Message ID | 1354776711-12449-2-git-send-email-wdongxu@linux.vnet.ibm.com |
---|---|
State | New |
Headers | show |
Am 06.12.2012 07:51, schrieb Dong Xu Wang: > Document for add-cow format, the usage and spec of add-cow are introduced. > > Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com> > --- > docs/specs/add-cow.txt | 154 ++++++++++++++++++++++++++++++++++++++++++++++++ > 1 files changed, 154 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..24e9a11 > --- /dev/null > +++ b/docs/specs/add-cow.txt > @@ -0,0 +1,154 @@ > +== General == > + > +The raw file format does not support backing files or copy on write feature. > +The add-cow image format makes it possible to use backing files with a 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. > + > +An example usage of add-cow would look like:: Double colon. > +(ubuntu.img is a disk image which has an 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 from the size of test.raw. > + > +image_fmt can be omitted, in that case image_fmt should be set as "raw". By "should be set as" you mean "is assumed to be"? > +backing_fmt can also be omitted, add-cow should do a probe operation and determine This line takes more than 80 characters. More follow, I won't comment on each. > +what the backing file's format is. > + > +=Specification= > + > +The file format looks like this: > + > + +---------------+-------------------------------+ > + | Header | COW bitmap | > + +---------------+-------------------------------+ > + > +All numbers in add-cow are stored in Little Endian byte order. > + > +== Header == > + > +The Header is included in the first bytes: > +(HEADER_SIZE is defined in 44-47 bytes.) > + Byte 0 - 3: magic > + add-cow magic string ("ACOW"). > + > + 4 - 7: version > + Version number (only valid value is 1 now). > + > + 8 - 11: backing file name offset > + Offset in the add-cow file at which the backing file > + name is stored (NB: The string is not NUL-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). > + > + 12 - 15: 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. Backing file name offset + size > + must be no more than HEADER_SIZE. > + > + 16 - 19: image file name offset > + Offset in the add-cow file at which the image file name > + is stored (NB: The string is not NUL-terminated). It > + must be between 80 and [HEADER_SIZE - 2]. Image file > + name size + offset must be no more than HEADER_SIZE. > + > + 20 - 23: 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. > + > + 24 - 27: cluster bits > + Number of bits that are used for addressing an offset > + within a cluster (1 << cluster_bits is the cluster size). > + Must not be less than 9 (i.e. 512 byte clusters). > + > + Note: qemu as of today has an implementation limit of 2 MB > + as the maximum cluster size and won't be able to open images > + with larger cluster sizes. > + > + 28 - 35: features > + Bitmask of features. If a feature bit is set but not recognized, > + the add-cow file should be dropped. They are not used in v1. Does v1 mean header.version = 1? I think this is wrong, we will want to add incompatible feature flags without increasing header.version (that's the whole point of them) > + > + Bits 0-63: Reserved (set to 0) > + > + 36 - 43: compatible features > + Bitmask of compatible features. An implementation can > + safely ignore any unknown bits that are set. > + Bit 0: All allocated bit. If this bit is set then > + backing file and COW bitmap will not be used, > + and can read from or write to image file directly. > + > + Bits 1-63: Reserved (set to 0) > + > + 44 - 47: HEADER_SIZE > + The header field is variable-sized. This field indicates > + how many bytes will be used to store add-cow header. > + In add-cow v1, it is fixed to 4096. Same question about v1. If it's fixed, why have a field for it? > + > + 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 non-empty, it must be non-empty. It is coded > + in free-form ASCII, and is not NUL-terminated. Zero > + padded on the right. > + > + 64 - 79: image file format > + Format of image file. It must be non-empty. It is coded > + in free-form ASCII, and is not NUL-terminated. Zero > + padded on the right. > + > + 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. The bytes that are not pointing to > + backing file and image file names must be set to 0. > + > +== COW bitmap == > + > +The "COW bitmap" field starts at offset HEADER_SIZE, stores a bitmap related to > +backing file and image file. It is tracking whether the sector in image file > +is allocated or not. > + > +Each bit in the bitmap tracks one cluster's status. For example, if cluster > +bit is 16, then each bit