Re: [pve-devel] [PATCH v3 pve-storage 1/3] qcow2: add external snapshot support

["DERUMIER, Alexandre via pve-devel" <pve-devel@lists.proxmox.com>]

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From: "DERUMIER, Alexandre" <alexandre.derumier@groupe-cyllene.com>
To: "pve-devel@lists.proxmox.com" <pve-devel@lists.proxmox.com>, "f.gruenbichler@proxmox.com" <f.gruenbichler@proxmox.com>
Subject: Re: [pve-devel] [PATCH v3 pve-storage 1/3] qcow2: add external snapshot support
Date: Fri, 10 Jan 2025 13:14:48 +0000
Message-ID: <32c2f6a7978e2c37bb5b51f44de2261dc12446e8.camel@groupe-cyllene.com>

>>but you don't know up front that you want to collapse all the
>>snapshots. for each single removal, you have to merge the delta
>>towards the overlay, not the base, else the base contents is no
>>longer matching its name.
>>
>>think about it this way:
>>
>>you take a snapshot B at time X. this snapshot must never contain a
>>modification that happened after X. that means you cannot ever commit
>>a newer snapshot into B, unless you are removing and renaming B.

>>if you start with a chain A -> B -> C -> D (with A being the first
>>snapshot/base, and D being the current active overlay. if you want to
>>remove B, you can either
>>- stream B into C, remove B
>>- commit C into B, remove C, rename B to C
>>
>>in both cases you will end up with a chain A -> C' -> D where C' is
>>the combination of the old B and C.
>>
>>the downside of the streaming variant is that if B's delta is bigger
>>than C's, you have more I/O. the upside is that there is no inbetween
>>state where the backing chain is broken and error handling can go
>>very wrong.
>>
>>what you are doing right now is:
>>
>>chain A->B->C->D as before. remove B by commiting B into A and then
>>rebasing C on top of A. that means you end up with:
>>A'->C->D where A' is A+B. but now this snapshot A contains writes
>>that happened after the original A was taken. this is wrong.


Ah yes, you are right. 

I was just thinking about it, and have the same conclusion

for example:

A 1TB  (12:00)---->  B 500MB  (13:00) ----> C 10MB (14:00) ---> current
(now)

if I delete B, If I merge it to A, it'll not be the A view at 12:00,
but 13:00.
so we indeed need to merge it to C.

(Sorry, I'm using zfs/ceph for too long, where merge never occur, and
block are only referenced  && destroyed in background.)


Ok,I'll rework with stream implementation. (I need to do it anyway for
multi-branch, but later please




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