Re: [pve-devel] [PATCH v3 qemu-server 11/11] qcow2: add external snapshot support

[Fabio Fantoni via pve-devel <pve-devel@lists.proxmox.com>]

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From: Fabio Fantoni <fabio.fantoni@m2r.biz>
To: "Proxmox VE development discussion" <pve-devel@lists.proxmox.com>, "Fabian Grünbichler" <f.gruenbichler@proxmox.com>
Subject: Re: [pve-devel] [PATCH v3 qemu-server 11/11] qcow2: add external snapshot support
Date: Thu, 9 Jan 2025 14:19:38 +0100
Message-ID: <483058af-44d9-441c-98df-fd7150184ebe@m2r.biz>

Il 09/01/2025 12:57, Fabian Grünbichler ha scritto:
>> Alexandre Derumier via pve-devel <pve-devel@lists.proxmox.com> hat am 16.12.2024 10:12 CET geschrieben:
> it would be great if there'd be a summary of the design choices and a high level summary of what happens to the files and block-node-graph here. it's a bit hard to judge from the code below whether it would be possible to eliminate the dynamically named block nodes, for example ;)
>
> a few more comments documenting the behaviour and ideally also some tests (mocking the QMP interactions?) would be nice

@Alexandre Derumier: Thanks for add external snapshot support, I have 
not looked at the implementation in detail because I do not have enough 
time but I think external snapshot support would be useful.

I used it outside of proxmox years ago, on Debian servers with VMs 
managed with libvirt, I managed external snapshots completely manually 
from cli with multiple commands because they were not implemented in 
virtmanager and they were useful to save a lot of time (instead of 
backup/restore) in some high-risk operations on VMs with large disks, 
raw pre-allocated on hdd disks.

I used them very little and kept them only the minimum time necessary 
for delicate maintenance operations, if there were unforeseen events it 
returned to the situation before the snapshot, I deleted the external 
snapshot and created another one to try again, if instead everything was 
ok in the end I did the commit, and went back to using only the 
pre-allocated raw image. With high disk usage as in the operations I was 
doing the performance decrease with external qcow2 snapshots compared to 
just pre-allocated raw disks was huge if I remember correctly (which is 
why I used them for the minimum amount of time possible).

If it hasn't already been planned I think it could be useful to warn 
users (atleast in documentation) to avoid them underestimating their 
possible impact on performance (especially if they basically have 
pre-allocated raw on hdd disks for greater performance and minimal 
defragmentation) and avoid use or keep them for a long time without real 
need. Another important thing to notify users is the increase in space 
usage (again mainly for those who are used to pre-allocated disks where 
they usually don't risk increases in space).

In this implementation I don't see the possibility of using them on raw 
disks (on files) from a fast look, or am I wrong? If so, why? I think 
the main use would be in cases like that where you don't have snapshot 
support by default


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