On 04/28/2017 01:00 PM, Max Reitz wrote: > On 27.04.2017 03:46, Eric Blake wrote: >> Similar to discard_single_l2(), we should try to avoid dirtying >> the L2 cache when the cluster we are changing already has the >> right characteristics. >>
>> - /* Update L2 entries */
>> + /*
>> + * Minimize L2 changes if the cluster already reads back as
>> + * zeroes with correct allocation.
>> + */
>> + if (qcow2_get_cluster_type(old_offset) == QCOW2_CLUSTER_ZERO &&
>> + !(unmap && old_offset & L2E_OFFSET_MASK)) {
>> + continue;
>> + }
>> +
>> qcow2_cache_entry_mark_dirty(bs, s->l2_table_cache, l2_table);
>> - if (old_offset & QCOW_OFLAG_COMPRESSED || flags &
>> BDRV_REQ_MAY_UNMAP) {
>> + if (old_offset & QCOW_OFLAG_COMPRESSED || unmap) {
>
> Works, but I'd like it better to store the cluster type somewhere and
> then use it here instead of checking the COMPRESSED flag.
Indeed, qcow2_get_cluster_type() checks QCOW_OFLAG_COMPRESSED first -
but I like the idea of storing it in a temporary variable now that we
are using the cluster type in more than one place.
>
> But it works, so it's up to you:
>
> Reviewed-by: Max Reitz <mre...@redhat.com>
>
>> l2_table[l2_index + i] = cpu_to_be64(QCOW_OFLAG_ZERO);
>> qcow2_free_any_clusters(bs, old_offset, 1,
>> QCOW2_DISCARD_REQUEST);
>> } else {
>>
>
>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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