Re: [Qemu-devel] [PATCH] block: delete cow block driver

[Stefan Hajnoczi]

On Tue, Sep 16, 2014 at 03:24:24PM +0100, Stefan Hajnoczi wrote:
> This patch removes support for the cow file format.
> 
> Normally we do not break backwards compatibility but in this case there
> is no impact and it is the most logical option.  Extraordinary claims
> require extraordinary evidence so I will show why removing the cow block
> driver is the right thing to do.
> 
> The cow file format is the disk image format for Usermode Linux, a way
> of running a Linux system in userspace.  The performance of UML was
> never great and it was hacky, but it enjoyed some popularity before
> hardware virtualization support became mainstream.
> 
> QEMU's block/cow.c is supposed to read this image file format.
> Unfortunately the file format was underspecified:
> 
> 1. Earlier Linux versions used the MAXPATHLEN constant for the backing
>    filename field.  The value of MAXPATHLEN can change, so Linux
>    switched to a 4096 literal but QEMU has a 1024 literal.
> 
> 2. Padding was not used on the header struct (both in the Linux kernel
>    and in QEMU) so the struct layout varied across architectures.  In
>    particular, i386 and x86_64 were different due to int64_t alignment
>    differences.  Linux now uses __attribute__((packed)), QEMU does not.
> 
> Therefore:
> 
> 1. QEMU cow images do not conform to the Linux cow image file format.
> 
> 2. cow images cannot be shared between different host architectures.
> 
> This means QEMU cow images are useless and QEMU has not had bug reports
> from users actually hitting these issues.
> 
> Let's get rid of this thing, it serves no purpose and no one will be
> affected.
> 
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  block/Makefile.objs          |   2 +-
>  block/cow.c                  | 432 
> -------------------------------------------
>  qapi/block-core.json         |   5 +-
>  qemu-doc.texi                |   9 -
>  qemu-img.texi                |   4 +-
>  qmp-commands.hx              |   2 +-
>  tests/image-fuzzer/runner.py |   5 +-
>  tests/qemu-iotests/069       |   2 +-
>  tests/qemu-iotests/072       |   2 +-
>  tests/qemu-iotests/099       |   2 +-
>  tests/qemu-iotests/common    |   6 -
>  ui/cocoa.m                   |   2 +-
>  12 files changed, 12 insertions(+), 461 deletions(-)
>  delete mode 100644 block/cow.c

Applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

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