tracks one cluster, (1 << 16) = 65536 bytes. The clusters bit_s_ > +image file size is rounded up to cluster size (where any bytes in the > +last cluster that do not fit in the image are ignored), then if the > +number of clusters is not a multiple of 8, then remaining bits in the > +bitmap will be set to 0. > + > +The size of bitmap is calculated according to virtual size of image file, and > +the size of bitmap should be multiple of add-cow file's cluster size, the bits > +not used will be set to 0. Within each byte, the least significant bit covers > +the first cluster. Bit orders in one byte look like: > + +----+----+----+----+----+----+----+----+ > + | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | > + +----+----+----+----+----+----+----+----+ > + > +If the bit is 0, it indicates the sector has not been allocated in image file, s/sector/cluster/ More instances follow, not commenting on each. > +data should be loaded from backing file while reading; if the bit is 1, it > +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. > +If there is no backing file, or if the image file is larger than the backing > +file and the offset is beyond the end of the backing file, then the data should > +be read as all zero bytes instead. > + > +If raw image is not an even multiple of cluster bytes, bits that correspond to > +bytes beyond the raw file size in add-cow must be written as 0 and must be > +ignored when reading. Don't refer to a "raw image", it could be any image format. > + > +Image file name and backing file name must NOT be the same, we prevent this > +while creating add-cow files via qemu-img. If image file name and backing file > +name are the same, the add-cow image must be treated as invalid. Kevin
On Mon, Dec 10, 2012 at 11:39 PM, Kevin Wolf <kwolf@redhat.com> wrote: > Am 06.12.2012 07:51, schrieb Dong Xu Wang: >> Document for add-cow format, the usage and spec of add-cow are introduced. >> >> Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com> >> --- >> docs/specs/add-cow.txt | 154 ++++++++++++++++++++++++++++++++++++++++++++++++ >> 1 files changed, 154 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..24e9a11 >> --- /dev/null >> +++ b/docs/specs/add-cow.txt >> @@ -0,0 +1,154 @@ >> +== General == >> + >> +The raw file format does not support backing files or copy on write feature. >> +The add-cow image format makes it possible to use backing files with a 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. >> + >> +An example usage of add-cow would look like:: > > Double colon. Okay. > >> +(ubuntu.img is a disk image which has an 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 from the size of test.raw. >> + >> +image_fmt can be omitted, in that case image_fmt should be set as "raw". > > By "should be set as" you mean "is assumed to be"? > Okay. Will fix. >> +backing_fmt can also be omitted, add-cow should do a probe operation and determine > > This line takes more than 80 characters. More follow, I won't comment on > each. > Okay, will fix. >> +what the backing file's format is. >> + >> +=Specification= >> + >> +The file format looks like this: >> + >> + +---------------+-------------------------------+ >> + | Header | COW bitmap | >> + +---------------+-------------------------------+ >> + >> +All numbers in add-cow are stored in Little Endian byte order. >> + >> +== Header == >> + >> +The Header is included in the first bytes: >> +(HEADER_SIZE is defined in 44-47 bytes.) >> + Byte 0 - 3: magic >> + add-cow magic string ("ACOW"). >> + >> + 4 - 7: version >> + Version number (only valid value is 1 now). >> + >> + 8 - 11: backing file name offset >> + Offset in the add-cow file at which the backing file >> + name is stored (NB: The string is not NUL-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). >> + >> + 12 - 15: 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. Backing file name offset + size >> + must be no more than HEADER_SIZE. >> + >> + 16 - 19: image file name offset >> + Offset in the add-cow file at which the image file name >> + is stored (NB: The string is not NUL-terminated). It >> + must be between 80 and [HEADER_SIZE - 2]. Image file >> + name size + offset must be no more than HEADER_SIZE. >> + >> + 20 - 23: 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. >> + >> + 24 - 27: cluster bits >> + Number of bits that are used for addressing an offset >> + within a cluster (1 << cluster_bits is the cluster size). >> + Must not be less than 9 (i.e. 512 byte clusters). >> + >> + Note: qemu as of today has an implementation limit of 2 MB >> + as the maximum cluster size and won't be able to open images >> + with larger