On Tue, 18 Jul 2017 16:57:10 +0500
Roman Mamedov <rm@xxxxxxxxxxx> wrote:
> if a block written consists of zeroes entirely, instead of writing zeroes to
> the backing storage, converts that into an "unmap" operation
> (FALLOC_FL_PUNCH_HOLE[1]).
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
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