btrfs-convert man page
btrfs-convert — convert from ext2/3/4 filesystem to btrfs
btrfs-convert [options] <device>
btrfs-convert is used to convert existing ext2/3/4 filesystem image to a btrfs filesystem in-place. The original filesystem image is accessible subvolume named ext2_saved as file image.
If you are going to perform rollback to ext2/3/4, you should not execute btrfs balance command on the converted filesystem. This will change the extent layout and make btrfs-convert unable to rollback.
The conversion utilizes free space of the original filesystem. The exact estimate of the required space cannot be foretold. The final btrfs metadata might occupy several gigabytes on a hundreds-gigabyte filesystem.
If you decide not to rollback anymore, it is recommended to perform a few more steps to transform the btrfs filesystem to a more compact layout. The conversion inherits the original data block fragmentation and the metadata blocks are bound to the original free space layout.
REMOVE THE ORIGINAL FILESYSTEM METADATA
By removing the ext2_saved subvolume, all metadata of the original filesystem will be removed:
# btrfs subvolume delete /mnt/ext2_saved
At this point it’s not possible to do rollback. The filesystem is usable but may be impacted by the fragmentation.
MAKE FILE DATA MORE CONTIGUOUS
An optional but recommended step is to run defragmentation on the entire filesystem. This will attempt to make file extents more contiguous.
# btrfs filesystem defrag -v -r -f -t 32M /mnt/btrfs
Verbose recursive defragmentation (-v, -r), flush data per-file (-f) with target extent size 32M (-t).
ATTEMPT TO MAKE BTRFS METADATA MORE COMPACT
Optional but recommended step.
The metadata block groups after conversion may be smaller than the default size (256MiB or 1GiB). Running a balance will attempt to merge the block groups. This depends on the free space layout (and fragmentation) and may fail. This is a soft error leaving the filesystem usable but the block group layout may remain unchanged.
Note that balance operation takes a lot of time.
# btrfs balance start -m /mnt/btrfs
- disable data checksum calculations and set NODATASUM file flag, this can speed up the conversion
- ignore xattrs and ACLs of files
- disable inlining of small files to metadata blocks, this will decrease the metadata consumption and may help to convert a filesystem with low free space
- -N|--nodesize <SIZE>
- set filesystem nodesize, the tree block size in which btrfs stores its metadata. The default value is 16KB (16384) or the page size, whichever is bigger. Must be a multiple of the sectorsize, but not larger than 65536. Se
mkfs.btrfs(8) for more details.
- rollback to the original ext2/3/4 filesystem if possible
- -l|--label <LABEL>
- set filesystem label during conversion
- use label from the converted filesystem
- show progress of conversion, on by default
- disable detailed progress and show only the main phases of conversion
btrfs-convert will return 0 if no error happened. If any problems happened, 1 will be returned.