cluster sizes. >> + >> + 28 - 35: features >> + Bitmask of features. If a feature bit is set but not recognized, >> + the add-cow file should be dropped. They are not used in v1. > > Does v1 mean header.version = 1? I think this is wrong, we will want to > add incompatible feature flags without increasing header.version (that's > the whole point of them) Okay, will fix. > >> + >> + Bits 0-63: Reserved (set to 0) >> + >> + 36 - 43: compatible features >> + Bitmask of compatible features. An implementation can >> + safely ignore any unknown bits that are set. >> + Bit 0: All allocated bit. If this bit is set then >> + backing file and COW bitmap will not be used, >> + and can read from or write to image file directly. >> + >> + Bits 1-63: Reserved (set to 0) >> + >> + 44 - 47: HEADER_SIZE >> + The header field is variable-sized. This field indicates >> + how many bytes will be used to store add-cow header. >> + In add-cow v1, it is fixed to 4096. > > Same question about v1. If it's fixed, why have a field for it? Okay, will make more clear in next version. > >> + >> + 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 non-empty, it must be non-empty. It is coded >> + in free-form ASCII, and is not NUL-terminated. Zero >> + padded on the right. >> + >> + 64 - 79: image file format >> + Format of image file. It must be non-empty. It is coded >> + in free-form ASCII, and is not NUL-terminated. Zero >> + padded on the right. >> + >> + 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. The bytes that are not pointing to >> + backing file and image file names must be set to 0. >> + >> +== COW bitmap == >> + >> +The "COW bitmap" field starts at offset HEADER_SIZE, stores a bitmap related to >> +backing file and image file. It is tracking whether the sector in image file >> +is allocated or not. >> + >> +Each bit in the bitmap tracks one cluster's status. For example, if cluster >> +bit is 16, then each bit tracks one cluster, (1 << 16) = 65536 bytes. The > > clusters bit_s_ > Okay. "cluster_bits is". >> +image file size is rounded up to cluster size (where any bytes in the >> +last cluster that do not fit in the image are ignored), then if the >> +number of clusters is not a multiple of 8, then remaining bits in the >> +bitmap will be set to 0. >> + >> +The size of bitmap is calculated according to virtual size of image file, and >> +the size of bitmap should be multiple of add-cow file's cluster size, the bits >> +not used will be set to 0. Within each byte, the least significant bit covers >> +the first cluster. Bit orders in one byte look like: >> + +----+----+----+----+----+----+----+----+ >> + | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | >> + +----+----+----+----+----+----+----+----+ >> + >> +If the bit is 0, it indicates the sector has not been allocated in image file, > > s/sector/cluster/ > > More instances follow, not commenting on each. > Okay. >> +data should be loaded from backing file while reading; if the bit is 1, it >> +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. >> +If there is no backing file, or if the image file is larger than the backing >> +file and the offset is beyond the end of the backing file, then the data should >> +be read as all zero bytes instead. >> + >> +If raw image is not an even multiple of cluster bytes, bits that correspond to >> +bytes beyond the raw file size in add-cow must be written as 0 and must be >> +ignored when reading. > > Don't refer to a "raw image", it could be any image format. > Okay, will fix. >> + >> +Image file name and backing file name must NOT be the same, we prevent this >> +while creating add-cow files via qemu-img. If image file name and backing file >> +name are the same, the add-cow image must be treated as invalid. > > Kevin >
diff --git a/docs/specs/add-cow.txt b/docs/specs/add-cow.txt new file mode 100644 index 0000000..24e9a11 --- /dev/null +++ b/docs/specs/add-cow.txt @@ -0,0 +1,154 @@ +== General == + +The raw file format does not support backing files or copy on write feature. +The add-cow image format makes it possible to use backing files with a 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. + +An example usage of add-cow would look like:: +(ubuntu.img is a disk image which has an 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 from the size of test.raw. + +image_fmt can be omitted, in that case image_fmt should be set as "raw". +backing_fmt can also be omitted, add-cow should do a probe operation and determine +what the backing file's format is. + +=Specification= + +The file format looks like this: + + +---------------+-------------------------------+ + | Header | COW