Patchwork [1/6,v10] docs: spec for add-cow file format

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Submitter Robert Wang
Date June 13, 2012, 2:36 p.m.
Message ID <1339598189-17933-1-git-send-email-wdongxu@linux.vnet.ibm.com>
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Permalink /patch/164698/
State New
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Robert Wang - June 13, 2012, 2:36 p.m.
Introduce a new file format:add-cow. The usage can be found at this patch.

Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
---
 docs/specs/add-cow.txt |   87 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 87 insertions(+), 0 deletions(-)
 create mode 100644 docs/specs/add-cow.txt
Eric Blake - June 13, 2012, 3:10 p.m.
On 06/13/2012 08:36 AM, Dong Xu Wang wrote:
> Introduce a new file format:add-cow. The usage can be found at this patch.
> 
> Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
> ---
>  docs/specs/add-cow.txt |   87 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 87 insertions(+), 0 deletions(-)
>  create mode 100644 docs/specs/add-cow.txt
> 
> diff --git a/docs/specs/add-cow.txt b/docs/specs/add-cow.txt
> new file mode 100644
> index 0000000..e077fc2
> --- /dev/null
> +++ b/docs/specs/add-cow.txt
> @@ -0,0 +1,87 @@
> +== General ==
> +
> +Raw file format does not support backing_file and copy on write feature.
> +The add-cow image format makes it possible to use backing files with raw
> +image by keeping a separate .add-cow metadata file.  Once all sectors
> +have been written into the raw image it is safe to discard the .add-cow
> +and backing files, then we can use the raw image directly.
> +
> +While using add-cow, procedures may like this:
> +(ubuntu.img is a disk image which has been installed OS.)
> +    1)  Create a raw image with the same size of ubuntu.img
> +            qemu-img create -f raw test.raw 8G

Make sure we also support a raw file larger than the backing file.

Does it matter whether the raw file starts life sparse?

> +    2)  Create an add-cow image which will store dirty bitmap
> +            qemu-img create -f add-cow test.add-cow \
> +                -o backing_file=ubuntu.img,image_file=test.raw
> +    3)  Run qemu with add-cow image
> +            qemu -drive if=virtio,file=test.add-cow

How does this interact with live snapshots/live disk mirroring?  Is this
something where I have to call 'block-stream' to pull data into the new
raw file on-demand?  I take it that test.add-cow is required until the
block-stream completes?  Is there a way, while qemu is still running,
but after the block-stream is complete, to reassociate the drive with
the actual raw file instead of the add-cow, or does the add-cow have to
remain around as long as the qemu process is still running?

> +
> +=Specification=
> +
> +The file format looks like this:
> +
> + +---------------+-------------+-----------------+
> + |     Header    |   Reserved  |    COW bitmap   |
> + +---------------+-------------+-----------------+
> +
> +All numbers in add-cow are stored in Little Endian byte order.

Okay, but different than network byte order, which means we can't use
htonl() and friends to convert host bytes into the proper format.
<endian.h> is not (yet) standardized, although there is talk of adding
it to the next version of POSIX, in which case htole32() and friends
would be guaranteed.

> +
> +== Header ==
> +
> +The Header is included in the first bytes:
> +
> +    Byte    0 -  7:     magic
> +                        add-cow magic string ("ADD_COW\xff")
> +
> +            8 -  11:    version
> +                        Version number (only valid value is 1 now)
> +
> +            12 - 15:    backing_filename_offset
> +                        Offset in the add-cow file at which the backing file name
> +                        is stored (NB: The string is not null terminated). 0 if the
> +                        image doesn't have a backing file.

Mention that if this is not 0, then it must be between 36 and 4094 (a
file name must be at least 1 byte).  What are the semantics if the
filename is relative?

> +
> +            16 - 19:    backing_filename_size
> +                        Length of the backing file name in bytes. Undefined if the
> +                        image doesn't have a backing file.

Better to require 0 if backing_filename_offset is 0, than to leave this
field undefined; also if backing_filename_offset is non-zero, then this
must be non-zero.  Must be less than 4096-36 to fit in the reserved part
of the header.

