even if you run defragment multiple times. Btrfs tries to merge smaller extents into extents of this size.

The target extent size is an important option. (e.g., files copied with 'cp -reflink', snapshots) which may causeĬonsiderable increase of space usage. Warning: most Linux kernels will break up the ref-links of COW data v deprecated, alias for global -v option t size target extent size hint (default: 32M) s start defragment only from byte onward f flush data to disk immediately after defragmenting Especially on spinning HDD's, this can lead to extra seeks and slowness.ītrfs filesystem defragment is used to defragment files, but it is also used to apply compression to them.ītrfs fi defrag -help # btrfs filesystem defragment -help usage: btrfs filesystem defragment | This is because for each new write, Btrfs have to write to a new location.

The Copy-on-Write nature of Btrfs can lead to higher amounts of fragmentation in files compared to ext4.

3 Defragmenting the subvolume and extent trees.

Defrag free space