The QEMU Copy-On-Write (QCOW) image file format is used by the QEMU Open Source Process Emulator to store disk images. This specification is based on available documentation and was enhanced by analyzing test data.
This document is intended as a working document of the data format specification for the libqcow project.
Author(s): |
Joachim Metz <joachim.metz@gmail.com> |
Abstract: |
This document contains information about the QEMU Copy-On-Write file format. |
Classification: |
Public |
Keywords: |
QEMU Copy-On-Write file, QCOW |
Copyright (C) 2010-2020, Joachim Metz <joachim.metz@gmail.com>. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".
| Version | Author | Date | Comments |
|---|---|---|---|
0.0.1 |
J.B. Metz |
December 2010 |
Initial version |
0.0.2 |
J.B. Metz |
May 2012 |
Small update. |
0.0.3 |
J.B. Metz |
April 2013 |
Additional information regarding QCOW version 1, compression and encryption. |
0.0.4 |
J.B. Metz |
May 2013 |
Additional information regarding QCOW version 2. |
0.0.5 |
J.B. Metz |
December 2013 |
Additional information regarding compressed QCOW version 2. |
0.0.6 |
J.B. Metz |
October 2014 |
Switched to asccidoc format. |
0.0.7 |
J.B. Metz |
December 2014 |
Additional information regarding QCOW version 3. |
0.0.8 |
J.B. Metz |
July 2020 |
Formatting changes. |
0.0.9 |
J.B. Metz |
December 2020 |
Additional information regarding QCOW version 3. |
The QEMU Copy-On-Write (QCOW) image file format is used by the QEMU Open Source Process Emulator to store disk images (storage media).
The QCOW image file consists of:
-
the file header
-
optional file header extensions
-
-
the level 1 table (cluster aligned)
-
the reference count table (cluster aligned)
-
reference count blocks
-
snapshot headers (8-byte aligned on cluster boundary)
-
clusters containing:
-
level 2 tables
-
storage media data
-
The storage media data is stored in clusters. Each cluster is a multitude of 512 bytes. The level 1 (L1) table contains level 1 reference of level 2 (L2) tables. The level 2 tables contain level 2 references of the storage media clusters.
There are multiple versions of the QCOW image file format. QCOW (version 1) and QCOW2 (version) are sometimes considered even as separate image formats, where version 3 is considered as an extended version of QCOW2.
| Characteristics | Description |
|---|---|
Byte order |
big-endian in most cases, note that some values are in little-endian. |
Date and time values |
Number of seconds since Jan 1, 1970 00:00:00 UTC (POSIX epoch) |
Character strings |
Narrow character strings assumed stored in UTF-8 |
The QCOW image format can encrypted the media data stored in the image format. Currently supported encryption methods are:
-
AES-CBC 128-bit
-
Linux Unified Key Setup (LUKS)
If no encryption is used the encryption method in the file header is set to none (0).
It seems that compression and encryption at the same time is not supported, at least at the moment by qemu-img.
Both encryption and decryption use:
-
AES-CBC with a 128-bits key decryption of sector data
The key is direct copy of the first 16 characters of a user provided (narrow character) password. If the password is smaller than 16 characters. The remaining key data is set to 0-byte values.
It is unclear which character sets are allowed and how characters outside the 7-bit ASCII set should be handled.
The initialization vector of the AES-CBC is using media data sector number (relative to the start of the disk) in little-endian format as the first 64 bits of the 128 bit initialization vector. The remaining initialization vector data is set to 0-byte values. The first sector number is 0 and the bytes per sector are 512.
The version 1 file header is 48 bytes of size and consist of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
4 |
"QFI\xfb" |
The signature (magic identifier) |
4 |
4 |
1 |
Version |
8 |
8 |
Backing filename offset |
|
16 |
4 |
Backing filename size |
|
20 |
4 |
Modification date and time |
|
24 |
8 |
Storage media size |
|
32 |
1 |
Number of cluster block bits |
|
33 |
1 |
Number of level 2 table bits |
|
34 |
2 |
Unknown (empty values) |
|
36 |
4 |
Encryption method |
|
40 |
8 |
Level 1 table offset |
The cluster block size is calculated as:
cluster block size = 1 << number of cluster block bits
The level table 2 size is calculated as:
level table 2 size = ( 1 << number of level 2 table bits ) * 8
The level 1 table size is calculated as:
level 1 table size = cluster block size
* ( 1 << number of level 2 table bits )
if( media size % level 1 table size != 0 )
{
level 1 table size = ( media size / level 1 table size ) + 1
}
else
{
level 1 table size = media size / level 1 table size
}
level 1 table size *= 8
The backing filename is set in snapshot image files and is normally stored after the file header.