> +
> +            20 - 23:    image_filename_offset
> +                        Offset in the add-cow file at which the image_file name
> +                        is stored (NB: The string is not null terminated).

Mention that this must be between 36 and 4094 (a file name must be at
least 1 byte). What are the semantics if the filename is relative?

> +
> +            24 - 27:    image_filename_size
> +                        Length of the image_file name in bytes.

If backing_filename_offset is non-zero, then this must be non-zero.
Must be less than 4096-36 to fit in the reserved part of the header.

May image_filename and backing_filename overlap (possible if one is a
suffix of the other)?  Are there any constraints to prevent infinite
loops, such as forbidding backing_filename and image_filename from
resolving either to the same file or to the add-cow file?

> +
> +            28 - 35:    features
> +                        Currently only 2 feature bits are used:
> +                        Feature bits:
> +                            The image uses a backing file:
> +                            * ADD_COW_F_BACKING_FILE = 0x01.
> +                            The backing file's format is raw:
> +                            * ADD_COW_F_BACKING_FORMAT_NO_PROBE = 0x02.

Should this follow the qcow2v3 proposal of splitting into mandatory vs.
optional feature bits?

I agree that ADD_COW_F_BACKING_FORMAT_NO_PROBE is sufficient to avoid
security implications, but do we want the extra flexibility of
specifying the backing format file format rather than just requiring
probes on all but raw?

> +
> +== Reserved ==
> +
> +    Byte    36 - 4095:  Reserved field:
> +                        It is used to make sure COW bitmap field starts at the
> +                        4096th byte, backing_file name and image_file name will
> +                        be stored here.

Do we want to keep a fixed-size header, or should we be planning on the
possibility of future extensions requiring enough other header
extensions that a variable-sized header would be wiser?  That is, I'm
fine with requiring that the header be a multiple of 4k, but maybe it
would make sense to have a mandatory header field that states how many
header pages are present before the COW bitmap begins.  In the first
round of implementation, this header field can be required to be 1 (that
is, for now, we require exactly 4k header), but having the field would
let us change in the future to a design with an 8k header to hold more
metadata as needed.

> +
> +== COW bitmap ==
> +
> +The "COW bitmap" field starts at the 4096th byte, stores a bitmap related to
> +backing_file and image_file. The bitmap will track whether the sector in
> +backing_file is dirty or not.
> +
> +Each bit in the bitmap indicates one cluster's status. One cluster includes 128
> +sectors, then each bit indicates 512 * 128 = 64k bytes, So the size of bitmap is
> +calculated according to virtual size of image_file. In each byte, bit 0 to 7
> +will track the 1st to 7th cluster in sequence, bit orders in one byte look like:
> + +----+----+----+----+----+----+----+----+
> + | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 |
> + +----+----+----+----+----+----+----+----+
> +
> +If the bit is 0, indicates the sector has not been allocated in image_file, data
> +should be loaded from backing_file while reading; if the bit is 1,  indicates the
> +related sector has been dirty, should be loaded from image_file while reading.
> +Writing to a sector causes the corresponding bit to be set to 1.

So basically an add-cow image is thin as long as at least one bit is 0,
and the add-cow wrapper can only be discarded when all bits are 1.

