Re: [Qemu-devel] [PATCH v6] spec: add qcow2 bitmaps extension specification

[John Snow <jsnow@redhat.com>]

Since Max didn't offer a grammatical review, here's my attempt at some
suggestions.

On 12/23/2015 12:49 PM, 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 <vsementsov@virtuozzo.com>
> ---
> 
> 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 | 161 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 160 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 121dfc8..b23966a 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -103,7 +103,19 @@ 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.

For consistency, no period after this.

> +                                This bit is responsible for Bitmaps extension
> +                                consistency.
> +

I might phrase it as: "This bit indicates consistency for the Bitmaps
extension data."

> +                                If it is set, but there is no Bitmaps
> +                                extension, this should be considered as an
> +                                error.
> +

"This should be considered as an error" can be shortened to just "This
is an error." This makes the sentence top-heavy though, so how about:

"It is an error if this bit is set without the Bitmaps extension present."

> +                                If it is not set, but there is a Bitmaps
> +                                extension, its data should be considered as
> +                                inconsistent.
> +

Let's remove "considered" here. The data *is* inconsistent if this has
happened.

"If the Bitmaps extension is present but this bit is unset, the Bitmaps
extension data is inconsistent."

> +                    Bits 1-63:  Reserved (set to 0)
>  
>           96 -  99:  refcount_order
>                      Describes the width of a reference count block entry (width
> @@ -123,6 +135,7 @@ 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 +179,34 @@ the header extension data. Each entry look like this:
>                      terminated if it has full length)
>  
>  
> +== Bitmaps extension ==
> +
> +Bitmaps extension is an optional header 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.
> +

I think "Bitmaps extension" is awkward without "The" leading it, so:

"The Bitmaps extension is an optional header extension."

And from Eric's suggestion:

"It provides the ability to store bitmaps related to a virtual disk.
For now, there is only one bitmap type: Dirty Tracking Bitmap, which
tracks virtual disk changes from some moment."

> +The data of the extension should be considered as consistent only if
> +corresponding auto-clear feature bit is set (see autoclear_features above).
> +

I might remove the parenthetical.

"The data in this extension should be considered consistent only if the
corresponding auto-clear feature bit is set, see autoclear_features above."

> +The fields of Bitmaps extension are:
> +

Adding 'the' again:

"The fields of the Bitmaps extension are:"

> +          0 -  3:  nb_bitmaps
> +                   The number of bitmaps contained in the image. Must be
> +                   greater or equal to 1.
> +

"Greater than or equal to" is a common phrasing -- adding the 'than'.

> +                   Note: Qemu currently only supports up to 65535 bitmaps per
> +                   image.
> +
> +          4 -  7:  bitmap_directory_size
> +                   Size of the Bitmap Directory in bytes. It is a cumulative
> +                   size of all (nb_bitmaps) bitmap headers.
> +
> +          8 - 15:  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 +401,121 @@ Snapshot table entry:
>  
>          variable:   Padding to round up the snapshot table entry size to the
>                      next multiple of 8.
> +
> +
> +== Bitmaps ==
> +
> +The feature supports storing bitmaps in a qcow2 image. All bitmaps are related
> +to the virtual disk, stored in this image.
> +

Maybe like Eric's earlier suggestion, we can avoid "in a qcow2 image"
here. We can probably just repeat some of the earlier description here.

> +=== Bitmap Directory ===
> +
> +Each bitmap saved in the image is described in a Bitmap Directory entry. Bitmap

"The Bitmap Directory"

> +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
> +entries are also called bitmap headers.
> +
> +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'.
> +
> +                    Bits 2 - 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 are: 0 - 63.
> +
> +                    Note: Qemu currently doesn't support granularity_bits
> +                    greater than 31.
> +
> +                    Granularity is calculated as
> +                        granularity = 1 << granularity_bits
> +
> +                    Granularity of the bitmap is how many bytes of the image
> +                    accounts for one bit of the bitmap.
> +
> +        18 - 19:    name_size
> +                    Size of the bitmap name. Valid values: 1 - 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 must be zero.
> +
> +        variable:   Type-specific extra data for the bitmap.
> +

And I guess we don't have any definitions for the "Dirty Tracking" type,
so this is zero.

Should we add a table of the bitmap types and their corresponding
definitions for the "Type-specific extra data"?

e.g.

=== Bitmap Types ===

0: Reserved
   extra_data_size: 0

   This bitmap type is invalid and must not be used.

1: Dirty Tracking Bitmap
   extra_data_size: 0

   This bitmap type represents data changed since a point in time
   and has no extra data.

2: Lorem Ipsum Bitmap
   extra_data_size: 4

   Lorem ipsum, dolor sit amet...
   This bitmap type has four byte of extra data:

   0-1: Identifier
   2-3: Padding

> +        variable:   The name of the bitmap (not null terminated). 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.
> +
> +=== Bitmap Table ===
> +
> +Bitmaps are stored using a one-level (not two-level like refcounts and guest
> +clusters mapping) structure for the mapping of bitmaps data to host clusters.
> +It is called Bitmap Table.
> +

I might add "the" before Bitmap Table above.

> +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.
> +
> +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.
> +
> +         1 -  8:    Reserved and must be zero.
> +
> +         9 - 55:    Bits 9 - 55 of host cluster offset. Must be aligned to a
> +                    cluster boundary. If the offset is 0, the cluster is
> +                    unallocated, see bit 0 description.
> +
> +        56 - 63:    Reserved and must be zero.
> +
> +=== Bitmap Data ===
> +
> +As noted above, bitmap data is stored in several (or may be one, exactly
> +bitmap_table_size) separate clusters, described by 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)
> +
> +Taking into account the granularity of the bitmap, an offset in bits into the
> +image file, corresponding to byte number byte_nr of the virtual disk can be
> +calculated like this:
> +
> +    bit_offset =
> +        image_offset(byte_nr / granularity / 8) * 8 +
> +            (byte_nr / granularity) % 8
> 

That's all the language commentary I have on this. Hopefully not long
before consensus.

Thank you,
--John