On Tue, Jul 31, 2012 at 5:51 PM, Dong Xu Wang <wdon...@linux.vnet.ibm.com> wrote: > Introduce a new file format:add-cow. The usage can be found at this patch. > > Signed-off-by: Dong Xu Wang <wdon...@linux.vnet.ibm.com> > --- > Now add-cow is still using QEMUOptionParameter, not QemuOpts, I will send a > seperate patch series to convert.
I suggest including a cover letter in future patch series: git format-patch --cover-letter --numbered -o my-series/ master.. Or to do it in a single command with git-send-email(1) use the --compose option. The cover letter makes it easy for reviewers to add their Reviewed-by: to the entire series by replying only once. It's also a good place to include a changelog that tells reviewers what you changed from the last published version. > docs/specs/add-cow.txt | 128 > ++++++++++++++++++++++++++++++++++++++++++++++++ > 1 files changed, 128 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..4793a3e > --- /dev/null > +++ b/docs/specs/add-cow.txt > @@ -0,0 +1,128 @@ > +== 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 > + > +test.raw may be larger than ubuntu.img, in that case, the size of > test.add-cow > +will be calculated by the size of ubuntu.img, test.raw will be used from the > +1st byte, the rest part can be used for other purpose. This is not how backing files normally work. With qcow2 or qed a smaller backing file just means that the guest reads zeroes from the areas beyond the end of the backing file. Is there a special reason why you want to implement the behavior you described in the spec? Otherwise I suggest implementing the same behavior as qcow2/qed. > + > +=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: > +(#define HEADER_SIZE (4096 * header_pages_size)) > + 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 file name offset > + Offset in the add-cow file at which the backing file > + name is stored (NB: The string is not null > terminated). > + If backing file name does NOT exist, this field will > be > + 0. Must be between 80 and [HEADER_SIZE - 2](a file > name > + must be at least 1 byte). > + > + 16 - 19: backing file name size > + Length of the backing file name in bytes. It will be > 0 > + if the backing file name offset is 0. If backing file > + name offset is non-zero, then it must be non-zero. > Must > + be less than [HEADER_SIZE - 80] to fit in the > reserved > + part of the header. > + > + 20 - 23: image file name offset > + Offset in the add-cow file at which the image file > name > + is stored (NB: The string is not null terminated). It > + must be between 80 and [HEADER_SIZE - 2]. > + > + 24 - 27: image file name size > + Length of the image file name in bytes. > + Must be less than [HEADER_SIZE - 80] to fit in the > reserved > + part of the header. > + > + 28 - 35: features > + Currently only 3 feature bit is used: > + Feature bits: > + The image uses a backing file: > + * ADD_COW_F_BACKING_FILE = 0x01. > + The image uses a image file: > + * ADD_COW_F_IMAGE_FILE = 0x02. > + All bits in bitmap have been set to 1, add-cow > wrapper > + can be discarded. > + * ADD_COW_F_All_ALLOCATED = 0x04. > + > + 36 - 43: optional features > + Not used now. Researved for future use. s/Researved/Reserved/ > + > + 44 - 47: header pages size > + The header field is variable-sized. This field > indicates > + how many pages(4k) will be used to store add-cow > header. > + In add-cow v1, it is fixed to 1, so the header size > will > + be 4k * 1 = 4096 bytes. > + > +Image file name and backing file name must NOT be the same, we prevent this > +while creating add-cow files. > + > +Image file and backing file are interpreted relative to the qcow2 file, not > +to the current working directory of the process that opened the qcow2 file. > + > +== Reserved == > + > + Byte 48 - 63: backing file format > + format of backing file. It will be filled with 0 if > + backing file name offset is 0. If backing file name > + offset is none-zero, it must be non-zero. s/none-zero/non-zero/ > + > + 64 - 79: image file format > + format of image file. It must be non-zero. > + > + 80 - [HEADER_SIZE - 1]: > + It is used to make sure COW bitmap field starts at > the > + HEADER_SIZE 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 I would say it starts at offset HEADER_SIZE. 4096th byte == offset 0x0fff, 4097th byte == offset 0x1000. Stefan