Re: Can the BTRFS transparent compression mess with RPM's free space checks?

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On Mon, Apr 19, 2021 at 4:42 PM Lyes Saadi <fedora@xxxxxxx> wrote:
>
> I think I realized what went wrong: I compressed my filesystem _after_
> already having done some snapshots. I think it then duplicated all my
> files and basically filled my filesystem... And I did

Yeah defragmentation unshares shared extents. So all the deduplication
of reflinks and snapshots is undone. It's best to narrow the
defragmentation to be limited to a particular subvolume, or
directories in that subvolume, rather than all snapshots. Or just
enable compression in fstab and don't compress already written things,
just let it compress new writes.

The autodefrag mount option is a bit different because it's only
triggered on small writes less than 64KiB, either inserts or appends
into an existing file. The target extent size is 128KiB which is also
the max compression block size. So as a file is modified with this
write pattern, it's already unsharing any shared extents from
snapshots or reflinks. Since the whole file isn't (re)defragmented
every time autodefrag is triggered by the write pattern, it's not
going to add much (if at all) to unsharing extents.



-- 
Chris Murphy
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