qcow2: Add bdrv_discard support

This adds a bdrv_discard function to qcow2 that frees the discarded clusters.
It does not yet pass the discard on to the underlying file system driver, but
the space can be reused by future writes to the image.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-cluster.c |   78 +++++++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.c         |    8 +++++
 block/qcow2.h         |    2 +
 3 files changed, 88 insertions(+), 0 deletions(-)

On Thu, Jan 27, 2011 at 01:40:21PM +0100, Kevin Wolf wrote:
> +/*
> + * This discards as many clusters of nb_clusters as possible at once (i.e.
> + * all clusters in the same L2 table) and returns the number of discarded
> + * clusters.
> + */
> +static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
> +    unsigned int nb_clusters)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    uint64_t l2_offset, *l2_table;
> +    int l2_index;
> +    int ret;
> +    int i;
> +
> +    ret = get_cluster_table(bs, offset, &l2_table, &l2_offset, &l2_index);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    /* Limit nb_clusters to one L2 table */
> +    nb_clusters = MIN(nb_clusters, s->l2_size - l2_index);
> +
> +    for (i = 0; i < nb_clusters; i++) {
> +        uint64_t old_offset;
> +
> +        old_offset = be64_to_cpu(l2_table[l2_index + i]);
> +        old_offset &= ~QCOW_OFLAG_COPIED;
> +
> +        if (old_offset == 0) {
> +            continue;
> +        }
> +
> +        /* First remove L2 entries */
> +        qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
> +        l2_table[l2_index + i] = cpu_to_be64(0);
> +
> +        /* Then decrease the refcount */
> +        qcow2_free_any_clusters(bs, old_offset, 1);
> +    }
> +
> +    ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
> +    if (ret < 0) {
> +        return ret;
> +    }

There is no loop to continue discards across L2 boundaries.  Guests
could use discard on the entire disk from an installer, for example.

> +
> +    return nb_clusters;
> +}
> +
> +int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
> +    int nb_sectors)

qcow2_discard_sectors() since units are in sectors not clusters?

> +{
> +    BDRVQcowState *s = bs->opaque;
> +    uint64_t end_offset;
> +    unsigned int nb_clusters;
> +    int ret;
> +

When offset=0x10200, nb_sectors=1, and cluster_size=65536...

> +    end_offset = offset + (nb_sectors << BDRV_SECTOR_BITS);
> +
> +    /* Round start up and end down */
> +    offset = align_offset(offset, s->cluster_size);
> +    end_offset &= ~(s->cluster_size - 1);

offset=0x20000
end_offset=0x10000

> +
> +    nb_clusters = size_to_clusters(s, end_offset - offset);

nb_clusters=4294967295

...and the loop will discard almost 256TB of data.  We need to check
against overflow/underflow or do this in the block layer.

Stefan

On Fri, Jan 28, 2011 at 9:57 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Thu, Jan 27, 2011 at 01:40:21PM +0100, Kevin Wolf wrote:
>> +/*
>> + * This discards as many clusters of nb_clusters as possible at once (i.e.
>> + * all clusters in the same L2 table) and returns the number of discarded
>> + * clusters.
>> + */
>> +static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
>> +    unsigned int nb_clusters)
>> +{
>> +    BDRVQcowState *s = bs->opaque;
>> +    uint64_t l2_offset, *l2_table;
>> +    int l2_index;
>> +    int ret;
>> +    int i;
>> +
>> +    ret = get_cluster_table(bs, offset, &l2_table, &l2_offset, &l2_index);
>> +    if (ret < 0) {
>> +        return ret;
>> +    }
>> +
>> +    /* Limit nb_clusters to one L2 table */
>> +    nb_clusters = MIN(nb_clusters, s->l2_size - l2_index);
>> +
>> +    for (i = 0; i < nb_clusters; i++) {
>> +        uint64_t old_offset;
>> +
>> +        old_offset = be64_to_cpu(l2_table[l2_index + i]);
>> +        old_offset &= ~QCOW_OFLAG_COPIED;
>> +
>> +        if (old_offset == 0) {
>> +            continue;
>> +        }
>> +
>> +        /* First remove L2 entries */
>> +        qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
>> +        l2_table[l2_index + i] = cpu_to_be64(0);
>> +
>> +        /* Then decrease the refcount */
>> +        qcow2_free_any_clusters(bs, old_offset, 1);
>> +    }
>> +
>> +    ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
>> +    if (ret < 0) {
>> +        return ret;
>> +    }
>
> There is no loop to continue discards across L2 boundaries.  Guests
> could use discard on the entire disk from an installer, for example.

Sorry, please ignore this comment.  This function discards as many
clusters as possible at once but gets called from a loop so that
eventually we discard the full amount.

Stefan