Re: [Qemu-devel] [PATCH v9] spec: add qcow2 bitmaps extension specification

[Fam Zheng]

On Tue, 02/02 09:35, Vladimir Sementsov-Ogievskiy wrote:
> The new feature for qcow2: storing bitmaps.
> 
> This patch adds new header extension to qcow2 - Bitmaps Extension. It
> provides an ability to store virtual disk related bitmaps in a qcow2
> image. For now there is only one type of such bitmaps: Dirty Tracking
> Bitmap, which just tracks virtual disk changes from some moment.
> 
> Note: Only bitmaps, relative to the virtual disk, stored in qcow2 file,
> should be stored in this qcow2 file. The size of each bitmap
> (considering its granularity) is equal to virtual disk size.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
> 
> v9
>     - rewordings, thanks to Max
> 
> v8
>     - rewordings
>     - bitmap_directory_size: 4b -> 8b
>     - add more descriptive description in == Bitmaps == section
>     - add paragraph "Dirty tracking bitmaps"
> 
>   Bitmap directory entry:
>     - extra data should not allocate additional clusters
>     - padding must be all-bytes-zero
>     - add extra_data_compatible flag (now behavior in case of unknown
>       extra data is defined by this flag)
> 
> v7:
> 
> - Rewordings, grammar.
>   Max, Eric, John, thank you very much.
> 
> - add last paragraph: remaining bits in bitmap data clusters must be
>   zero.
> 
> - s/Bitmap Directory/bitmap directory/ and other names like this at
>   the request of Max.
> 
> v6:
> 
> - reword bitmap_directory_size description
> - bitmap type: make 0 reserved
> - extra_data_size: resize to 4bytes
>   Also, I've marked this field as "must be zero". We can always change
>   it, if we decide allowing managing app to specify any extra data, by
>   defining some magic value as a top of user extra data.. So, for now
>   non zeor extra_data_size should be considered as an error.
> - swap name and extra_data to give good alignment to extra_data.
> 
> 
> v5:
> 
> - 'Dirty bitmaps' renamed to 'Bitmaps', as we may have several types of
>   bitmaps.
> - rewordings
> - move upper bounds to "Notes about Qemu limits"
> - s/should/must somewhere. (but not everywhere)
> - move name_size field closer to name itself in bitmap header
> - add extra data area to bitmap header
> - move bitmap data description to separate section
> 
> 
>  docs/specs/qcow2.txt | 223 
> ++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 222 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index f236d8c..db5e666 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -103,7 +103,18 @@ in the description of a field.
>                      write to an image with unknown auto-clear features if it
>                      clears the respective bits from this field first.
>  
> -                    Bits 0-63:  Reserved (set to 0)
> +                    Bit 0:      Bitmaps extension bit
> +                                This bit indicates consistency for the 
> bitmaps
> +                                extension data.
> +
> +                                It is an error if this bit is set without the
> +                                bitmaps extension present.
> +
> +                                If the bitmaps extension is present but this
> +                                bit is unset, the bitmaps extension data 
> must be
> +                                considered inconsistent.
> +
> +                    Bits 1-63:  Reserved (set to 0)
>  
>           96 -  99:  refcount_order
>                      Describes the width of a reference count block entry 
> (width
> @@ -123,6 +134,7 @@ be stored. Each extension has a structure like the 
> following:
>                          0x00000000 - End of the header extension area
>                          0xE2792ACA - Backing file format name
>                          0x6803f857 - Feature name table
> +                        0x23852875 - Bitmaps extension
>                          other      - Unknown header extension, can be safely
>                                       ignored
>  
> @@ -166,6 +178,36 @@ the header extension data. Each entry look like this:
>                      terminated if it has full length)
>  
>  
> +== Bitmaps extension ==
> +
> +The bitmaps extension is an optional header extension. It provides the 
> ability
> +to store bitmaps related to a virtual disk. For now, there is only one bitmap
> +type: the dirty tracking bitmap, which tracks virtual disk changes from some
> +point in time.
> +
> +The data of the extension should be considered consistent only if the
> +corresponding auto-clear feature bit is set, see autoclear_features above.
> +
> +The fields of the bitmaps extension are:
> +
> +    Byte  0 -  3:  nb_bitmaps
> +                   The number of bitmaps contained in the image. Must be
> +                   greater than or equal to 1.
> +
> +                   Note: Qemu currently only supports up to 65535 bitmaps per
> +                   image.
> +
> +          4 -  7:  Reserved, must be zero.
> +
> +          8 - 15:  bitmap_directory_size
> +                   Size of the bitmap directory in bytes. It is the 
> cumulative
> +                   size of all (nb_bitmaps) bitmap headers.
> +
> +         16 - 23:  bitmap_directory_offset
> +                   Offset into the image file at which the bitmap directory
> +                   starts. Must be aligned to a cluster boundary.
> +
> +
>  == Host cluster management ==
>  
>  qcow2 manages the allocation of host clusters by maintaining a reference 
> count
> @@ -360,3 +402,182 @@ Snapshot table entry:
>  
>          variable:   Padding to round up the snapshot table entry size to the
>                      next multiple of 8.
> +
> +
> +== Bitmaps ==
> +
> +As mentioned above, the bitmaps extension provides the ability to store 
> bitmaps
> +related to a virtual disk. This section describes how these bitmaps are 
> stored.
> +
> +All stored bitmaps are related to the virtual disk stored in the same image, 
> so
> +each bitmap size is equal to the virtual disk size.
> +
> +Each bit of the bitmap is responsible for strictly defined range of the 
> virtual
> +disk. For bit number bit_nr the corresponding range (in bytes) will be:
> +
> +    [bit_nr * bitmap_granularity .. (bit_nr + 1) * bitmap_granularity - 1]
> +
> +Granularity is a property of the concrete bitmap, see below.
> +
> +
> +=== Bitmap directory ===
> +
> +Each bitmap saved in the image is described in a bitmap directory entry. The
> +bitmap directory is a contiguous area in the image file, whose starting 
> offset
> +and length are given by the header extension fields bitmap_directory_offset 
> and
> +bitmap_directory_size. The entries of the bitmap directory have variable
> +length, depending on the length of the bitmap name and extra data. These

