Re: Using send/receive to keep two rootfs-partitions in sync fails with "ERROR: snapshot: cannot find parent subvolume"

[Andrei Borzenkov <arvidjaar@gmail.com>]

On 07.01.2024 11:10, Clemens Eisserer wrote:
> Hello Andrei,
> 
> Thanks for taking a look.
> 
> The exact commands executed where:
> 
> mkdir intern
> mkdir extern
> 
> mount /dev/mapper/ext extern
> mout /dev/mapper/int intern
> 
> ls extern/
> # output: root-ro
> 
> ls intern/
> # output: -> empty
> 
> btrfs send extern/root-ro | btrfs receive intern/ #initial ext -> int
> btrfs sub snap intern/root-ro intern/root-rw # rw snapshot to modify int
> touch intern/root-rw/newfile #actual modification
> 
> umount intern
> umount extern
> 
> sh sync_int_to_ext.sh #source of script at bottom
> 
> ls extern/root-ro/newfile
> # output: extern/root-ro/newfile -> sync int->ext was successful
> 
> # now modify ext fs
> mount /dev/mapper/ext extern/
> touch extern/root-rw/anothernewfile
> umount extern
> 
> sh sync_ext_to_int.sh
> 
> mount /dev/mapper/int intern/
> 
> ls intern/root-ro/anothernewfile
> # output intern/root-ro/anothernewfile -> sync ext->int was successful
> 
> umount intern
> 
> sh sync_int_to_ext.sh # just to make sure
> 
> # boot into extern/root-rw with rootflags=subvol/root-rw
> # both newfile and anothernewfile are visible in the root-fs
> 
> # reboot into other OS used for syncing disks
> 
> sh sync_ext_to_int.sh #to mirror modifications made in ext during it
> was rootfs back to intern, worked
> sh sync_int_to_ext.sh # just to make sure
> 
> # boot into extern/root-rw with rootflags=subvol/root-rw
> # perform a few modifications
> 
> # reboot into other OS used for syncing disks
> 
> sh sync_ext_to_int.sh
> # command btrfs send -p extern/root-ro extern/root-ro-new | btrfs
> receive intern/ fails with:
> ERROR: clone: cannot find source subvol 29fca96e-ca6a-3d4b-b7c9-566f1240d978
> 
> 
> btrfs sub list -pqu intern/
> ID 330 gen 426 parent 5 top level 5 parent_uuid
> 29fca96e-ca6a-3d4b-b7c9-566f1240d978 uuid
> 6409bfb7-1af0-7e4b-8a0f-d5a44e34a15c path root-ro
> ID 331 gen 426 parent 5 top level 5 parent_uuid
> 6409bfb7-1af0-7e4b-8a0f-d5a44e34a15c uuid
> 258c1fe5-b14e-654b-8ad5-5591268c9095 path root-rw
> 
> btrfs sub list -pqu extern/
> ID 291 gen 418 parent 5 top level 5 parent_uuid
> 1c738933-0bb2-2547-ad5f-326bfc6263b3 uuid
> 939c1ed9-9589-6c4b-ace7-2fcdeb970303 path root-rw
> ID 292 gen 418 parent 5 top level 5 parent_uuid
> 939c1ed9-9589-6c4b-ace7-2fcdeb970303 uuid
> 155f4370-32f0-5f4b-b288-2e8d7302d279 path root-ro
> 
> Best regards, Clemens
> 
> 
> 
> script sync_ext_to_int.sh:
> 
> unalias cp
> mount -o compress=zstd:6 /dev/mapper/ext extern/
> mount -o compress=zstd:6 /dev/mapper/int intern/
> 
> btrfs sub snap -r extern/root-rw extern/root-ro-new
> btrfs send -p extern/root-ro extern/root-ro-new | btrfs receive intern/
> 
> btrfs sub del extern/root-ro
> mv extern/root-ro-new extern/root-ro
> 
> btrfs sub del intern/root-ro
> mv intern/root-ro-new intern/root-ro
> btrfs sub del intern/root-rw
> btrfs sub snap intern/root-ro intern/root-rw
> 
> #cp cfgint/fstab intern/root-rw/etc/
> #cp cfgint/crypttab intern/root-rw/etc/
> #cp cfgint/grub intern/root-rw/etc/default/
> 
> umount extern;
> umount intern;
> 
> 
> 
> script sync_int_to_ext.sh:
> 
> unalias cp
> mount -o compress=zstd:6 /dev/mapper/ext extern/
> mount -o compress=zstd:6 /dev/mapper/int intern/
> 
> btrfs sub snap -r intern/root-rw intern/root-ro-new
> btrfs send -p intern/root-ro intern/root-ro-new | btrfs receive extern/
> 
> btrfs sub del intern/root-ro
> mv intern/root-ro-new intern/root-ro
> 
> btrfs sub del extern/root-ro
> mv extern/root-ro-new extern/root-ro
> btrfs sub del extern/root-rw
> btrfs sub snap extern/root-ro extern/root-rw
> 
> cp cfgext/fstab extern/root-rw/etc/
> cp cfgext/crypttab extern/root-rw/etc/
> cp cfgext/grub extern/root-rw/etc/default/
> 
> umount extern;
> umount intern;
> 
> Am So., 7. Jan. 2024 um 08:19 Uhr schrieb Andrei Borzenkov
> <arvidjaar@gmail.com>:
>>
>> On 07.01.2024 10:06, Clemens Eisserer wrote:
>>> Hi,
>>>
