Re: btrfs send and receive showing errors

[Andrei Borzenkov <arvidjaar@gmail.com>]

On 05.01.2023 19:47, Randy Nürnberger wrote:
> On Thu, Jan 5, 2023 at 17:35, Randy Nürnberger wrote:
>> On Thu, Jan 5, 2023 at 17:01, Andrei Borzenkov wrote:
>>> On 05.01.2023 18:55, Randy Nürnberger wrote:
>>>>
>>>> I’ve attached the output of ‘btrfs subvolume list’ for the source and
>>>> the target filesystem.
>>>>
>>>
>>> You have a lot of source subvolumes with the same received_uuid and
>>> you have at least two destination subvolumes with the same
>>> received_uuid. This is wrong, received_uuid is supposed to be unique
>>> identification which explains your errors (incorrect subvolume is
>>> selected).
>>
>> I guess many of my source subvolumes have the same received_uuid,
>> because I created new snapshots from a snapshot that was previously
>> received and the received_uuid just did get copied.
>>
>> I just did a small experiment on another system, created a snapshot
>> from a snapshot that previously was received and could confirm that
>> the received_uuid does indeed get copied. Is this a problem?
> 

No, it is not a problem by itself. Subvolumes used for send/receive are 
supposed to be a remain read-only, so any clone of any subvolume should 
have identical content and you can use any of the clones with the same 
received_uuid. But as soon as you made subvolume read-write, you got 
multiple subvolumes with different content and the same receive_uuid. 
"Doctor, it hurts when I stab myself in the eye".

Never, NEVER, ever, change  subvolume involved in send/receive to 
read-write to use it; clone it (create writable snapshot) and use clone.

> Ha! This seems to be the problem! I’m able to write a small script that
> reproduces the bug! Give me a couple minutes.
> 
> If this is a bug in the kernel,

It is not so much a bug as the consequence of unfortunate design.

> may I write the patch? :D Would be my
> first one :)

The problem is known for years (just search this list) and there are 
patches to clear received_uuid on ro -> rw transition but they are still 
not applied. Personally I would just prohibit such transition completely.