Re: [Qemu-devel] [PATCH 3/4] qcow2: Discard preallocated zero clusters

[Eric Blake]

On 05/03/2017 06:11 PM, Max Reitz wrote:
> In discard_single_l2(), we completely discard normal clusters instead of
> simply turning them into preallocated zero clusters. That means we
> should probably do the same with such preallocated zero clusters:
> Discard them instead of keeping them allocated.
> 
> Reported-by: Eric Blake 
> Signed-off-by: Max Reitz 
> ---
>  block/qcow2-cluster.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Reviewed-by: Eric Blake 

> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index fb91fd8979..31077d8102 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -1511,7 +1511,8 @@ static int discard_single_l2(BlockDriverState *bs, 
> uint64_t offset,
>  break;
>  
>  case QCOW2_CLUSTER_ZERO:
> -if (!full_discard) {
> +/* Preallocated zero clusters should be discarded in any 
> case */
> +if (!full_discard && (old_l2_entry & L2E_OFFSET_MASK) == 0) {

Again, if we had a separate QCOW2_CLUSTER_* for plain vs. pre-allocated
zero, this might be easier to write.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.   +1-919-301-3266
Virtualization:  qemu.org | libvirt.org



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[Qemu-devel] [PATCH 3/4] qcow2: Discard preallocated zero clusters

[Max Reitz]

In discard_single_l2(), we completely discard normal clusters instead of
simply turning them into preallocated zero clusters. That means we
should probably do the same with such preallocated zero clusters:
Discard them instead of keeping them allocated.

Reported-by: Eric Blake 
Signed-off-by: Max Reitz 
---
 block/qcow2-cluster.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index fb91fd8979..31077d8102 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1511,7 +1511,8 @@ static int discard_single_l2(BlockDriverState *bs, 
uint64_t offset,
 break;
 
 case QCOW2_CLUSTER_ZERO:
-if (!full_discard) {
+/* Preallocated zero clusters should be discarded in any case 
*/
+if (!full_discard && (old_l2_entry & L2E_OFFSET_MASK) == 0) {
 continue;
 }
 break;
-- 
2.12.2