Re: is back and forth incremental send/receive supported/stable?

[Andrei Borzenkov <arvidjaar@gmail.com>]

02.02.2021 00:53, Chris Murphy пишет:
> It needs testing but I think -c option can work for this case, because
> the parent on both source and destination are identical, even if the
> new destination (the old source) has an unexpected received subvolume
> uuid.
> 

Incremental send requires base snapshot. It is given either as -p option
or "btrfs send" tries to find "best" base snapshot among those given by
-c. This works only if both target snapshot and (some) snapshots given
by -c are created from the same subvolume. So "btrfs send -c foo bar" is
entirely equivalent to "btrfs send -p foo bar" as long as both foo and
bar have the same parent subvolume.

> At least for me, it worked once and I didn't explore it further. I
> also don't know if it'll set received uuid, such that subsequent send
> can use -p instead of -c.
> 

On receive side "btrfs receive" will always set received_uuid to
snapshot given as "btrfs send" argument. The question is whether receive
side will be able to find base (parent) snapshot. As far as I can tell,
"btrfs receive" will first search for subvolume with matching
received_uuid and if nothing is found it will repeat searching for
subvolume with matching uuid. "btrfs send" will send received_uuid
instead of uuid if it is set.

Which means that in case

btrfs subvolume snapshot -r source base
btrfs send base
btrfs subvolume snapshot -r source diff
btrfs send -p base diff

you should be able to reverse replication by doing on destination

btrfs subvolume snapshot diff new-source

this creates writable clone of replication snapshot that will be used to
actually store data. You cannot use replication snapshot itself because
you need its uuid and it must remain read-only

btrfs subvolume snapshot -r new-source diff2
btrfs send -p diff diff2

This should find "diff" on source that has uuid matching received_uuid
of "diff" on destination.

The key here is to not attempt to directly use snapshots involved in
replication. They must remain immutable.

> -c generally still confuses me... in particular multiple instances of -c
> 


Well, it allows you to optimize transfer. if you know that large amount
of data in subvolume foo is reflinked to data in subvolume bar, you can use

btrfs send -c bar foo

and instead of sending (duplicate copy) of data in foo "btrfs receive"
will reflink it from bar on destination. Consider

tw:/mnt/src # btrfs subvolume create source
Create subvolume './source'
tw:/mnt/src # btrfs subvolume create clone
Create subvolume './clone'
tw:/mnt/src # dd if=/dev/urandom of=clone/cln bs=8k count=2
2+0 records in
2+0 records out
16384 bytes (16 kB, 16 KiB) copied, 0.000495937 s, 33.0 MB/s
tw:/mnt/src # btrfs sub snap -r source source-base
Create a readonly snapshot of 'source' in './source-base'
tw:/mnt/src # btrfs sub snap -r clone clone-base
Create a readonly snapshot of 'clone' in './clone-base'
tw:/mnt/src # btrfs send source-base/ | btrfs receive ../dst
At subvol source-base/
At subvol source-base
tw:/mnt/src # btrfs send clone-base/ | btrfs receive ../dst
At subvol clone-base/
At subvol clone-base
tw:/mnt/src # cp --reflink=always clone/cln source
tw:/mnt/src # btrfs sub snap -r source source-diff
Create a readonly snapshot of 'source' in './source-diff'
tw:/mnt/src # btrfs send -p source-base source-diff | btrfs receive --dump
At subvol source-diff
...
write           ./source-diff/cln               offset=0 len=16384
...
tw:/mnt/src # btrfs send -p source-base -c clone-base source-diff |
btrfs receive --dump
At subvol source-diff
...
clone           ./source-diff/cln               offset=0 len=16384
from=./source-diff/cln clone_offset=0
...
tw:/mnt/src #

Ignore weird clone from= in "btrfs receive --dump". It should have
printed clone uuid instead (it does not really have name at this point).