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Re: [Qemu-devel] [RFC PATCH 0/6] block: Add driver specific options
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [RFC PATCH 0/6] block: Add driver specific options |
Date: |
Thu, 28 Feb 2013 11:23:57 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Wed, Feb 27, 2013 at 06:25:49PM +0100, Kevin Wolf wrote:
> Instead of constantly keeping complaining that we need this big -blockdev
> things, let's start attacking individual parts of the project. Here is the
> first part, allowing block drivers to provide additional -drive options.
>
> Any options that isn't processed by the block layer is passed to the driver
> which can use them. If it doesn't, you get an error message like today. At the
> end of this series you can specify:
>
> -drive file=test.qcow2,lazy_refcounts=on
>
> This is still quite rough and more of a quick hack, but I'd like to get some
> feedback on the general approach and for example if at least the user
> interface
> is acceptable. I thought -drive already mixes so many things, messing a bit
> more with it can't make things much worse, but you may disagree.
Quick comment before diving into the patches:
The consequence of this syntax is that option name collisions may become
a problem in the future. The chance is low but when you string together
a protocol with a format there is already a chance that both
BlockDrivers want an option. This is the multiple inheritance problem
in object oriented language: did the user mean to set both options?
Just one and which? etc
However, when we get -blockdev this is no longer an issue. And in the
meantime this patch series seems to add a useful feature.
> Before merging this, I'll look into using QemuOpts a bit more (the manual
> parsing of the option in qcow2 definitely has to go away) and possibly allow
> specifying options for backing files or the protocol as well with a syntax
> like
> "backing.lazy_refcounts=on". If all options starts using this, maybe even
> something like "backing.cache=writeback" could be made to work.
Introducing a new <foo>.<option> namespace pushes the necessity for
-blockdev farther into the future. I'm reminded that GlusterFS has a
working module graph which can be configured (plus CLI tools for common
use cases). It's not an impossible problem, we're just not very good at
or keen on defining interfaces :).
Stefan
- Re: [Qemu-devel] [RFC PATCH 2/6] block: Add options QDict to bdrv_open() prototype, (continued)
- [Qemu-devel] [RFC PATCH 1/6] block: Add options QDict to .bdrv_open(), Kevin Wolf, 2013/02/27
- [Qemu-devel] [RFC PATCH 5/6] block: Support driver specific options in drive_init(), Kevin Wolf, 2013/02/27
- [Qemu-devel] [RFC PATCH 6/6] qcow2: Allow lazy refcounts to be enabled on the command line, Kevin Wolf, 2013/02/27
- [Qemu-devel] [RFC PATCH 4/6] block: Add options QDict to bdrv_open_common(), Kevin Wolf, 2013/02/27
- Re: [Qemu-devel] [RFC PATCH 0/6] block: Add driver specific options, Eric Blake, 2013/02/27
- Re: [Qemu-devel] [RFC PATCH 0/6] block: Add driver specific options,
Stefan Hajnoczi <=