Re: [Qemu-devel] [PATCH 1/8] spec: add qcow2-dirty-bitmaps specification

[Stefan Hajnoczi]

On Mon, Jun 08, 2015 at 06:21:19PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> From: Vladimir Sementsov-Ogievskiy <address@hidden>
> 
> Persistent dirty bitmaps will be saved into qcow2 files. It may be used
> as 'internal' bitmaps (for qcow2 drives) or as 'external' bitmaps for
> other drives (there may be qcow2 file with zero disk size but with
> several dirty bitmaps for other drives).
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
>  docs/specs/qcow2.txt | 66 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 66 insertions(+)
> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 121dfc8..0fffba2 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -123,6 +123,7 @@ 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 - Dirty bitmaps
>                          other      - Unknown header extension, can be safely
>                                       ignored
>  
> @@ -166,6 +167,19 @@ the header extension data. Each entry look like this:
>                      terminated if it has full length)
>  
>  
> +== Dirty bitmaps ==
> +
> +Dirty bitmaps is an optional header extension. It provides a possibility of
> +storing dirty bitmaps in qcow2 image. The fields are:
> +
> +          0 -  3:  nb_dirty_bitmaps
> +                   Number of dirty bitmaps contained in the image

Is there a maximum?

> +
> +          4 - 11:  dirty_bitmaps_offset
> +                   Offset into the image file at which the dirty bitmaps 
> table
> +                   starts. Must be aligned to a cluster boundary.

The autoclear feature bit is undocumented.

>  == Host cluster management ==
>  
>  qcow2 manages the allocation of host clusters by maintaining a reference 
> count
> @@ -360,3 +374,55 @@ Snapshot table entry:
>  
>          variable:   Padding to round up the snapshot table entry size to the
>                      next multiple of 8.
> +
> +
> +== Dirty bitmaps ==
> +
> +The feature supports storing several dirty bitmaps in the qcow2 file.
> +
> +=== Cluster mapping ===
> +
> +Dirty bitmaps are stored using a ONE-level structure for the mapping of
> +bitmaps to host clusters. There is only an L1 table.
> +
> +The L1 table has a variable size (stored in the Bitmap table entry) and may
> +use multiple clusters, however it must be contiguous in the image file.

The use of "L1 table" could be confusing.  The refcount metadata uses
"refcount table" and "refcount block" to describe a one-level table.

> +
> +Given an offset into the bitmap, the offset into the image file can be
> +obtained as follows:
> +
> +    offset = l1_table[offset / cluster_size] + (offset % cluster_size)

It might help to add granularity to this formula.

Instead of "offset", "bit_number" or "bitnr" might be clearer since
"offset" means something different in other parts of the document.

> +
> +L1 table entry:
> +
> +    Bit  0 -  61:   Standard cluster descriptor
> +
> +        62 -  63:   Reserved

Do you really want to use the standard cluster descriptor with it's zero
bit?

Since bitmaps don't honor backing files there doesn't seem much point in
using the zero bit, things are simpler if just bits 9-55 are contain the
host cluster offset and 0 means the cluster is unallocated.

By honoring the zero bit there are three states:
1. Zero bit set, read zeroes
2. Zero bit not set, host cluster offset != 0, bits valid
3. Zero bit not set, host cluster offset == 0, unallocated

State 1 is not useful.

> +=== Bitmap table ===
> +
> +A directory of all bitmaps is stored in the bitmap table, a contiguous area 
> in
> +the image file, whose starting offset and length are given by the header 
> fields
> +dirty_bitmaps_offset and nb_dirty_bitmaps. The entries of the bitmap table 
> have
> +variable length, depending on the length of name and extra data.
> +
> +Bitmap table entry:
> +
> +    Byte 0 -  7:    Offset into the image file at which the L1 table for the
> +                    bitmap starts. Must be aligned to a cluster boundary.
> +
> +         8 - 11:    Number of entries in the L1 table of the bitmap
> +
> +        12 - 15:    Bitmap granularity in bytes
> +
> +        16 - 23:    Bitmap size in sectors
> +
> +        24 - 25:    Size of the bitmap name
> +
> +        variable:   The name of the bitmap (not null terminated)
> +
> +        variable:   Padding to round up the bitmap table entry size to the
> +                    next multiple of 8.
> +
> +The fields "size", "granularity" and "name" are corresponding with the fields
> +in struct BdrvDirtyBitmap.

Referring to the internals of a C struct in QEMU is not appropriate for
a file format specification.  Please document the fields fully including
their constraints, minimums, maximums, etc.

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