How do you handle the case where the raw image is not an even multiple
of cluster bytes?  That is, do bits that correspond to bytes beyond the
raw file size have to be in a certain state?
Robert Wang - June 14, 2012, 3:06 a.m.
On Wed, Jun 13, 2012 at 11:10 PM, Eric Blake <eblake@redhat.com> wrote:
> On 06/13/2012 08:36 AM, Dong Xu Wang wrote:
>> Introduce a new file format:add-cow. The usage can be found at this patch.
>>
>> Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
>> ---
>>  docs/specs/add-cow.txt |   87 ++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 files changed, 87 insertions(+), 0 deletions(-)
>>  create mode 100644 docs/specs/add-cow.txt
>>
>> diff --git a/docs/specs/add-cow.txt b/docs/specs/add-cow.txt
>> new file mode 100644
>> index 0000000..e077fc2
>> --- /dev/null
>> +++ b/docs/specs/add-cow.txt
>> @@ -0,0 +1,87 @@
>> +== General ==
>> +
>> +Raw file format does not support backing_file and copy on write feature.
>> +The add-cow image format makes it possible to use backing files with raw
>> +image by keeping a separate .add-cow metadata file.  Once all sectors
>> +have been written into the raw image it is safe to discard the .add-cow
>> +and backing files, then we can use the raw image directly.
>> +
>> +While using add-cow, procedures may like this:
>> +(ubuntu.img is a disk image which has been installed OS.)
>> +    1)  Create a raw image with the same size of ubuntu.img
>> +            qemu-img create -f raw test.raw 8G
>
> Make sure we also support a raw file larger than the backing file.
>

Okay, in this version, I just truncate raw file with the same size of
backing_file,
I will correct this in v11.

> Does it matter whether the raw file starts life sparse?

No, it can work with sparse raw file.

>
>> +    2)  Create an add-cow image which will store dirty bitmap
>> +            qemu-img create -f add-cow test.add-cow \
>> +                -o backing_file=ubuntu.img,image_file=test.raw
>> +    3)  Run qemu with add-cow image
>> +            qemu -drive if=virtio,file=test.add-cow
>
> How does this interact with live snapshots/live disk mirroring?  Is this
> something where I have to call 'block-stream' to pull data into the new
> raw file on-demand?  I take it that test.add-cow is required until the
> block-stream completes?  Is there a way, while qemu is still running,
> but after the block-stream is complete, to reassociate the drive with
> the actual raw file instead of the add-cow, or does the add-cow have to
> remain around as long as the qemu process is still running?
>

I did not touch snapshots/live disk mirroring code much, so have to call
"block_stream".

Now, add-cow has to remain until qemu process is still running, I did not
implement a way to use raw file directly after the block-stream is complete.

>> +
>> +=Specification=
>> +
>> +The file format looks like this:
>> +
>> + +---------------+-------------+-----------------+
>> + |     Header    |   Reserved  |    COW bitmap   |
>> + +---------------+-------------+-----------------+
>> +
>> +All numbers in add-cow are stored in Little Endian byte order.
>
> Okay, but different than network byte order, which means we can't use
> htonl() and friends to convert host bytes into the proper format.
> <endian.h> is not (yet) standardized, although there is talk of adding
> it to the next version of POSIX, in which case htole32() and friends
> would be guaranteed.
>
>> +
>> +== Header ==
>> +
>> +The Header is included in the first bytes:
>> +
>> +    Byte    0 -  7:     magic
>> +                        add-cow magic string ("ADD_COW\xff")
>> +
>> +            8 -  11:    version
>> +                        Version number (only valid value is 1 now)
>> +
>> +            12 - 15:    backing_filename_offset
>> +                        Offset in the add-cow file at which the backing file name
>> +                        is stored (NB: The string is not null terminated). 0 if the
>> +                        image doesn't have a backing file.
>
> Mention that if this is not 0, then it must be between 36 and 4094 (a
> file name must be at least 1 byte).  What are the semantics if the
> filename is relative?

relative filename is ok, I tested it just now.

>
>> +
>> +            16 - 19:    backing_filename_size
>> +                        Length of the backing file name in bytes. Undefined if the
>> +                        image doesn't have a backing file.
>
> Better to require 0 if backing_filename_offset is 0, than to leave this
> field undefined; also if backing_filename_offset is non-zero, then this
> must be non-zero.  Must be less than 4096-36 to fit in the reserved part
> of the header.
>

Okay.

>> +
>> +            20 - 23:    image_filename_offset
>> +                        Offset in the add-cow file at which the image_file name
>> +                        is stored (NB: The string is not null terminated).
>
> Mention that this must be between 36 and 4094 (a file name must be at
> least 1 byte). What are the semantics if the filename is relative?

relative filename is ok, I tested it just now.

