Re: [Qemu-block] [PATCH v2] qcow2: discard bitmap when removed

[Vladimir Sementsov-Ogievskiy]

28.02.2019 12:26, Andrey Shinkevich wrote:
> When a bitmap is removed, we can clean some space on the disk. The size
> of a cluster may be larger, so is the size of the bitmap that includes
> many clusters. Some bitmaps can be as large as tens of megabytes.
> The flag QCOW2_DISCARD_ALWAYS allows a call to the raw_co_pdiscard()
> that does the actual cleaning of the image on disk, while with the flag
> QCOW2_DISCARD_OTHER, a reference count of the cluster is updated only.

not quite right, as it depends on configuration.

So I'd reword the whole message to be simpler, like:

Bitmap data may take a lot of disk space, so it's better to discard it always.

> 
> Signed-off-by: Andrey Shinkevich <address@hidden>


Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>

> ---
> v1: Discard old bitmap directories in QCOW2 image
> 
> In the first version of the patch, any call to qcow2_free_clusters() in
> block/qcow2-bitmap.c was with the flag QCOW2_DISCARD_ALWAYS, even in the
> cases when the cleaned space is insignificant, particularly, in case of
> bitmap directories.
> Discussed in the email thread with the message ID
> <address@hidden>
> 
>   block/qcow2-bitmap.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c
> index 3ee524d..162744e 100644
> --- a/block/qcow2-bitmap.c
> +++ b/block/qcow2-bitmap.c
> @@ -202,7 +202,7 @@ static void clear_bitmap_table(BlockDriverState *bs, 
> uint64_t *bitmap_table,
>               continue;
>           }
>   
> -        qcow2_free_clusters(bs, addr, s->cluster_size, QCOW2_DISCARD_OTHER);
> +        qcow2_free_clusters(bs, addr, s->cluster_size, QCOW2_DISCARD_ALWAYS);
>           bitmap_table[i] = 0;
>       }
>   }
> 


-- 
Best regards,
Vladimir