On 2018-05-19 04:54, Niccolò Belli wrote:
On venerdì 18 maggio 2018 20:33:53 CEST, Austin S. Hemmelgarn wrote:
With a bit of work, it's possible to handle things sanely. You can
deduplicate data from snapshots, even if they are read-only (you need
to pass the `-A` option to duperemove and run it as root), so it's
perfectly reasonable to only defrag the main subvolume, and then
deduplicate the snapshots against that (so that they end up all being
reflinks to the main subvolume). Of course, this won't work if you're
short on space, but if you're dealing with snapshots, you should have
enough space that this will work (because even without defrag, it's
fully possible for something to cause the snapshots to suddenly take
up a lot more space).
Been there, tried that. Unfortunately even if I skip the defreg a simple
duperemove -drhA --dedupe-options=noblock --hashfile=rootfs.hash rootfs
is going to eat more space than it was previously available (probably
due to autodefrag?).
It's not autodefrag (that doesn't trigger on use of the EXTENT_SAME
ioctl). There's two things involved here:
* BTRFS has somewhat odd and inefficient handling of partial extents.
When part of an extent becomes unused (because of a CLONE ioctl, or an
EXTENT_SAME ioctl, or something similar), that part stays allocated
until the whole extent would be unused.
* You're using the default deduplication block size (128k), which is
larger than your filesystem block size (which is at most 64k, most
likely 16k, but might be 4k if it's an old filesystem), so deduplicating
can split extents.
Because of this, if a duplicate region happens to overlap the front of
an already shared extent, and the end of said shared extent isn't
aligned with the deduplication block size, the EXTENT_SAME call will
deduplicate the first part, creating a new shared extent, but not the
tail end of the existing shared region, and all of that original shared
region will stick around, taking up extra space that it wasn't before.
Additionally, if only part of an extent is duplicated, then that area of
the extent will stay allocated, because the rest of the extent is still
referenced (so you won't necessarily see any actual space savings).
You can mitigate this by telling duperemove to use the same block size
as your filesystem using the `-b` option. Note that using a smaller
block size will also slow down the deduplication process and greatly
increase the size of the hash file.
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