>
>> +
>> +            24 - 27:    image_filename_size
>> +                        Length of the image_file name in bytes.
>
> If backing_filename_offset is non-zero, then this must be non-zero.
> Must be less than 4096-36 to fit in the reserved part of the header.
>

Yes,

> May image_filename and backing_filename overlap (possible if one is a
> suffix of the other)?  Are there any constraints to prevent infinite
> loops, such as forbidding backing_filename and image_filename from
> resolving either to the same file or to the add-cow file?
>

Sorry, I should add the code that judge if image_file is valid. will fix.

>> +
>> +            28 - 35:    features
>> +                        Currently only 2 feature bits are used:
>> +                        Feature bits:
>> +                            The image uses a backing file:
>> +                            * ADD_COW_F_BACKING_FILE = 0x01.
>> +                            The backing file's format is raw:
>> +                            * ADD_COW_F_BACKING_FORMAT_NO_PROBE = 0x02.
>
> Should this follow the qcow2v3 proposal of splitting into mandatory vs.
> optional feature bits?
>
> I agree that ADD_COW_F_BACKING_FORMAT_NO_PROBE is sufficient to avoid
> security implications, but do we want the extra flexibility of
> specifying the backing format file format rather than just requiring
> probes on all but raw?

Kevin, or Stefan, can you give some comments for this? thanks.

>
>> +
>> +== Reserved ==
>> +
>> +    Byte    36 - 4095:  Reserved field:
>> +                        It is used to make sure COW bitmap field starts at the
>> +                        4096th byte, backing_file name and image_file name will
>> +                        be stored here.
>
> Do we want to keep a fixed-size header, or should we be planning on the
> possibility of future extensions requiring enough other header
> extensions that a variable-sized header would be wiser?  That is, I'm
> fine with requiring that the header be a multiple of 4k, but maybe it
> would make sense to have a mandatory header field that states how many
> header pages are present before the COW bitmap begins.  In the first
> round of implementation, this header field can be required to be 1 (that
> is, for now, we require exactly 4k header), but having the field would
> let us change in the future to a design with an 8k header to hold more
> metadata as needed.
>

Okay.

>> +
>> +== COW bitmap ==
>> +
>> +The "COW bitmap" field starts at the 4096th byte, stores a bitmap related to
>> +backing_file and image_file. The bitmap will track whether the sector in
>> +backing_file is dirty or not.
>> +
>> +Each bit in the bitmap indicates one cluster's status. One cluster includes 128
>> +sectors, then each bit indicates 512 * 128 = 64k bytes, So the size of bitmap is
>> +calculated according to virtual size of image_file. In each byte, bit 0 to 7
>> +will track the 1st to 7th cluster in sequence, bit orders in one byte look like:
>> + +----+----+----+----+----+----+----+----+
>> + | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 |
>> + +----+----+----+----+----+----+----+----+
>> +
>> +If the bit is 0, indicates the sector has not been allocated in image_file, data
>> +should be loaded from backing_file while reading; if the bit is 1,  indicates the
>> +related sector has been dirty, should be loaded from image_file while reading.
>> +Writing to a sector causes the corresponding bit to be set to 1.
>
> So basically an add-cow image is thin as long as at least one bit is 0,
> and the add-cow wrapper can only be discarded when all bits are 1.
>
> How do you handle the case where the raw image is not an even multiple
> of cluster bytes?  That is, do bits that correspond to bytes beyond the
> raw file size have to be in a certain state?
>

Now, bits correspond to bytes beyond the raw file size are set to 0.

Really thanks for your reviewing, Eric.