The version 2 file header is 72 or 112 bytes of size and consist of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
4 |
"QFI\xfb" |
The signature (magic identifier) |
4 |
4 |
2 or 3 |
Version |
8 |
8 |
Backing filename offset |
|
16 |
4 |
Backing filename size |
|
20 |
4 |
Number of cluster block bits |
|
24 |
8 |
Storage media size |
|
32 |
4 |
Encryption method |
|
36 |
4 |
Number of level 1 table references |
|
40 |
8 |
Level 1 table offset |
|
48 |
8 |
Reference count table offset |
|
56 |
4 |
Reference count table clusters |
|
60 |
4 |
Number of snapshots |
|
64 |
8 |
Snapshots offset |
|
Introduced in version 3 |
|||
72 |
8 |
Incompatible feature flags |
|
80 |
8 |
Compatible feature flags |
|
88 |
8 |
Auto-clear feature flags |
|
96 |
4 |
Reference count order |
|
100 |
4 |
104 |
File header size |
If file header size > 104 |
|||
104 |
1 |
Compression type |
|
105 |
7 |
Unknown (padding) |
|
The cluster block size is calculated as:
cluster block size = 1 << number of cluster block bits
The number of level 2 table bits is calculated as:
number of level 2 table bits = number of cluster block bits - 3
The level table 2 size is calculated as:
level table 2 size = ( 1 << number of level 2 table bits ) * 8
The level 1 table size is calculated as:
level 1 table size = number of level 1 table references * 8
The backing filename is set in snapshot image files and is normally stored after the file header.
| Value | Identifier | Description |
|---|---|---|
0 |
QCOW_CRYPT_NONE |
No encryption |
1 |
QCOW_CRYPT_AES |
AES-CBC 128-bits encryption |
2 |
QCOW_CRYPT_LUKS |
Linux Unified Key Setup (LUKS) encryption |
| Value | Identifier | Description |
|---|---|---|
0x00000001 |
QCOW2_INCOMPAT_DIRTY |
|
0x00000002 |
QCOW2_INCOMPAT_CORRUPT |
|
0x00000004 |
QCOW2_INCOMPAT_DATA_FILE |
|
0x00000008 |
QCOW2_INCOMPAT_COMPRESSION |
|
0x00000010 |
QCOW2_INCOMPAT_EXTL2 |
| Value | Identifier | Description |
|---|---|---|
0x00000001 |
QCOW2_AUTOCLEAR_BITMAPS |
|
0x00000002 |
QCOW2_AUTOCLEAR_DATA_FILE_RAW |
A file header extension consist of:
-
file header extension header
-
file header extension data
The file header extension header is 8 bytes of size and consist of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
4 |
The extension type (signature) |
|
4 |
4 |
The extension data size |
| Value | Identifier | Description |
|---|---|---|
0x0537be77 |
QCOW2_EXT_MAGIC_CRYPTO_HEADER |
Crypto header |
0x23852875 |
QCOW2_EXT_MAGIC_BITMAPS |
Bitmaps |
0x44415441 |
QCOW2_EXT_MAGIC_DATA_FILE |
Data-file |
0x6803f857 |
QCOW2_EXT_MAGIC_FEATURE_TABLE |
Feature table |
0xe2792aca |
QCOW2_EXT_MAGIC_BACKING_FORMAT |
Backing format |
The backing format file header extension header is variable of size and consist of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
… |
Backing format identifier |
The crypto header file header extension header is 16 bytes of size and consist of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
8 |
The crypto data offset |
|
8 |
8 |
The crypto data size |
The data-file file header extension header is variable of size and consist of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
… |
Data-file filename |
The level 1 table contains level 2 table references.
A reference value of 0 represents unused or unallocated and is considered as sparse or stored in a corresponding backing file.
The level 2 table reference is 8-bytes of size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0.0 |
63 bits |
Level 2 table offset |
|
7.7 |
1 bit |
Is compressed flag |
The level 2 table reference is 8-bytes of size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0.0 |
62 bits |
Level 2 table offset |
|
7.6 |
1 bit |
Is compressed flag |
|
7.7 |
1 bit |
Is copied flag |
The is copied flag indicates that the reference count of the corresponding level 2 table is exactly one.
The level 2 table contains cluster block references.
The level 2 table size is calculated as:
level 2 table size = ( 1 << number of level 2 table bits ) * 8
A reference value of 0 represents unused or unallocated and is considered as sparse or stored in a corresponding backing file.
The cluster block reference is 8-bytes of size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0.0 |
63 bits |
Cluster block offset |
|
7.7 |
1 bit |
Is compressed flag |
The cluster block reference is 8-bytes of size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0.0 |
62 bits |
Cluster block offset |
|
7.6 |
1 bit |
Is compressed flag |
|
7.7 |
1 bit |
Is copied flag |
The is copied flag indicates that the reference count of the corresponding cluster block is exactly one.
The cluster data blocks are referenced counted. For every cluster data block a 16-bit reference count is stored in the reference count table.
The reference count table is stored in cluster block sizes. The file header contains the number of blocks (or reference count table clusters).
