btrfs-progs/Documentation/btrfs-convert.asciidoc

btrfs-convert(8)
================

NAME
----
btrfs-convert - convert from ext2/3/4 or reiserfs filesystem to btrfs in-place

SYNOPSIS
--------
*btrfs-convert* [options] <device>

DESCRIPTION
-----------
*btrfs-convert* is used to convert existing source filesystem image to a btrfs
filesystem in-place.  The original filesystem image is accessible in subvolume
named like 'ext2_saved' as file 'image'.

Supported filesystems:

* ext2, ext3, ext4 -- original feature, always built in

* reiserfs -- since version 4.13, optionally built, requires libreiserfscore 3.6.27

The list of supported source filesystem by a given binary is listed at the end
of help (option '--help').

WARNING: If you are going to perform rollback to the original filesystem, 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 the ability to rollback is no longer important, the it is recommended to
perform a few more steps to transition the btrfs filesystem to a more compact
layout. This is because the conversion inherits the original data blocks'
fragmentation, and also because the metadata blocks are bound to the original
free space layout.

Due to different constraints, it is only possible to convert filesystems that
have a supported data block size (ie. the same that would be valid for
'mkfs.btrfs'). This is typically the system page size (4KiB on x86_64
machines).

NOTE: The source filesystem should be clean, you are encouraged to run the
'fsck' tool if you're not sure.

**REMOVE THE ORIGINAL FILESYSTEM METADATA**

By removing the subvolume named like 'ext2_saved' or 'reiserfs_saved', all
metadata of the original filesystem will be removed:

   # btrfs subvolume delete /mnt/ext2_saved

At this point it is not possible to do a rollback. The filesystem is usable but
may be impacted by the fragmentation inherited from the original filesystem.

**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 32MiB ('-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 due to
lack of enough work space. 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, please see also
`btrfs-balance`(8).

   # btrfs balance start -m /mnt/btrfs

OPTIONS
-------
--csum <type>::
--checksum <type>::
Specify the checksum algorithm. Default is 'crc32c'. Valid values are 'crc32c',
'xxhash', 'sha256' or 'blake2'. To mount such filesystem kernel must support the 
checksums as well.
-d|--no-datasum::
disable data checksum calculations and set the NODATASUM file flag, this can speed
up the conversion
-i|--no-xattr::
ignore xattrs and ACLs of files
-n|--no-inline::
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. See
`mkfs.btrfs`(8) for more details.
-r|--rollback::
rollback to the original ext2/3/4 filesystem if possible
-l|--label <LABEL>::
set filesystem label during conversion
-L|--copy-label::
use label from the converted filesystem
-O|--features <feature1>[,<feature2>...]::
A list of filesystem features enabled the at time of conversion. Not all features
are supported by old kernels. To disable a feature, prefix it with '^'.
Description of the features is in section 'FILESYSTEM FEATURES' of
`mkfs.btrfs`(8).
+
To see all available features that btrfs-convert supports run:
+
+btrfs-convert -O list-all+
+
-p|--progress::
show progress of conversion (a heartbeat indicator and number of inodes
processed), on by default
--no-progress::
disable progress and show only the main phases of conversion

EXIT STATUS
-----------
*btrfs-convert* will return 0 if no error happened.
If any problems happened, 1 will be returned.

SEE ALSO
--------
`mkfs.btrfs`(8)