bitmap | + +---------------+-------------------------------+ + +All numbers in add-cow are stored in Little Endian byte order. + +== Header == + +The Header is included in the first bytes: +(HEADER_SIZE is defined in 44-47 bytes.) + Byte 0 - 3: magic + add-cow magic string ("ACOW"). + + 4 - 7: version + Version number (only valid value is 1 now). + + 8 - 11: backing file name offset + Offset in the add-cow file at which the backing file + name is stored (NB: The string is not NUL-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). + + 12 - 15: 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. Backing file name offset + size + must be no more than HEADER_SIZE. + + 16 - 19: image file name offset + Offset in the add-cow file at which the image file name + is stored (NB: The string is not NUL-terminated). It + must be between 80 and [HEADER_SIZE - 2]. Image file + name size + offset must be no more than HEADER_SIZE. + + 20 - 23: 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. + + 24 - 27: cluster bits + Number of bits that are used for addressing an offset + within a cluster (1 << cluster_bits is the cluster size). + Must not be less than 9 (i.e. 512 byte clusters). + + Note: qemu as of today has an implementation limit of 2 MB + as the maximum cluster size and won't be able to open images + with larger cluster sizes. + + 28 - 35: features + Bitmask of features. If a feature bit is set but not recognized, + the add-cow file should be dropped. They are not used in v1. + + Bits 0-63: Reserved (set to 0) + + 36 - 43: compatible features + Bitmask of compatible features. An implementation can + safely ignore any unknown bits that are set. + Bit 0: All allocated bit. If this bit is set then + backing file and COW bitmap will not be used, + and can read from or write to image file directly. + + Bits 1-63: Reserved (set to 0) + + 44 - 47: HEADER_SIZE + The header field is variable-sized. This field indicates + how many bytes will be used to store add-cow header. + In add-cow v1, it is fixed to 4096. + + 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 non-empty, it must be non-empty. It is coded + in free-form ASCII, and is not NUL-terminated. Zero + padded on the right. + + 64 - 79: image file format + Format of image file. It must be non-empty. It is coded + in free-form ASCII, and is not NUL-terminated. Zero + padded on the right. + + 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. The bytes that are not pointing to + backing file and image file names must be set to 0. + +== COW bitmap == + +The "COW bitmap" field starts at offset HEADER_SIZE, stores a bitmap related to +backing file and image file. It is tracking whether the sector in image file +is allocated or not. + +Each bit in the bitmap tracks one cluster's status. For example, if cluster +bit is 16, then each bit tracks one cluster, (1 << 16) = 65536 bytes. The +image file size is rounded up to cluster size (where any bytes in the +last cluster that do not fit in the image are ignored), then if the +number of clusters is not a multiple of 8, then remaining bits in the +bitmap will be set to 0. + +The size of bitmap is calculated according to virtual size of image file, and +the size of bitmap should be multiple of add-cow file's cluster size, the bits +not used will be set to 0. Within each byte, the least significant bit covers +the first cluster. Bit orders in one byte look like: + +----+----+----+----+----+----+----+----+ + | b7 | b6 | b5 | b4 | b3 | b2 | b1 | b0 | + +----+----+----+----+----+----+----+----+ + +If the bit is 0, it indicates the sector has not been allocated in image file, +data should be loaded from backing file while reading; if the bit is 1, it +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. +If there is no backing file, or if the image file is larger than the backing +file and the offset is beyond the end of the backing file, then the data should +be read as all zero bytes instead. + +If raw image is not an even multiple of cluster bytes, bits that correspond to +bytes beyond the raw file size in add-cow must be written as 0 and must be +ignored when reading. + +Image file name and backing file name must NOT be the same, we prevent this +while creating add-cow files via qemu-img. If image file name and backing file +name are the same, the add-cow image must be treated as invalid.
Document for add-cow format, the usage and spec of add-cow are introduced. Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com> --- docs/specs/add-cow.txt | 154 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 154 insertions(+), 0 deletions(-) create mode 100644 docs/specs/add-cow.txt