Re: [Qemu-devel] [PATCH] qcow2: Add bdrv_discard support

[Stefan Hajnoczi]

On Fri, Jan 28, 2011 at 9:57 AM, Stefan Hajnoczi <address@hidden> 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