> --
> Eric Blake   eblake@redhat.com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>
Kevin Wolf - June 14, 2012, 10:47 a.m.
Am 14.06.2012 05:06, schrieb Dong Xu Wang:
> On Wed, Jun 13, 2012 at 11:10 PM, Eric Blake <eblake@redhat.com> wrote:
>> On 06/13/2012 08:36 AM, Dong Xu Wang wrote:
>>> Introduce a new file format:add-cow. The usage can be found at this patch.
>>>
>>> Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
>>> ---
>>>  docs/specs/add-cow.txt |   87 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>  1 files changed, 87 insertions(+), 0 deletions(-)
>>>  create mode 100644 docs/specs/add-cow.txt

>>> +
>>> +== Header ==
>>> +
>>> +The Header is included in the first bytes:
>>> +
>>> +    Byte    0 -  7:     magic
>>> +                        add-cow magic string ("ADD_COW\xff")
>>> +
>>> +            8 -  11:    version
>>> +                        Version number (only valid value is 1 now)
>>> +
>>> +            12 - 15:    backing_filename_offset
>>> +                        Offset in the add-cow file at which the backing file name
>>> +                        is stored (NB: The string is not null terminated). 0 if the
>>> +                        image doesn't have a backing file.
>>
>> Mention that if this is not 0, then it must be between 36 and 4094 (a
>> file name must be at least 1 byte).  What are the semantics if the
>> filename is relative?
> 
> relative filename is ok, I tested it just now.

I believe Eric wanted to know what a relative path means, i.e. that it's
relative to the image file rather than relative to the working directory.

>>> +
>>> +            16 - 19:    backing_filename_size
>>> +                        Length of the backing file name in bytes. Undefined if the
>>> +                        image doesn't have a backing file.
>>
>> Better to require 0 if backing_filename_offset is 0, than to leave this
>> field undefined; also if backing_filename_offset is non-zero, then this
>> must be non-zero.  Must be less than 4096-36 to fit in the reserved part
>> of the header.
>>
> 
> Okay.

Does an add-cow image without a backing file even make sense?

>>> +            28 - 35:    features
>>> +                        Currently only 2 feature bits are used:
>>> +                        Feature bits:
>>> +                            The image uses a backing file:
>>> +                            * ADD_COW_F_BACKING_FILE = 0x01.
>>> +                            The backing file's format is raw:
>>> +                            * ADD_COW_F_BACKING_FORMAT_NO_PROBE = 0x02.
>>
>> Should this follow the qcow2v3 proposal of splitting into mandatory vs.
>> optional feature bits?
>>
>> I agree that ADD_COW_F_BACKING_FORMAT_NO_PROBE is sufficient to avoid
>> security implications, but do we want the extra flexibility of
>> specifying the backing format file format rather than just requiring
>> probes on all but raw?
> 
> Kevin, or Stefan, can you give some comments for this? thanks.

I tend to agree that a format name is better than relying on probing.

Also, I think we need the same thing for image_file. add-cow is not only
useful for raw images, but also for other image format types for which
we don't support backing files.

>>> +== Reserved ==
>>> +
>>> +    Byte    36 - 4095:  Reserved field:
>>> +                        It is used to make sure COW bitmap field starts at the
>>> +                        4096th byte, backing_file name and image_file name will
>>> +                        be stored here.
>>
>> Do we want to keep a fixed-size header, or should we be planning on the
>> possibility of future extensions requiring enough other header
>> extensions that a variable-sized header would be wiser?  That is, I'm
>> fine with requiring that the header be a multiple of 4k, but maybe it
>> would make sense to have a mandatory header field that states how many
>> header pages are present before the COW bitmap begins.  In the first
>> round of implementation, this header field can be required to be 1 (that
>> is, for now, we require exactly 4k header), but having the field would
>> let us change in the future to a design with an 8k header to hold more
>> metadata as needed.

I have the impression that this simple add-cow hack is starting to get
seriously overengineered... :-)

