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Re: [Qemu-block] [PATCH v6 2/2] qemu-img: Document --force-share / -U

From: Stefan Hajnoczi
Subject: Re: [Qemu-block] [PATCH v6 2/2] qemu-img: Document --force-share / -U
Date: Thu, 1 Feb 2018 11:44:25 +0000
User-agent: Mutt/1.9.1 (2017-09-22)

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

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.

For anyone else reading this, here is the code that protects qcow2 and
other image formats:

  void bdrv_format_default_perms(BlockDriverState *bs, BdrvChild *c,
                                 const BdrvChildRole *role,
                                 BlockReopenQueue *reopen_queue,
                                 uint64_t perm, uint64_t shared,
                                 uint64_t *nperm, uint64_t *nshared)

      /* bs->file always needs to be consistent because of the metadata. We
       * can never allow other users to resize or write to it. */
      if (!(flags & BDRV_O_NO_IO)) {
          perm |= BLK_PERM_CONSISTENT_READ;
      shared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);


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