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samozrejme ze sem si prosel "man btrfs-convert"

V mém man btrfs-convert se hned v úvodu píše:

       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.

           Warning
           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.

       Due to different constraints, it’s possible to convert only filesystem that have
       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 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 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

Tj. bude potřeba hodně místa a nelze to odhadnout dopředu, je dobré provést defrag, nejen pro spojení souborů, ale také pro změnu velikosti extentů a nakonec je dobré provést balance.

Myslím, si, že každého admina asi trkne, že to nebude jednoduchá operace (z hlediska toho, co ty tooly dělají, ne pro admina).

potreba vystudovat "Vysokou skolu Btrfsackou"

To je nutné u všech složitějších systému. Vždycky je dobré vědět, co to umí, co to neumí, jak se věci dělají, aby člověk nebyl překvapen.