aboutsummaryrefslogtreecommitdiff
path: root/qemu-img.texi
diff options
context:
space:
mode:
authorStefan Hajnoczi <stefanha@redhat.com>2014-09-16 15:24:24 +0100
committerStefan Hajnoczi <stefanha@redhat.com>2014-09-22 11:39:45 +0100
commit550830f9351291c585c963204ad9127998b1c1ce (patch)
treec0836eb22f9ddd14e5c805599cefbbab8417fc6c /qemu-img.texi
parent3e7e6f186f6180c5900dbc8db04abbbda1275dad (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.texi4
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