"detect-zeroes=unmap" support in Btrfs?

Hello,

Qemu/KVM has this nice feature in its storage layer, "detect-zeroes=unmap". Basically the VM host detects if a block written by the guest consists of zeroes entirely, and instead of writing zeroes to the backing storage, converts that into an "unmap" operation (FALLOC_FL_PUNCH_HOLE[1]).

I wonder if the same could be added into Btrfs directly? With a CoW filesystem there is really no reason to store long runs of zeroes, or even spend compression cycles on them (even if they compress really well), it would be more efficient to turn all zero-filled blocks into file holes. (In effect forcing all files with zero blocks into always being "sparse" files.)

You could say this increases fragmentation, but given the CoW nature of Btrfs, any write to a file increases fragmentation already (except with "nocow"), and converting zeroes into holes would be beneficial due to not requiring any actual IO when those need to be read (reading back zeroes which are not stored anywhere, as opposed to reading actual zeroes from disk).

[1] http://man7.org/linux/man-pages/man2/fallocate.2.html

With respect,
Roman

BTW I found that it is very easy to "offline" process preexisting files for this, using "fallocate -d".

-d, --dig-holes
Detect and dig holes. Makes the file sparse in-place, without using extra disk space. The minimal size of the hole depends on filesystem I/O block size (usually 4096 bytes). Also, when using this option, --keep-size is implied. If no range is specified by --offset and --length, then all file is analyzed for holes.

You can think of this as doing a "cp --sparse" and renaming the dest file as the original, without the need for extra disk space.

So my suggestion is to implement an "online" counterpart to such forced-sparsifying, i.e. the same thing done on FS I/O in-band. (the analogy is with offline vs in-band dedup).

With respect,
Roman