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

[Eric Blake]

On 01/13/2015 10:02 AM, Vladimir Sementsov-Ogievskiy wrote:
> 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 | 59 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 121dfc8..b29de40 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -116,6 +116,13 @@ in the description of a field.
>                      Length of the header structure in bytes. For version 2
>                      images, the length is always assumed to be 72 bytes.
>  
> +        104 - 107:  nb_dirty_bitmaps
> +                    Number of dirty bitmaps contained in the image
> +
> +        108 - 115:  dirty_bitmaps_offset
> +                    Offset into the image file at which the dirty bitmaps 
> table
> +                    starts. Must be aligned to a cluster boundary.

NACK.  You cannot add new fields directly into the header without
bumping the version number; but that is overkill.  Instead, stick the
dirty bitmaps into a new header extension.  The existing fields of the
header are sufficient to record that a new extension is present.  Also,
you need to decide whether dirty bitmaps are an ignorable feature or a
mandatory feature, and set the appropriate feature bit in the header
flag fields (thus when an older qemu that doesn't know about dirty
bitmaps opens the file, it will either reject the open due to not
knowing how to keep dirty bitmaps sane, or will accept the open but
clear the ignorable bit so that the next time a new qemu opens the file
it will know that the dirty bitmap is no longer reliable).

> +=== Cluster mapping ===
> +
> +Dirty bitmaps are stored using a ONE-level structure for the mapping of
> +bitmaps to host clusters. There only an L1 table.

s/There/There is/

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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