s/length/lengths/ ?

> +entries are also called bitmap headers.
> +
> +Structure of a bitmap directory entry:
> +
> +    Byte 0 -  7:    bitmap_table_offset
> +                    Offset into the image file at which the bitmap table
> +                    (described below) for the bitmap starts. Must be aligned 
> to
> +                    a cluster boundary.
> +
> +         8 - 11:    bitmap_table_size
> +                    Number of entries in the bitmap table of the bitmap.
> +
> +        12 - 15:    flags
> +                    Bit
> +                      0: in_use
> +                         The bitmap was not saved correctly and may be
> +                         inconsistent.
> +
> +                      1: auto
> +                         The bitmap must reflect all changes of the virtual
> +                         disk by any application that would write to this 
> qcow2
> +                         file (including writes, snapshot switching, etc.). 
> The
> +                         type of this bitmap must be 'dirty tracking bitmap'.
> +
> +                      2: extra_data_compatible
> +                         This flags is meaningful when the extra data is
> +                         unknown to the software (currently any extra data is
> +                         unknown to Qemu).
> +                         If it is set, the bitmap may be used as expected, 
> extra
> +                         data must be left as is.
> +                         If it is not set, the bitmap must not be used, but
> +                         both it and its extra data be left as is.
> +
> +                    Bits 3 - 31 are reserved and must be 0.
> +
> +             16:    type
> +                    This field describes the sort of the bitmap.
> +                    Values:
> +                      1: Dirty tracking bitmap
> +
> +                    Values 0, 2 - 255 are reserved.
> +
> +             17:    granularity_bits
> +                    Granularity bits. Valid values: 0 - 63.
> +
> +                    Note: Qemu currently doesn't support granularity_bits
> +                    greater than 31.
> +
> +                    Granularity is calculated as
> +                        granularity = 1 << granularity_bits
> +
> +                    A bitmap's granularity is how many bytes of the image
> +                    accounts for one bit of the bitmap.
> +
> +        18 - 19:    name_size
> +                    Size of the bitmap name. Must be non-zero.
> +
> +                    Note: Qemu currently doesn't support values greater than
> +                    1023.
> +
> +        20 - 23:    extra_data_size
> +                    Size of type-specific extra data.
> +
> +                    For now, as no extra data is defined, extra_data_size is
> +                    reserved and should be zero. If it is non-zero the
> +                    behavior is defined by extra_data_compatible flag.
> +
> +        variable:   extra_data
> +                    Extra data for the bitmap, occupying extra_data_size 
> bytes.
> +                    Extra data must never contain references to clusters or 
> in
> +                    some other way allocate additional clusters.
> +
> +        variable:   name
> +                    The name of the bitmap (not null terminated), occupying
> +                    name_size bytes. Must be unique among all bitmap names
> +                    within the bitmaps extension.
> +
> +        variable:   Padding to round up the bitmap directory entry size to 
> the
> +                    next multiple of 8. All bytes of the padding must be 
> zero.

