Re: hierarchical, tree-like structure of snapshots

From: Forza <forza@tnonline.net>
To: Andrei Borzenkov <arvidjaar@gmail.com>,
	john terragon <jterragon@gmail.com>
Cc: Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: hierarchical, tree-like structure of snapshots
Date: Thu, 31 Dec 2020 11:00:12 +0100	[thread overview]
Message-ID: <4febd23e-94d2-6aa3-8d3f-77731eb83ad0@tnonline.net> (raw)
In-Reply-To: <f472181d-d6a4-f5f4-df7f-03bc7788b45a@gmail.com>

On 2020-12-31 08:05, Andrei Borzenkov wrote:
> 30.12.2020 20:39, john terragon пишет:
>> On Wed, Dec 30, 2020 at 6:24 PM sys <system@lechevalier.se> wrote:
>>>
>>>
>>>
>> [...]
>>> You should simply make a 'read-write' snapshot (Y-rw) of the 'read-only'
>>> snapshot (Y) that is part of your backup/send scheme. Do not modify
>>> read-only snapshots to be rw.
>>>
>>
>> OK, but then could I use Y as parent of the rw snapshot, let's call it
>> W, in a send?
> 
> No
> 
>> So I would have this tree where Y is still the root.
>>
>> Y-W
>>   \
>>    Z-X
>>
>> Can I do a send -p Y W ?
> 
> No. All subvolumes used in send/receive must be read-only. And they must
> remain read-only from the moment they are created - we have seen quite a
> lot of reports when users removed read-only property from subvolume used
> in the past as send source, modified it, set as read-only again and
> tried to continue replication. This resulted in complete mess on receive
> side. Also if you try to modify destination snapshots it will break at
> some point.
> 
> The general rule - everything used for replication must remain
> read-only. If you want to use any snapshot that is part of replication
> you clone it and use its clone.
> 
>> Because I thought it was other way around, that is I do a readonly
>> snapshot W of Y and that will be the base for incrementally sending
>> the future modified Y to another  FS (provided of course W is already
>> there).
>>
> 
> If you want to capture changes in W since it was cloned from Y you
> create another read-only snapshot of W and use it.
> 
> btrfs subvolume snapshot -r W V
> btrfs send -p Y V
> 
> It is possible that btrfs implementation is optimized for sequential
> snapshots from the same subvolume so the send stream size will be
> larger. I am not familiar with these low level details. From the naïve
> end-user point of view there should be no difference between
> 
> btrfs subvolume snapshot -r W R1
> btrfs send R1
> modify W
> btrfs subvolume snapshot -r W R2
> btrfs send -p R1 R2
> 
> and
> 
> btrfs send R1
> btrfs subvolume snapshot R1 W
> modify W
> btrfs subvolume snapshot -r W R2
> btrfs send -p R1 R2
> 

I think you are correct. The man page specifies that all snapshots must 
be read-only, but it is rather unclear if you modify some snaps in between.

https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-send