Re: difference between -c and -p for send-receive?

[Andrei Borzenkov <arvidjaar@gmail.com>]

20.09.2017 22:05, Antoine Belvire пишет:
> Hello,
> 
>> All snapshots listed in -c options and snapshot that we want to
>> transfer must have the same parent uuid, unless -p is explicitly
>> provided.
> 
> It's rather the same mount point than the same parent uuid, like cp
> --reflink, isn't it?

Sorry, I do not understand this sentence. Could you rephrase?

> 
> ~# btrfs subvolume create /test2/
> Create subvolume '//test2'
> ~# btrfs subvolume create /test2/foo
> Create subvolume '/test2/foo'
> ~# cd /test2   
> ~# btrfs subvolume snapshot -r . .1
> Create a readonly snapshot of '.' in './.1'
> ~#
> ~# # a: 40 MiB in /test2/
> ~# dd if=/dev/urandom of=a bs=4k count=10k
> 10240+0 records in
> 10240+0 records out
> 41943040 bytes (42 MB, 40 MiB) copied, 0.198961 s, 211 MB/s
> ~#
> ~# # b: 80 MiB in /test2/foo
> ~# dd if=/dev/urandom of=foo/b bs=4k count=20k
> 0480+0 records in
> 20480+0 records out
> 83886080 bytes (84 MB, 80 MiB) copied, 0.393823 s, 213 MB/s
> ~#
> ~# # copy-clone /test2/foo/b to /test2/b
> ~# cp --reflink foo/b .
> ~#
> ~# btrfs subvolume -s . .2
> Create a readonly snapshot of '.' in './.2'
> ~#
> ~# # Sending .2 with only .1 as parent (.1 already sent)
> ~# btrfs send -p .1 .2 | wc -c
> At subvol .2
> 125909258 # 120 Mio = 'a' + 'b'
> ~#
> ~# # Sending .2 with .1 and foo as clone sources (.1 and foo already
> ~# # sent), .1 is automatically picked as parent
> ~# btrfs property set foo ro true
> ~# btrfs send -c .1 -c foo .2 | wc -c
> At subvol .2
> 41970349 # 40 Mio, only 'a'
> ~#
> 
> UUIDs on the sending side:
> 
> ~# btrfs subvolume list -uq / | grep test2
> ID 6141 gen 454658 top level 6049 parent_uuid - uuid
> bbf936dd-ca84-f749-9b9b-09f7081879a2 path test2
> ID 6142 gen 454658 top level 6141 parent_uuid - uuid
> 54a7cdea-6198-424a-9349-8116172d0c17 path test2/foo

Yes, sorry, I misread the code. We need at least one snapshot from -c
option that has the same parent uuid (i.e. is snapshot of the same
subvolume) as the snapshot we want to transfer, not all of them. In your
case

btrfs send -c .1 -c foo .2

it will select .1 as base snapshot and additionally try to clone from
foo if possible. In wiki example the only two snapshots are from
completely different subvolumes which will fail. In discussion that lead
to this mail snapshots probably did not have any parent uuid at all.

> ID 6143 gen 454655 top level 6141 parent_uuid
> bbf936dd-ca84-f749-9b9b-09f7081879a2 uuid
> 28f1d7db-7341-f545-a2ac-d8819d22a5b5 path test2/.1
> ID 6144 gen 454658 top level 6141 parent_uuid
> bbf936dd-ca84-f749-9b9b-09f7081879a2 uuid
> db9ad2b1-aee1-544c-b368-d698b4a05119 path test2/.2
> ~#
> 
> On the receiving side, .1 is used as parent:
> 
> ~# btrfs subvolume list -uq /var/run/media/antoine/backups/ | grep dest
> ID 298 gen 443 top level 5 parent_uuid - uuid
> 7695cba7-dfbf-2f44-bd79-18c9820fdb2f path dest/.1
> ID 299 gen 443 top level 5 parent_uuid - uuid
> c32c06ec-0a17-cf42-9b04-3804ad72f836 path dest/foo
> ID 300 gen 446 top level 5 parent_uuid
> 7695cba7-dfbf-2f44-bd79-18c9820fdb2f uuid
> 552b1d51-38bf-d546-a47f-4bc667ec4128 path dest/.2
> ~#
> 
> Regards,
> 
> -- 
> Antoine