>>> I would like to use send/receive to keep two root-filesystems in sync,
>>> as I've been using it for years now for backups where it really does
>>> wonders (thanks a lot!).
>>>
>>> Both disks contain a read-only snapshot which is kept in-sync between
>>> the filesystems (int and ext are the mountpoints of the two disks,
>>> original_disk is just used for initial data):
>>>      btrfs send original_disk/root-ro | btrfs receive int/ #send
>>> snapshot of the original disk to the first of the two filesystens
>>> (disk "int")
>>>      btrfs send int/root-ro | btrfs receive ext/ #now replicate the same
>>> to disk "ext"
>>> so both disks start with a snapshot "root-ro" with equal content.
>>>
>>> in case I would like to work with one of the two disks, I create a rw
>>> snapshot based no root-ro:
>>>     btrfs sub snap ext/root-ro ext/root-rw
>>>
>>>     touch ext/root-wr/create-a-new-file # perform some modifications
>>>
>>
>> There was no "root-wr" before.
>>
>>> once modifications in root-rw are done, the following steps are
>>> performed to sync the filesystems:
>>>     btrfs sub snap -r ext/root-rw ext/root-ro-new #create a root-ro-new
>>> read-only snapshot based on the rw-snapshot with modfications (so it
>>> can be used with btrfs send)
>>>     btrfs send -p ext/root-ro extern/root-ro-new | btrfs receive int/
>>> #send root-ro-new to "int" filesystem
>>
>> There was no "extern" before.
>>
>> Never describe computer commands. Copy and paste them in full with
>> complete output.
>>
>>>     btrfs sub del ext/root-ro # delete the original root-ro snapshot, as
>>> it is no longer needed for differential btrfs send
>>>     mv ext/root-ro-new ext/root-ro #rename root-ro-new to root-ro, as
>>> this is the current state of the other (int) filesystem
>>>     btrfs sub del int/root-ro # delete root-ro in "int" too, as it is no
>>> longer needed for differential btrfs receive
>>>     mv int/root-ro-new int/root-ro #rename root-ro-new to root-ro
>>>     btrfs sub snap int/root-ro int/root-rw # create a working copy of
>>> root-ro which is writeable
>>>
>>> this works great - i can add/modify files in one root-rw folder, call
>>> the synchronization script and everything is found on the other
>>> filesystem.
>>> When exchanging int and ext in the script above it actually works in
>>> both directions, so this is exactly what I was hoping to achieve.
>>> Even when executing the script multiple times int->ext, ext->int,
>>> int->ext ... with modifications in between, everything works as
>>> expected.
>>> Awesome :)
>>>
>>> However, when actually using the file-systems as rootfs, this seem to
>>> break when performing the following steps:
>>> - create rw snapshot of root-ro on disk "ext": btrfs sub snap
>>> ext/root-ro ext/root-rw
>>> - boot the system with rootfs=ext-uuid and rootflags=subvol=/root-rw
>>> (etc/fstab was adapted accordingly)
>>> - use the system, modify file system etc and shutdown again
>>> - start separate system to synchronize disks (not based on int or ext
>>> rootfs) and call sync script ext->int (shown above)
>>>
>>
>> It is absolutely unclear what it means. As mentioned, provide full
>> transcript of your actions as well as the output of
>>
>> btrfs subvolume list -pqu /mount-point
>>

Apologies, could you post

btrfs subvolume list -pqRu ...

to get received UUID.

>> for all filesystems involved in these commands.
>>
>>> it now suddenly fails at btrfs receive with:
>>> btrfs send -p ext/root-ro ext/root-ro-new | btrfs receive intern/
>>> ERROR: snapshot: cannot find parent subvolume
>>> 4ed11491-7563-fb49-99e7-86cb47cfb510
>>>
>>> which I, to be honest, don't understand.
>>> Exactly the same sequence of commands worked multiple times when the
>>> root-rw snapshot was not booted from but modified directly on the sync
>>> system, even with umounts between modification & send/receive.
>>> Does it make a difference for btrfs if it is used as rootfs vs normal
>>> writeable mount?
>>> Or does it just work in the non-boot case because of some side-effects?
>>>
>>> Thanks & best regards, Clemens
>>>
>>
>