From 550830f9351291c585c963204ad9127998b1c1ce Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Tue, 16 Sep 2014 15:24:24 +0100 Subject: 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 Reviewed-by: Markus Armbruster Message-id: 1410877464-20481-1-git-send-email-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi --- tests/image-fuzzer/runner.py | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) (limited to 'tests/image-fuzzer/runner.py') diff --git a/tests/image-fuzzer/runner.py b/tests/image-fuzzer/runner.py index b7002905e1..0a8743ef41 100755 --- a/tests/image-fuzzer/runner.py +++ b/tests/image-fuzzer/runner.py @@ -150,8 +150,7 @@ class TestEnv(object): 'discard $off $len'], ['qemu-io', '$test_img', '-c', 'truncate $off']] - for fmt in ['raw', 'vmdk', 'vdi', 'cow', 'qcow2', 'file', - 'qed', 'vpc']: + for fmt in ['raw', 'vmdk', 'vdi', 'qcow2', 'file', 'qed', 'vpc']: self.commands.append( ['qemu-img', 'convert', '-f', 'qcow2', '-O', fmt, '$test_img', 'converted_image.' + fmt]) @@ -178,7 +177,7 @@ class TestEnv(object): by 'qemu-img create'. """ # All formats supported by the 'qemu-img create' command. - backing_file_fmt = random.choice(['raw', 'vmdk', 'vdi', 'cow', 'qcow2', + backing_file_fmt = random.choice(['raw', 'vmdk', 'vdi', 'qcow2', 'file', 'qed', 'vpc']) backing_file_name = 'backing_img.' + backing_file_fmt backing_file_size = random.randint(MIN_BACKING_FILE_SIZE, -- cgit v1.2.3-55-g7522