Isn't it clearer to find the next entry, if you add an "entry_size" in the
beginning, before bitmap_table_offset in each record?

> +
> +
> +=== Bitmap table ===
> +
> +Bitmaps are stored using a one-level structure (as opposed to two-level
> +structure like for refcounts and guest clusters mapping) for the mapping of

s/structure/structures/

> +bitmap data to host clusters. This structure is called the bitmap table.
> +
> +Each bitmap table has a variable size (stored in the bitmap directory entry)
> +and may use multiple clusters, however, it must be contiguous in the image
> +file.
> +
> +Structure of a bitmap table entry:
> +
> +    Bit       0:    Reserved and must be zero if bits 9 - 55 are non-zero.
> +                    If bits 9 - 55 are zero:
> +                      0: Cluster should be read as all zeros.
> +                      1: Cluster should be read as all ones.

Once bits 9 - 55 are non-zero, this bit goes useless? That doesn't make much
sense to me. In which case bit 0 is set but 9-55 are zero?

> +
> +         1 -  8:    Reserved and must be zero.
> +
> +         9 - 55:    Bits 9 - 55 of the host cluster offset. Must be aligned 
> to
> +                    a cluster boundary. If the offset is 0, the cluster is
> +                    unallocated; in that case, bit 0 determines how this
> +                    cluster should be treated during reads.
> +
> +        56 - 63:    Reserved and must be zero.
> +
> +
> +=== Bitmap data ===
> +
> +As noted above, bitmap data is stored in separate clusters, described by the
> +bitmap table. Given an offset (in bytes) into the bitmap data, the offset 
> into
> +the image file can be obtained as follows:
> +
> +    image_offset =
> +        bitmap_table[bitmap_data_offset / cluster_size] +
> +            (bitmap_data_offset % cluster_size)

In this pseudo code, image_offset looks like an variable, but...

> +
> +This offset is not defined if bits 9 - 55 of bitmap table entry are zero (see
> +above).
> +
> +Given an offset byte_nr into the virtual disk and the bitmap's granularity, 
> the
> +bit offset into the bitmap can be calculated like this:
> +
> +    bit_offset =
> +        image_offset(byte_nr / granularity / 8) * 8 +
> +            (byte_nr / granularity) % 8

... here it looks like a function. Could you make it consistent?

> +
> +If the size of the bitmap data is not a multiple of the cluster size then the
> +last cluster of the bitmap data contains some unused tail bits. These bits 
> must
> +be zero.

What defines the size of the bitmap data?

> +
> +
> +=== Dirty tracking bitmaps ===
> +
> +Bitmaps with 'type' field equal to one are dirty tracking bitmaps.
> +
> +When the virtual disk is in use dirty tracking bitmap may be 'enabled' or
> +'disabled'. 
> While the bitmap is 'enabled', all writes to the virtual disk
> +should be reflected in the bitmap. A set bit in the bitmap means that the
> +corresponding range of the virtual disk (see above) was written to while the
> +bitmap was 'enabled'. An unset bit means that this range was not written to.
> +
> +The software should not sync the bitmap in the image file with its
> +representation in RAM after each write. Flag 'in_use' should be set while the
> +bitmap is not synced.

I think this is an implementation detail. IMO a software *can* keep the bitmap
synced, "should not" is an obsecure and unnecessary constraint.

> +
> +In the image file the 'enabled' state is reflected by the 'auto' flag. If 
> this
> +flag is set, the software must consider the bitmap as 'enabled' and start
> +tracking virtual disk changes to this bitmap from the first write to the
> +virtual disk. If this flag is not set then the bitmap is disabled.
> +
> +To maintain bitmap consistency, the only software which is allowed to change
> +the value of the 'auto' flag is the one which has created the bitmap.

How does one software know if the image is created by it or not?

Fam