Re: [Qemu-devel] [PATCH v8 03/14] qcow2: Optimize bdrv_make_empty()

[Max Reitz <mreitz@redhat.com>]

On 30.06.2014 13:33, Kevin Wolf wrote:
> Am 07.06.2014 um 20:51 hat Max Reitz geschrieben:
>> bdrv_make_empty() is currently only called if the current image
>> represents an external snapshot that has been committed to its base
>> image; it is therefore unlikely to have internal snapshots. In this
>> case, bdrv_make_empty() can be greatly sped up by creating an empty L1
>> table and dropping all data clusters at once by recreating the refcount
>> structure accordingly instead of normally discarding all clusters.
>>
>> If there are snapshots, fall back to the simple implementation (discard
>> all clusters).
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> Reviewed-by: Eric Blake <eblake@redhat.com>
> This approach looks a bit too complicated to me, and calulating the
> required metadata size seems error-prone.
>
> How about this:
>
> 1. Set the dirty flag in the header so we can mess with the L1 table
>     without keeping the refcounts consistent

Hm, I didn't think about this. *g*

> 2. Overwrite the L1 table with zeros
>
> 3. Overwrite the first n clusters after the header with zeros
>     (n = 2 + l1_clusters).
>
> 4. Update the header:
>     refcount_table_offset = cluster_size
>     refcount_table_clusters = 1
>     l1_table_offset = 3 * cluster_size
>
> 6. bdrv_truncate to n + 1 clusters
>
> 7. Now update the first 8 bytes at cluster_size (the first new refcount
>     table entry) to point to 2 * cluster_size (new refcount block)
>
> 8. Reset refcount block and L2 cache
>
> 9. Allocate n + 1 clusters (the header, too) and make sure you get
>     offset 0
>
> 10. Remove the dirty flag
>
> Surprisingly (or not) this is much like an ordinary image creation. The
> main difference is that we keep the full size of the L1 table so the
> image stays always valid (the spec would even allow us to temporarily
> set l1_size = 0, but qcow2_open() doesn't seem to like that)

Yes, I noticed. ;-)

> and all
> areas where the L1 table could be are zeroed (this includes the new
> refcount table/block until the header is updated).
>
>
> I wanted to check whether this would still give the preallocation=full
> series what it needs, but a v11 doesn't seem to be on the list yet and
> v10 doesn't have the dependency on this series yet.

Well, as far as I see it, the preallocation=full series will need a 
function to calculate the required image size (if it doesn't, 
preallocation=thin will). I don't really care whether this series 
introduces such a function or whether preallocation=full does.

Max

PS: I personally am reluctant to drop/change this patch, if only because 
I spent about a week getting it right. ;-)

I guess I'll just take a look into marking the image dirty and see how 
it goes.