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Re: [Qemu-devel] [PATCH] block: change default of .has_zero_init to 0
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH] block: change default of .has_zero_init to 0 |
Date: |
Fri, 28 Jun 2013 10:06:44 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 27.06.2013 um 15:52 hat Peter Lieven geschrieben:
> .has_zero_init defaults to 1 for all formats and protocols.
>
> this is a dangerous default since this means that all
> new added drivers need to manually overwrite it to 0 if
> they do not ensure that a device is zero initialized
> after bdrv_create().
>
> if a driver needs to explicitly set this value to
> 1 its easier to verify the correctness in the review process.
>
> during review of the existing drivers it turned out
> that ssh and gluster had a wrong default of 1.
> both protocols support host_devices as backend
> which are not by default zero initialized. this
> wrong assumption will lead to possible corruption
> if qemu-img convert is used to write to such a backend.
>
> a similar problem with the wrong default existed for
> iscsi mose likely because the driver developer did
> oversee the default value of 1.
>
> Signed-off-by: Peter Lieven <address@hidden>
> ---
> block.c | 8 +++++++-
> block/qcow.c | 1 +
> block/qcow2.c | 1 +
> block/qed.c | 1 +
> block/raw-posix.c | 10 +---------
> block/raw-win32.c | 7 +------
> block/rbd.c | 1 +
> block/sheepdog.c | 1 +
> block/vdi.c | 1 +
> block/vmdk.c | 1 +
> include/block/block.h | 1 +
> 11 files changed, 17 insertions(+), 16 deletions(-)
You forgot cow, which is also a simple case that can be handled in this
patch.
vpc is still easy, but a bit more complicated than a constant return 1,
because it depends on the subformat whether a new image will inherit the
has_zero_init property from the underlying storage or whether it always
produces zeros (for VHD_FIXED type images, it's basically raw + footer).
I'll send a separate patch for this.
A similar situation exists for vmdk, I think, just that the difference
can be per extent there. I guess we need to return 0 if one of the
extents is flat and has an underlying storage returning 0. I'll leave
this part to Fam. For now, please remove the bdrv_has_zero_init_1 for
vmdk from this patch as it's unsafe.
Kevin
Re: [Qemu-devel] [PATCH] block: change default of .has_zero_init to 0, Peter Lieven, 2013/06/28