Re: Why do full balance and deduplication reduce available free space?

[Kai Krakow <hurikhan77@gmail.com>]

Am Mon, 02 Oct 2017 12:02:16 +0200
schrieb Niccolò Belli <darkbasic@linuxsystems.it>:

> This is how I performe balance: btrfs balance start --full-balance
> rootfs This is how I perform deduplication (duperemove is from git
> master): duperemove -drh --dedupe-options=noblock
> --hashfile=../rootfs.hash <all_subvols_except_snapshots_ones>

Besides defragging removing the reflinks, duperemove will unshare your
snapshots when used in this way: If it sees duplicate blocks within the
subvolumes you give it, it will potentially unshare blocks from the
snapshots while rewriting extents.

BTW, you should be able to use duperemove with read-only snapshots if
used in read-only-open mode. But I'd rather suggest to use bees
instead: It works at whole-volume level, walking extents instead of
files. That way it is much faster, doesn't reprocess already
deduplicated extents, and it works with read-only snapshots.

Until my patch it didn't like mixed nodatasum/datasum workloads.
Currently this is fixed by just leaving nocow data alone as users
probably set nocow for exactly the reason to not fragment extents and
relocate blocks.


-- 
Regards,
Kai

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