Kevin
Robert Wang - June 18, 2012, 2:08 a.m.
On Thu, Jun 14, 2012 at 6:47 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 14.06.2012 05:06, schrieb Dong Xu Wang:
>> On Wed, Jun 13, 2012 at 11:10 PM, Eric Blake <eblake@redhat.com> wrote:
>>> On 06/13/2012 08:36 AM, Dong Xu Wang wrote:
>>>> Introduce a new file format:add-cow. The usage can be found at this patch.
>>>>
>>>> Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
>>>> ---
>>>>  docs/specs/add-cow.txt |   87 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>>  1 files changed, 87 insertions(+), 0 deletions(-)
>>>>  create mode 100644 docs/specs/add-cow.txt
>
>>>> +
>>>> +== Header ==
>>>> +
>>>> +The Header is included in the first bytes:
>>>> +
>>>> +    Byte    0 -  7:     magic
>>>> +                        add-cow magic string ("ADD_COW\xff")
>>>> +
>>>> +            8 -  11:    version
>>>> +                        Version number (only valid value is 1 now)
>>>> +
>>>> +            12 - 15:    backing_filename_offset
>>>> +                        Offset in the add-cow file at which the backing file name
>>>> +                        is stored (NB: The string is not null terminated). 0 if the
>>>> +                        image doesn't have a backing file.
>>>
>>> Mention that if this is not 0, then it must be between 36 and 4094 (a
>>> file name must be at least 1 byte).  What are the semantics if the
>>> filename is relative?
>>
>> relative filename is ok, I tested it just now.
>
> I believe Eric wanted to know what a relative path means, i.e. that it's
> relative to the image file rather than relative to the working directory.
>
Now it is relative to working directory.
>>>> +
>>>> +            16 - 19:    backing_filename_size
>>>> +                        Length of the backing file name in bytes. Undefined if the
>>>> +                        image doesn't have a backing file.
>>>
>>> Better to require 0 if backing_filename_offset is 0, than to leave this
>>> field undefined; also if backing_filename_offset is non-zero, then this
>>> must be non-zero.  Must be less than 4096-36 to fit in the reserved part
>>> of the header.
>>>
>>
>> Okay.
>
> Does an add-cow image without a backing file even make sense?
>

Okay, I think so, it will be v11.

>>>> +            28 - 35:    features
>>>> +                        Currently only 2 feature bits are used:
>>>> +                        Feature bits:
>>>> +                            The image uses a backing file:
>>>> +                            * ADD_COW_F_BACKING_FILE = 0x01.
>>>> +                            The backing file's format is raw:
>>>> +                            * ADD_COW_F_BACKING_FORMAT_NO_PROBE = 0x02.
>>>
>>> Should this follow the qcow2v3 proposal of splitting into mandatory vs.
>>> optional feature bits?
>>>
>>> I agree that ADD_COW_F_BACKING_FORMAT_NO_PROBE is sufficient to avoid
>>> security implications, but do we want the extra flexibility of
>>> specifying the backing format file format rather than just requiring
>>> probes on all but raw?
>>
>> Kevin, or Stefan, can you give some comments for this? thanks.
>
> I tend to agree that a format name is better than relying on probing.
>
> Also, I think we need the same thing for image_file. add-cow is not only
> useful for raw images, but also for other image format types for which
> we don't support backing files.
>

Okay.

>>>> +== Reserved ==
>>>> +
>>>> +    Byte    36 - 4095:  Reserved field:
>>>> +                        It is used to make sure COW bitmap field starts at the
>>>> +                        4096th byte, backing_file name and image_file name will
>>>> +                        be stored here.
>>>
>>> Do we want to keep a fixed-size header, or should we be planning on the
>>> possibility of future extensions requiring enough other header
>>> extensions that a variable-sized header would be wiser?  That is, I'm
>>> fine with requiring that the header be a multiple of 4k, but maybe it
>>> would make sense to have a mandatory header field that states how many
>>> header pages are present before the COW bitmap begins.  In the first
>>> round of implementation, this header field can be required to be 1 (that
>>> is, for now, we require exactly 4k header), but having the field would
>>> let us change in the future to a design with an 8k header to hold more
>>> metadata as needed.
>
> I have the impression that this simple add-cow hack is starting to get
> seriously overengineered... :-)
>
> Kevin
>
Eric Blake - June 18, 2012, 3:33 p.m.
On 06/17/2012 08:08 PM, Dong Xu Wang wrote:

>>>> Mention that if this is not 0, then it must be between 36 and 4094 (a
>>>> file name must be at least 1 byte).  What are the semantics if the
>>>> filename is relative?
>>>
>>> relative filename is ok, I tested it just now.
>>
>> I believe Eric wanted to know what a relative path means, i.e. that it's
>> relative to the image file rather than relative to the working directory.
>>
> Now it is relative to working directory.

