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Re: [Qemu-block] [PATCH v6 2/2] qemu-img: Document --force-share / -U
From: |
Fam Zheng |
Subject: |
Re: [Qemu-block] [PATCH v6 2/2] qemu-img: Document --force-share / -U |
Date: |
Fri, 9 Feb 2018 11:29:20 +0800 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
On Thu, 02/01 11:44, Stefan Hajnoczi wrote:
> On Wed, Jan 31, 2018 at 03:22:49PM +0100, Kevin Wolf wrote:
> > Am 31.01.2018 um 15:12 hat Stefan Hajnoczi geschrieben:
> > > There should be a separate paragraph in docs/qemu-block-drivers.texi
> > > explaining that share-rw=on can be used safely with format=raw if the
> > > guests are configured to safely access a shared disk. It should also
> > > mention that share-rw=on is unsafe for image formats.
> >
> > share-rw=on is a -device option and only about the guest, not about the
> > backend. It is never unsafe if the guest can cope with external writers.
> > It just doesn't prevent qemu from locking the image file for image
> > formats.
>
> Thanks for the explanation. Documentation on share-rw=on|off would be
> nice.
>
> Maybe something like:
>
> By default the guest has exclusive write access to its disk image.
> This is enforced by QEMU via file locking. If the guest can safely
> share the disk image with other writers the -device ...,share-rw=on
> parameter can be used. This is only safe if the guest is running
> software, such as a cluster file system, that coordinates disk
> accesses to avoid corruption.
>
> Note that share-rw=on only declares the guest's ability to share the
> disk. Some QEMU features, such as image file formats, require
> exclusive write access to the disk image and this is unaffected by the
> share-rw=on option.
I will add these paragraphs to the "image locking" seciton in
docs/qemu-block-drivers.texi.
Fam