Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification

[John Snow]

On 01/20/2016 07:34 AM, Vladimir Sementsov-Ogievskiy wrote:
> On 19.01.2016 20:48, Kevin Wolf wrote:
>> Am 11.01.2016 um 14:05 hat Vladimir Sementsov-Ogievskiy geschrieben:
>>> 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>
>>> ---
>>>
>>> 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 | 172
>>> ++++++++++++++++++++++++++++++++++++++++++++++++++-
>>>   1 file changed, 171 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
>>> index 121dfc8..997239d 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 is
>>> +                                inconsistent.
>> It may as well be consistent, but we don't know.
>>
>> Perhaps something like "must be considered inconsistent" or "is
>> potentially 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 +178,34 @@ 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.
>> I have one major problem with this patch, and it starts here.
>>
>> The spec talks about dirty tracking bitmaps all the way, but it never
>> really defines what a dirty tracking bitmap even contains. It has a few
>> hints here and there, but they aren't consistent.
>>
>> Here's the first hint: They track "virtual disk changes", which implies
>> they track guest clusters rather than host clusters.
>>
>>> +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:
>>> +
>>> +          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:  bitmap_directory_size
>>> +                   Size of the bitmap directory in bytes. It is the
>>> 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 +400,133 @@ 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 a virtual disk. This section describes how these bitmaps are
>>> stored.
>> s/related/related to/
>>
>>> +Note: all bitmaps are related to the virtual disk stored in this image.
>>> +
>>> +=== 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
>>> +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'.
>> This suggests that we can represent snapshot switching in a dirty
>> tracking bitmap. Which is almost impossible if we track guest clusters,
>> except if loading a snapshot should mean that all bits in the bitmap are
>> set. However, that feels a bit useless and dirty tracking across
>> snapshots doesn't seem to make a whole lot of sense anyway.
>>
>> Maybe what would make more sense is that a bitmap is tied to a specific
>> snapshot or bitmaps are included in a snapshot, so that you can revert
>> to the dirty status as it was when the snapshot was taken.
>>
>> Or, if you really meant, that the tracking words on a host cluster
>> level, what clusters would be included in the bitmap? Would only the
>> virtual disk be included? VM state? Any metadata?
>>
>> It seems we definitely need a new section on dirty tracking bitmaps that
>> describes what the bitmap actually means and how it's supposed to work
>> with snapshots. I guess we could also talk about how it works with other
>> changes to the image like resizing.
> 
> Bitmaps track guest clusters.
> Backup is not related to snapshots, so, I think set all bits in the
> dirty bitmap on snapshot switch is ok. Of course we can't just ignore
> snapshot switch, as it changes how user (and backup) sees the virtual
> disk. Deleting the bitmap on snapshot switch is not good option too I
> think.
> 
> Bitmap size is defined to be equal to virtual disk size. So on resize,
> the one who resize must resize the bitmap too, to maintain accordance of
> the image file and the spec.
> 
> I'll think about such section, ok.
> 
>>
>>> +                    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: 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 must be zero.
>>> +
>>> +        variable:   Type-specific extra data for the bitmap.
>> We talked about this in the other subthread.
>>
>>> +        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 structure (as opposed to two-level
>>> +structure like for refcounts and guest clusters mapping) for the
>>> mapping of
>>> +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.
>>> +
>>> +         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 when read from.
>>> +
>>> +        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)
>>> +
>>> +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
>>> +
>>> +If the size of the bitmap data is not a multiply of cluster size
>>> then the last
>>> +cluster of the bitmap data contains some unused tail bits. These
>>> bits must be
>>> +zero.
>> In which order are the bits stored in the bitmap?
> 
> What do you mean?

He means BE or LE. You can intuit it from the formula, but it's not
explicitly stated, I think.

> 
>>
>> Kevin
> 
> 

-- 
—js