Re: [Qemu-devel] [PATCH v2 01/12] qcow2: Add new overlap check functions

[Max Reitz]

On 2014-11-24 at 16:56, Max Reitz wrote:
> The existing qcow2 metadata overlap detection function used existing
> structures to determine the location of the image metadata, from plain
> fields such as l1_table_offset and l1_size in the BDRVQcowState, over
> image structures in memory such as the L1 table for the L2 tables'
> positions, or it even read the required data directly from disk for
> every requested check, such as the snapshot L1 tables for the inactive
> L2 tables' positions.
>
> These new functions instead keep a dedicated structure for keeping track
> of the metadata positions in memory. It consists of two parts: First,
> there is one structure which is basically a list of all metadata
> structures. Each entry has a bitmask of types (because some metadata
> structures may actually overlap, such as active and inactive L2 tables),
> a number of clusters occupied and the offset from the previous entry in
> clusters. This structure requires relatively few memory, but checking a
> certain point may take relatively long. Each entry is called a
> "fragment".
>
> Therefore, there is another representation which is a bitmap, or rather
> a bytemap, of metadata types. The previously described list is split
> into multiple windows with each describing a constant number of clusters
> (WINDOW_SIZE). If the list is to be queried or changed, the respective
> window is selected in constant time and the bitmap is generated from the
> fragments belonging to the window. This bitmap can then be queried in
> constant time and easily be changed.
>
> Because the bitmap representation requires more memory, it is only used
> as a cache. Whenever a window is removed from the cache, the fragment
> list will be rebuilt from the bitmap if the latter has been modified.
> Therefore, the fragment list is only used as the background
> representation to save memory, whereas the bitmap is used whenever
> possible.
>
> Regarding the size of the fragment list in memory: As one refcount block
> can handle cluster_size / 2 entries and one L2 table can handle
> cluster_size / 8 entries, for a qcow2 image with the standard cluster
> size of 64 kB, there is a ratio of data to metadata of about 1/6000
> (1/32768 refblocks and 1/8192 L2 tables) if one ignores the fact that
> not every cluster requires an L2 table entry. The refcount table and the
> L1 table is generally negligible. At the worst, each metadata cluster
> requires its own entry in the fragment list; each entry takes up four
> bytes, therefore, at the worst, the fragment list should take up (for an
> image with 64 kB clusters) (4 B) / (64 kB * 6000) of the image size,
> which is about 1.e-8 (i.e., 11 kB for a 1 TB image, or 11 MB for a 1 PB
> image).
>
> Signed-off-by: Max Reitz <address@hidden>
> ---
>   block/Makefile.objs   |   3 +-
>   block/qcow2-overlap.c | 404 ++++++++++++++++++++++++++++++++++++++++++++++++++
>   block/qcow2.h         |  13 ++
>   3 files changed, 419 insertions(+), 1 deletion(-)
>   create mode 100644 block/qcow2-overlap.c
>
> diff --git a/block/qcow2-overlap.c b/block/qcow2-overlap.c
> new file mode 100644
> index 0000000..9f3f2aa
> --- /dev/null
> +++ b/block/qcow2-overlap.c

[snip]

> +
> +/**
> + * Destroys the cached window bitmap. If it has been modified, the fragment 
> list
> + * will be rebuilt accordingly.
> + */
> +static void destroy_window_bitmap(Qcow2MetadataList *mdl,
> +                                  Qcow2MetadataWindow *window)
> +{
> +    if (!window->bitmap) {
> +        return;
> +    }
> +
> +    if (window->bitmap_modified) {
> +        int bitmap_i, fragment_i = 0;
> +        QCow2MetadataOverlap current_types = 0;
> +        int current_nb_clusters = 0;
> +
> +        /* Rebuild the fragment list; the case bitmap_i == WINDOW_SIZE is for
> +         * entering the last fragment at the bitmap end */
> +
> +        for (bitmap_i = 0; bitmap_i <= WINDOW_SIZE; bitmap_i++) {
> +            /* Qcow2MetadataFragment::nb_clusters is a uint8_t, so
> +             * current_nb_clusters may not exceed 255 */
> +            if (bitmap_i < WINDOW_SIZE &&
> +                current_types == window->bitmap[bitmap_i] &&
> +                current_nb_clusters < 255)
> +            {
> +                current_nb_clusters++;
> +            } else {
> +                if (current_types && current_nb_clusters) {
> +                    if (fragment_i >= window->fragments_array_size) {
> +                        window->fragments_array_size =
> +                            3 * window->fragments_array_size / 2 + 1;
> +
> +                        /* new_nb_fragments should be small enough, and there 
> is
> +                         * nothing we can do on failure anyway, so do not use
> +                         * g_try_renew() here */
> +                        window->fragments =
> +                            g_renew(Qcow2MetadataFragment, window->fragments,
> +                                    window->fragments_array_size);
> +                    }
> +
> +                    window->fragments[fragment_i++] = (Qcow2MetadataFragment){
> +                        .types          = current_types,
> +                        .nb_clusters    = current_nb_clusters,
> +                        .relative_start = bitmap_i - current_nb_clusters,
> +                    };
> +                }
> +
> +                current_nb_clusters = 0;

Because I don't want to write a new version every time I find a single 
bug myself, I'll just reply here, so you know which things I know about.

This should be 1, and not 0. At this point, we have found the first 
cluster of this fragment, which is why this should be 1.

Max

> +                if (bitmap_i < WINDOW_SIZE) {
> +                    current_types = window->bitmap[bitmap_i];
> +                }
> +            }
> +        }
> +
> +        window->nb_fragments = fragment_i;
> +    }
> +
> +    g_free(window->bitmap);
> +    window->bitmap = NULL;
> +}