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From: Christoph Hellwig <hch@infradead.org>
To: Kirill Tkhai <kirill.tkhai@openvz.org>
Cc: agk@redhat.com, snitzer@redhat.com, dm-devel@redhat.com,
	song@kernel.org, linux-kernel@vger.kernel.org,
	khorenko@virtuozzo.com
Subject: Re: [dm-devel] [PATCH 0/4] dm: Introduce dm-qcow2 driver to attach QCOW2 files as block device
Date: Tue, 29 Mar 2022 06:08:53 -0700	[thread overview]
Message-ID: <YkME5ZS2CpXuNmN6@infradead.org> (raw)
In-Reply-To: <164846619932.251310.3668540533992131988.stgit@pro>

On Mon, Mar 28, 2022 at 02:18:16PM +0300, Kirill Tkhai wrote:
> This patchset adds a new driver allowing to attach QCOW2 files
> as block devices. Its idea is to implement in kernel only that
> features, which affect runtime IO performance (IO requests
> processing functionality).

From a quick looks it seems to be like this should be a block driver
just like the loop driver and not use device mapper.  Why would
you use device mapper to basically reimplement a fancy loop driver
to start with?

> The maintenance operations are
> synchronously processed in userspace, while device is suspended.
> 
> Userspace is allowed to do only that operations, which never
> modifies virtual disk's data. It is only allowed to modify
> QCOW2 file metadata providing that disk's data. The examples
> of allowed operations is snapshot creation and resize.

And this sounds like a pretty fragile design.  It basically requires
both userspace and the kernel driver to access metadata on disk, which
sounds rather dangerous.

> This example shows the way of device-mapper infrastructure
> allows to implement drivers following the idea of
> kernel/userspace components demarcation. Thus, the driver
> uses advantages of device-mapper instead of implementing
> its own suspend/resume engine.

What do you need more than a queue freeze?

WARNING: multiple messages have this Message-ID (diff)
From: Christoph Hellwig <hch@infradead.org>
To: Kirill Tkhai <kirill.tkhai@openvz.org>
Cc: snitzer@redhat.com, linux-kernel@vger.kernel.org,
	song@kernel.org, dm-devel@redhat.com, agk@redhat.com,
	khorenko@virtuozzo.com
Subject: Re: [dm-devel] [PATCH 0/4] dm: Introduce dm-qcow2 driver to attach QCOW2 files as block device
Date: Tue, 29 Mar 2022 06:08:53 -0700	[thread overview]
Message-ID: <YkME5ZS2CpXuNmN6@infradead.org> (raw)
In-Reply-To: <164846619932.251310.3668540533992131988.stgit@pro>

On Mon, Mar 28, 2022 at 02:18:16PM +0300, Kirill Tkhai wrote:
> This patchset adds a new driver allowing to attach QCOW2 files
> as block devices. Its idea is to implement in kernel only that
> features, which affect runtime IO performance (IO requests
> processing functionality).

From a quick looks it seems to be like this should be a block driver
just like the loop driver and not use device mapper.  Why would
you use device mapper to basically reimplement a fancy loop driver
to start with?

> The maintenance operations are
> synchronously processed in userspace, while device is suspended.
> 
> Userspace is allowed to do only that operations, which never
> modifies virtual disk's data. It is only allowed to modify
> QCOW2 file metadata providing that disk's data. The examples
> of allowed operations is snapshot creation and resize.

And this sounds like a pretty fragile design.  It basically requires
both userspace and the kernel driver to access metadata on disk, which
sounds rather dangerous.

> This example shows the way of device-mapper infrastructure
> allows to implement drivers following the idea of
> kernel/userspace components demarcation. Thus, the driver
> uses advantages of device-mapper instead of implementing
> its own suspend/resume engine.

What do you need more than a queue freeze?

--
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dm-devel@redhat.com
https://listman.redhat.com/mailman/listinfo/dm-devel


  parent reply	other threads:[~2022-03-29 13:09 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-03-28 11:18 [PATCH 0/4] dm: Introduce dm-qcow2 driver to attach QCOW2 files as block device Kirill Tkhai
2022-03-28 11:18 ` [dm-devel] " Kirill Tkhai
2022-03-28 11:18 ` [PATCH 1/4] dm: Export dm_complete_request() Kirill Tkhai
2022-03-28 11:18   ` [dm-devel] " Kirill Tkhai
2022-03-28 11:18 ` [PATCH 2/4] dm: Process .io_hints for drivers not having underlying devices Kirill Tkhai
2022-03-28 11:18   ` [dm-devel] " Kirill Tkhai
2022-03-28 11:18 ` [PATCH 3/4] dm-qcow2: Introduce driver to create block devices over QCOW2 files Kirill Tkhai
2022-03-28 11:18   ` [dm-devel] " Kirill Tkhai
2022-03-28 20:03   ` kernel test robot
2022-03-28 20:03     ` kernel test robot
2022-03-28 23:42   ` kernel test robot
2022-03-28 23:42     ` [dm-devel] " kernel test robot
2022-03-29 10:42   ` [PATCH 3/4 v1.5] " Kirill Tkhai
2022-03-29 10:42     ` [dm-devel] " Kirill Tkhai
2022-03-29 13:34   ` [dm-devel] [PATCH 3/4] " Christoph Hellwig
2022-03-29 13:34     ` Christoph Hellwig
2022-03-29 15:24     ` Kirill Tkhai
2022-03-29 15:24       ` Kirill Tkhai
2022-03-29 22:30       ` Kirill Tkhai
2022-03-29 22:30         ` Kirill Tkhai
2022-03-28 11:18 ` [PATCH 4/4] dm-qcow2: Add helper for working with dm-qcow2 devices Kirill Tkhai
2022-03-28 11:18   ` [dm-devel] " Kirill Tkhai
2022-03-29 13:08 ` Christoph Hellwig [this message]
2022-03-29 13:08   ` [dm-devel] [PATCH 0/4] dm: Introduce dm-qcow2 driver to attach QCOW2 files as block device Christoph Hellwig
2022-03-29 15:14   ` Kirill Tkhai
2022-03-29 15:14     ` Kirill Tkhai

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