Yuck.  So I have to change directories before opening an add-cow image
in order to interpret it the same way as the creator?  You really need
to fix this to be like qcow2 backing files; relative path names there
are interpreted relative to the qcow2 file, not to the current working
directory of the process that opened the qcow2 file.

Patch

diff --git a/docs/specs/add-cow.txt b/docs/specs/add-cow.txt
new file mode 100644
index 0000000..e077fc2
--- /dev/null
+++ b/docs/specs/add-cow.txt
@@ -0,0 +1,87 @@ 
+== General ==
+
+Raw file format does not support backing_file and copy on write feature.
+The add-cow image format makes it possible to use backing files with raw
+image by keeping a separate .add-cow metadata file.  Once all sectors
+have been written into the raw image it is safe to discard the .add-cow
+and backing files, then we can use the raw image directly.
+
+While using add-cow, procedures may like this:
+(ubuntu.img is a disk image which has been installed OS.)
+    1)  Create a raw image with the same size of ubuntu.img
+            qemu-img create -f raw test.raw 8G
+    2)  Create an add-cow image which will store dirty bitmap
+            qemu-img create -f add-cow test.add-cow \
+                -o backing_file=ubuntu.img,image_file=test.raw
+    3)  Run qemu with add-cow image
+            qemu -drive if=virtio,file=test.add-cow
+
+=Specification=
+
+The file format looks like this:
+
+ +---------------+-------------+-----------------+
+ |     Header    |   Reserved  |    COW bitmap   |
+ +---------------+-------------+-----------------+
+
+All numbers in add-cow are stored in Little Endian byte order.
+
+== Header ==
+
+The Header is included in the first bytes:
+
+    Byte    0 -  7:     magic
+                        add-cow magic string ("ADD_COW\xff")
+
+            8 -  11:    version
+                        Version number (only valid value is 1 now)
+
+            12 - 15:    backing_filename_offset
+                        Offset in the add-cow file at which the backing file name
+                        is stored (NB: The string is not null terminated). 0 if the
+                        image doesn't have a backing file.
+
+            16 - 19:    backing_filename_size
+                        Length of the backing file name in bytes. Undefined if the
+                        image doesn't have a backing file.
+
+            20 - 23:    image_filename_offset
+                        Offset in the add-cow file at which the image_file name
+                        is stored (NB: The string is not null terminated).
+
+            24 - 27:    image_filename_size
+                        Length of the image_file name in bytes.
+
+            28 - 35:    features
+                        Currently only 2 feature bits are used:
+                        Feature bits:
+                            The image uses a backing file:
+                            * ADD_COW_F_BACKING_FILE = 0x01.
+                            The backing file's format is raw:
+                            * ADD_COW_F_BACKING_FORMAT_NO_PROBE = 0x02.
+
+== Reserved ==
+
+    Byte    36 - 4095:  Reserved field:
+                        It is used to make sure COW bitmap field starts at the
+                        4096th byte, backing_file name and image_file name will
+                        be stored here.
+
+== COW bitmap ==
+
+The "COW bitmap" field starts at the 4096th byte, stores a bitmap related to
+backing_file and image_file. The bitmap will track whether the sector in
+backing_file is dirty or not.
+
+Each bit in the bitmap indicates one cluster's status. One cluster includes 128
+sectors, then each bit indicates 512 * 128 = 64k bytes, So the size of bitmap is
+calculated according to virtual size of image_file. In each byte, bit 0 to 7
+will track the 1st to 7th cluster in sequence, bit orders in one byte look like:
+ +----+----+----+----+----+----+----+----+
+ | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 |
+ +----+----+----+----+----+----+----+----+
+
+If the bit is 0, indicates the sector has not been allocated in image_file, data
+should be loaded from backing_file while reading; if the bit is 1,  indicates the
+related sector has been dirty, should be loaded from image_file while reading.
+Writing to a sector causes the corresponding bit to be set to 1.