TODO
To retrieve a cluster data block corresponding a certain storage media offset:
Determine the level 1 table index from the offset:
level 1 table index bit shift = number of cluster block bits
+ number of level 2 table bits
Version 1
level 1 table index = ( offset & 0x7fffffffffffffffULL )
>> level 1 table index bit shift
Version 2
level 1 table index = ( offset & 0x3fffffffffffffffULL )
>> level 1 table index bit shift
Retrieve the level 2 table offset from the level 1 table. If the level table 2 offset is 0 and the image has a backing file the cluster data block is stored in the backing file otherwise the cluster block is considered sparse.
Read the corresponding level 2 table.
Determine the level 2 table index from the offset:
level 2 table index bit mask = ~( 0xffffffffffffffffULL
<< number of level 2 table bits )
level 2 table index = ( offset >> number of cluster block bits )
>> level 2 table index bit mask
Retrieve the cluster block offset from the level 2 table. If the cluster block offset is 0 and the image has a backing file the cluster data block is stored in the backing file otherwise the cluster block is considered sparse.
If the is compressed flag (QCOW_OFLAG_COMPRESSED) is not set:
cluster block bit mask = ~( 0xffffffffffffffffULL
<< number of cluster block bits )
cluster block data offset = ( offset & cluster block bit mask )
+ cluster block offset
Note that in version 2 or 3 the last cluster block in the file can be smaller than the cluster block size defined by the number of cluster block bits in the file header. This does not seem to be the case for version 1.
If the is compressed flag (QCOW_OFLAG_COMPRESSED) is set:
compressed size bit shift = 63 - number of cluster block bits
compressed block size = ( ( cluster block offset & 0x7fffffffffffffffULL )
>> compressed size bit shift
compressed block offset &= ~( 0xffffffffffffffffULL
<< compressed size bit shift )
|
Note
|
Multiple compressed cluster data blocks seem to be always stored together in cluster block sizes. The compressed cluster data blocks are sector (512 bytes) aligned. |
The compressed data uses a zlib inflate window bits value of -12
compressed size bit shift = 62 - ( number of cluster block bits – 8 )
[MCLOUGHLIN08] describes the compressed block size calculation as:
compressed block size = ( ( ( cluster block offset & 0x3fffffffffffffffULL )
>> compressed size bit shift ) + 1 ) * 512.
Since the compressed block size is stored in 512 byte sectors this value does not contain the exact byte size of the compressed cluster block data. It sometimes lacks the size of the last partially filled sector and one sector should be added if possible within the bounds of the cluster blocks size and the file size.
cluster block offset &= ~( 0xffffffffffffffffULL
<< compressed size bit shift )
|
Note
|
Multiple compressed cluster data blocks seem to be always stored together in cluster block sizes. Although the file size does not seem to be strictly a multitude of the the cluster block size. |
The compressed data uses a zlib inflate window bits value of -12
As of version 1 QCOW can use the backing filename in the file header to point to a parent image that contains the snapshot image where the current image only contains the modifications. Version 2 adds support to store snapshot inside the image.
An in-image snapshot is created by adding a snapshot header, copying the L1 table and incrementing the reference counts of all L2 tables and data clusters referenced by the L1 table.
The snapshot header is variable of size and consists of:
| Offset | Size | Value | Description |
|---|---|---|---|
0 |
8 |
Level 1 table offset |
|
8 |
4 |
Level 1 size |
|
12 |
2 |
Identifier string size |
|
14 |
2 |
Name size |
|
16 |
4 |
Date in seconds |
|
20 |
4 |
Date in nano seconds |
|
24 |
8 |
VM clock in nano seconds |
|
32 |
4 |
VM state size |
|
36 |
4 |
Extra data size |
|
40 |
… |
Extra data |
|
… |
… |
Identifier string size |
|
… |
… |
Name |
TODO
reference count cluster block offset = cluster data block offset / reference count table offset = cluster data block /
In order to obtain the reference count of a given cluster, you split the cluster offset into a refcount table offset and refcount block offset.
Since a refcount block is a single cluster of 2 byte entries, the lower cluster_size - 1 bits is used as the block offset and the rest of the bits are used as the table offset.
One optimization is that if any cluster pointed to by an L1 or L2 table entry has a refcount exactly equal to one, the most significant bit of the L1/L2 entry is set as a "copied" flag. This indicates that no snapshots are using this cluster and it can be immediately written to without having to make a copy for any snapshots referencing it.
[MCLOUGHLIN06]
| Title: | The QCOW Image Format |
|---|---|
Author(s): |
Mark McLoughlin |
Date: |
June 21, 2006 |
URL: |
http://people.gnome.org/~markmc/qcow-image-format-version-1.html |
[MCLOUGHLIN08]
| Title: | The QCOW2 Image Format |
|---|---|
Author(s): |
Mark McLoughlin |
Date: |
September 11, 2008 |
URL: |
[QEMU]
| Title: | QEMU Open Source Process Emulator |
|---|---|
URL: |
| Title: | Features/Qcow3 |
|---|---|
URL: |
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