diff options
| author | Stefan Hajnoczi <stefanha@redhat.com> | 2014-09-16 15:24:24 +0100 |
|---|---|---|
| committer | Stefan Hajnoczi <stefanha@redhat.com> | 2014-09-22 11:39:45 +0100 |
| commit | 550830f9351291c585c963204ad9127998b1c1ce (patch) | |
| tree | c0836eb22f9ddd14e5c805599cefbbab8417fc6c /qemu-img.texi | |
| parent | 3e7e6f186f6180c5900dbc8db04abbbda1275dad (diff) | |
block: delete cow block driver
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 <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1410877464-20481-1-git-send-email-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Diffstat (limited to 'qemu-img.texi')
| -rw-r--r-- | qemu-img.texi | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/qemu-img.texi b/qemu-img.texi index 50d2cca7a0..e3ef4a00e9 100644 --- a/qemu-img.texi +++ b/qemu-img.texi @@ -228,8 +228,8 @@ compression is read-only. It means that if a compressed sector is rewritten, then it is rewritten as uncompressed data. Image conversion is also useful to get smaller image when using a -growable format such as @code{qcow} or @code{cow}: the empty sectors -are detected and suppressed from the destination image. +growable format such as @code{qcow}: the empty sectors are detected and +suppressed from the destination image. @var{sparse_size} indicates the consecutive number of bytes (defaults to 4k) that must contain only zeros for qemu-img to create a sparse image during |