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

[Andrey Shinkevich]

On 19/03/2019 01:18, John Snow wrote:
> 
> 
> On 2/28/19 4:26 AM, 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.
>>
>> Signed-off-by: Andrey Shinkevich <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;
>>       }
>>   }
>>
> Looks like 4.1 material now. Kevin may have been expecting me to engage
> with this, even though I do not maintain the qcow2-bitmap.c file. (And
> I'm very behind on reading mail that was not directly addressed to me,
> sorry...!)
> 
> Please resend when the 4.1 window opens and we'll address this then.
> 
> --js
> 
Thank you, John.
Well noted.

Andrey