Re: [PATCH] qcow2: Expose bitmaps' size during measure

[Eric Blake <eblake@redhat.com>]

On 4/16/20 4:23 PM, Eric Blake wrote:
> It's useful to know how much space can be occupied by qcow2 persistent
> bitmaps, even though such metadata is unrelated to the guest-visible
> data.  Report this value as an additional field.
> 
> Reported-by: Nir Soffer <nsoffer@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
> 

Per https://bugzilla.redhat.com/show_bug.cgi?id=1779904#c0, I didn't 
quite round up in enough places:

> @@ -4739,6 +4742,26 @@ static BlockMeasureInfo *qcow2_measure(QemuOpts *opts, BlockDriverState *in_bs,
>               goto err;
>           }
> 
> +        FOR_EACH_DIRTY_BITMAP(in_bs, bm) {
> +            if (bdrv_dirty_bitmap_get_persistence(bm)) {
> +                uint64_t bmsize = bdrv_dirty_bitmap_size(bm);
> +                uint32_t granularity = bdrv_dirty_bitmap_granularity(bm);
> +                const char *name = bdrv_dirty_bitmap_name(bm);
> +                uint64_t bmclusters = DIV_ROUND_UP(bmsize / granularity
> +                                                   / CHAR_BIT, cluster_size);

All of these divisions need to round up.  For example, in an image with 
512-byte clusters and granularity, and a bitmap covering 512*512*8+512 
bytes (2097664), we need 2 clusters, not 1, for the bitmap itself. 
Fortunately, it is an edge case, and we usually have enough slop in the 
final round up to cluster size that most users won't trip on this.

> +
> +                /* Assume the entire bitmap is allocated */
> +                bitmaps_size += bmclusters * cluster_size;
> +                /* Also reserve space for the bitmap table entries */
> +                bitmaps_size += ROUND_UP(bmclusters * sizeof(uint64_t),
> +                                         cluster_size);
> +                /* Guess at contribution to bitmap directory size */
> +                bitmap_overhead += ROUND_UP(strlen(name) + 24,

And I don't like this magic number, but sizeof(Qcow2BitmapDirEntry) from 
qcow2-bitmap.c is a private struct not accessible here.

> +                                            sizeof(uint64_t));
> +            }
> +        }
> +        bitmaps_size += ROUND_UP(bitmap_overhead, cluster_size);
> +
-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org