qemu/block
Stefan Hajnoczi 550830f935 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>
2014-09-22 11:39:45 +01:00
..
Makefile.objs block: delete cow block driver 2014-09-22 11:39:45 +01:00
accounting.c block: Make the block accounting functions operate on BlockAcctStats 2014-09-10 10:41:29 +02:00
archipelago.c block/archipelago: Fix typo in qemu_archipelago_truncate() 2014-09-22 11:39:44 +01:00
backup.c block/backup: Fix hang for unaligned image size 2014-07-09 15:50:11 +02:00
blkdebug.c block: Rename qemu_aio_release -> qemu_aio_unref 2014-09-22 11:39:17 +01:00
blkverify.c block: Rename qemu_aio_release -> qemu_aio_unref 2014-09-22 11:39:17 +01:00
bochs.c block: Use g_new() & friends to avoid multiplying sizes 2014-08-20 11:51:28 +02:00
cloop.c cloop: Handle failure for potentially large allocations 2014-08-15 15:07:15 +02:00
commit.c block: extend block-commit to accept a string for the backing file 2014-07-01 10:47:01 +02:00
curl.c block: Rename qemu_aio_release -> qemu_aio_unref 2014-09-22 11:39:17 +01:00
dmg.c dmg: Handle failure for potentially large allocations 2014-08-15 15:07:15 +02:00
gluster.c block: don't convert file size to sector size 2014-09-12 15:43:06 +02:00
iscsi.c block: Rename qemu_aio_release -> qemu_aio_unref 2014-09-22 11:39:17 +01:00
linux-aio.c block: Rename qemu_aio_release -> qemu_aio_unref 2014-09-22 11:39:17 +01:00
mirror.c mirror: fix uninitialized variable delay_ns warnings 2014-08-28 13:42:25 +01:00
nbd-client.c nbd: implement .bdrv_detach/attach_aio_context() 2014-06-04 09:56:11 +02:00
nbd-client.h nbd: implement .bdrv_detach/attach_aio_context() 2014-06-04 09:56:11 +02:00
nbd.c nbd: Implement bdrv_refresh_filename() 2014-08-20 14:31:56 +02:00
nfs.c block: round up file size to nearest sector 2014-09-12 15:43:06 +02:00
null.c block: Introduce "null" drivers 2014-09-22 11:39:23 +01:00
parallels.c block: Use g_new() & friends to avoid multiplying sizes 2014-08-20 11:51:28 +02:00
qapi.c block: rename BlockAcctType members to start with BLOCK_ instead of BDRV_ 2014-09-10 10:41:29 +02:00
qcow.c block: don't convert file size to sector size 2014-09-12 15:43:06 +02:00
qcow2-cache.c qcow2: Use g_try_new0() for cache array 2014-08-20 11:51:28 +02:00
qcow2-cluster.c qcow2: Check L1/L2/reftable entries for alignment 2014-09-22 11:39:28 +01:00
qcow2-refcount.c qcow2: Check L1/L2/reftable entries for alignment 2014-09-22 11:39:28 +01:00
qcow2-snapshot.c block: Use g_new() & friends where that makes obvious sense 2014-08-20 11:51:28 +02:00
qcow2.c qcow2: Add overlap-check.template option 2014-09-22 11:39:34 +01:00
qcow2.h qcow2: Add overlap-check.template option 2014-09-22 11:39:34 +01:00
qed-check.c block: Use g_new() & friends to avoid multiplying sizes 2014-08-20 11:51:28 +02:00
qed-cluster.c Use glib memory allocation and free functions 2011-08-20 23:01:08 -05:00
qed-gencb.c Use glib memory allocation and free functions 2011-08-20 23:01:08 -05:00
qed-l2-cache.c qed: do not evict in-use L2 table cache entries 2012-03-12 15:14:06 +01:00
qed-table.c qed: use BlockDriverState's AioContext 2014-06-04 09:56:11 +02:00
qed.c block: Rename qemu_aio_release -> qemu_aio_unref 2014-09-22 11:39:17 +01:00
qed.h qed: Make qiov match request size until backing file EOF 2014-07-14 12:03:20 +02:00
quorum.c block: Rename qemu_aio_release -> qemu_aio_unref 2014-09-22 11:39:17 +01:00
raw-aio.h linux-aio: implement io plug, unplug and flush io queue 2014-07-07 11:05:17 +02:00
raw-posix.c raw-posix: Add falloc and full preallocation option 2014-09-12 15:43:06 +02:00
raw-win32.c block: don't convert file size to sector size 2014-09-12 15:43:06 +02:00
raw_bsd.c block: Add Error argument to bdrv_refresh_limits() 2014-07-18 13:18:43 +01:00
rbd.c block: Rename qemu_aio_release -> qemu_aio_unref 2014-09-22 11:39:17 +01:00
sheepdog.c block: Rename qemu_aio_release -> qemu_aio_unref 2014-09-22 11:39:17 +01:00
snapshot.c Use error_is_set() only when necessary 2014-02-17 11:57:23 -05:00
ssh.c block: round up file size to nearest sector 2014-09-12 15:43:06 +02:00
stream.c block: Add Error argument to bdrv_refresh_limits() 2014-07-18 13:18:43 +01:00
vdi.c block: round up file size to nearest sector 2014-09-12 15:43:06 +02:00
vhdx-endian.c block: VHDX endian fixes 2014-08-15 15:07:14 +02:00
vhdx-log.c block: Drop some superfluous casts from void * 2014-08-20 11:51:28 +02:00
vhdx.c block/vhdx.c: Mark parent_vhdx_guid variable as unused 2014-09-22 11:35:02 +01:00
vhdx.h block: VHDX endian fixes 2014-08-15 15:07:14 +02:00
vmdk.c block: round up file size to nearest sector 2014-09-12 15:43:06 +02:00
vpc.c block: round up file size to nearest sector 2014-09-12 15:43:06 +02:00
vvfat.c block/vvfat.c: remove debugging code to reinit stderr if NULL 2014-08-21 10:36:29 +02:00
win32-aio.c block: Rename qemu_aio_release -> qemu_aio_unref 2014-09-22 11:39:17 +01:00