Commit 550830f9 authored by Stefan Hajnoczi's avatar Stefan Hajnoczi

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.


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
Signed-off-by: default avatarStefan Hajnoczi <>
Reviewed-by: default avatarMarkus Armbruster <>
Signed-off-by: default avatarStefan Hajnoczi <>
parent 3e7e6f18
block-obj-y += raw_bsd.o cow.o qcow.o vdi.o vmdk.o cloop.o dmg.o bochs.o vpc.o vvfat.o
block-obj-y += raw_bsd.o qcow.o vdi.o vmdk.o cloop.o dmg.o bochs.o vpc.o vvfat.o
block-obj-y += qcow2.o qcow2-refcount.o qcow2-cluster.o qcow2-snapshot.o qcow2-cache.o
block-obj-y += qed.o qed-gencb.o qed-l2-cache.o qed-table.o qed-cluster.o
block-obj-y += qed-check.o
This diff is collapsed.
......@@ -197,7 +197,7 @@
# 'file', 'file', 'ftp', 'ftps', 'host_cdrom', 'host_device',
# 'host_floppy', 'http', 'https', 'nbd', 'parallels', 'qcow',
# 'qcow2', 'raw', 'tftp', 'vdi', 'vmdk', 'vpc', 'vvfat'
# 2.2: 'archipelago'
# 2.2: 'archipelago' added, 'cow' dropped
# @backing_file: #optional the name of the backing file (for copy-on-write)
......@@ -1148,7 +1148,7 @@
# Since: 2.0
{ 'enum': 'BlockdevDriver',
'data': [ 'archipelago', 'blkdebug', 'blkverify', 'bochs', 'cloop', 'cow',
'data': [ 'archipelago', 'blkdebug', 'blkverify', 'bochs', 'cloop',
'dmg', 'file', 'ftp', 'ftps', 'host_cdrom', 'host_device',
'host_floppy', 'http', 'https', 'null-aio', 'null-co', 'parallels',
'qcow', 'qcow2', 'qed', 'quorum', 'raw', 'tftp', 'vdi', 'vhdx',
......@@ -1565,7 +1565,6 @@
'blkverify': 'BlockdevOptionsBlkverify',
'bochs': 'BlockdevOptionsGenericFormat',
'cloop': 'BlockdevOptionsGenericFormat',
'cow': 'BlockdevOptionsGenericCOWFormat',
'dmg': 'BlockdevOptionsGenericFormat',
'file': 'BlockdevOptionsFile',
'ftp': 'BlockdevOptionsFile',
......@@ -654,15 +654,6 @@ File name of a base image (see @option{create} subcommand)
If this option is set to @code{on}, the image is encrypted.
@end table
@item cow
User Mode Linux Copy On Write image format. It is supported only for
compatibility with previous versions.
Supported options:
@table @code
@item backing_file
File name of a base image (see @option{create} subcommand)
@end table
@item vdi
VirtualBox 1.1 compatible image format.
Supported options:
......@@ -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
......@@ -2081,7 +2081,7 @@ Each json-object contain the following:
- "file": device file name (json-string)
- "ro": true if read-only, false otherwise (json-bool)
- "drv": driver format name (json-string)
- Possible values: "blkdebug", "bochs", "cloop", "cow", "dmg",
- Possible values: "blkdebug", "bochs", "cloop", "dmg",
"file", "file", "ftp", "ftps", "host_cdrom",
"host_device", "host_floppy", "http", "https",
"nbd", "parallels", "qcow", "qcow2", "raw",
......@@ -150,8 +150,7 @@ class TestEnv(object):
'discard $off $len'],
['qemu-io', '$test_img', '-c',
'truncate $off']]
for fmt in ['raw', 'vmdk', 'vdi', 'cow', 'qcow2', 'file',
'qed', 'vpc']:
for fmt in ['raw', 'vmdk', 'vdi', 'qcow2', 'file', 'qed', 'vpc']:
['qemu-img', 'convert', '-f', 'qcow2', '-O', fmt,
'$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,
......@@ -38,7 +38,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
. ./common.rc
. ./common.filter
_supported_fmt cow qed qcow qcow2 vmdk
_supported_fmt qed qcow qcow2 vmdk
_supported_proto file
_supported_os Linux
_unsupported_imgopts "subformat=monolithicFlat" "subformat=twoGbMaxExtentFlat"
......@@ -38,7 +38,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
. ./common.rc
. ./common.filter
_supported_fmt vpc vmdk vhdx vdi qed qcow2 qcow cow
_supported_fmt vpc vmdk vhdx vdi qed qcow2 qcow
_supported_proto file
_supported_os Linux
......@@ -41,7 +41,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
# Basically all formats, but "raw" has issues with _filter_imgfmt regarding the
# raw comparison image for blkverify; also, all images have to support creation
_supported_fmt cow qcow qcow2 qed vdi vhdx vmdk vpc
_supported_fmt qcow qcow2 qed vdi vhdx vmdk vpc
_supported_proto file
_supported_os Linux
......@@ -136,7 +136,6 @@ common options
check options
-raw test raw (default)
-bochs test bochs
-cow test cow
-cloop test cloop
-parallels test parallels
-qcow test qcow
......@@ -182,11 +181,6 @@ testlist options
......@@ -857,7 +857,7 @@ QemuCocoaView *cocoaView;
[op setPrompt:@"Boot image"];
[op setMessage:@"Select the disk image you want to boot.\n\nHit the \"Cancel\" button to quit"];
NSArray *filetypes = [NSArray arrayWithObjects:@"img", @"iso", @"dmg",
@"qcow", @"qcow2", @"cow", @"cloop", @"vmdk", nil];
@"qcow", @"qcow2", @"cloop", @"vmdk", nil];
[op setAllowedFileTypes:filetypes];
[op beginSheetModalForWindow:normalWindow
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