Re: [Qemu-devel] [PATCH v2 2/3] qemu-img: rebase: Reduce reads on in-chain rebase

[Sam Eiderman]

> On 23 May 2019, at 17:01, Max Reitz <address@hidden> wrote:
> 
> On 02.05.19 15:58, Sam Eiderman wrote:
>> In the following case:
>> 
>> (base) A <- B <- C (tip)
>> 
>> when running:
>> 
>>    qemu-img rebase -b A C
>> 
>> QEMU would read all sectors not allocated in the file being rebased (C)
>> and compare them to the new base image (A), regardless of whether they
>> were changed or even allocated anywhere along the chain between the new
>> base and the top image (B). This causes many unneeded reads when
>> rebasing an image which represents a small diff of a large disk, as it
>> would read most of the disk's sectors.
>> 
>> Instead, use bdrv_is_allocated_above() to reduce the number of
>> unnecessary reads.
>> 
>> Reviewed-by: Karl Heubaum <address@hidden>
>> Signed-off-by: Sam Eiderman <address@hidden>
>> Signed-off-by: Eyal Moscovici <address@hidden>
>> ---
>> qemu-img.c | 25 ++++++++++++++++++++++++-
>> 1 file changed, 24 insertions(+), 1 deletion(-)
>> 
>> diff --git a/qemu-img.c b/qemu-img.c
>> index d9b609b3f0..7f20858cb9 100644
>> --- a/qemu-img.c
>> +++ b/qemu-img.c
> 
> [...]
> 
>> @@ -3422,6 +3428,23 @@ static int img_rebase(int argc, char **argv)
>>                 continue;
>>             }
>> 
>> +            if (prefix_chain_bs) {
>> +                /*
>> +                 * If cluster wasn't changed since prefix_chain, we don't 
>> need
>> +                 * to take action
>> +                 */
>> +                ret = bdrv_is_allocated_above(bs, prefix_chain_bs,
>> +                                              offset, n, &n);
> 
> This will always return true because it definitely is allocated in @bs,
> or we wouldn’t be here.  (We just checked that with
> bdrv_is_allocated().)  I think @top should be backing_bs(bs).
> 
> Max

I don’t think that’s true:

Examine the case where we have the following chain:

A <- B <- C

When we rebase C directly over A: qemu-img rebase -b A C

We must check for every offset (sector): bdrv_is_allocated_above(C, A, offset, 
n, &n);

If a sector from C is allocated above A - it may have been changed - so we need 
to do a read from A and a read from C and compare.
If the sector is not allocated above, it was not changed - we don’t need to 
read from A or C.

Sam


> 
>> +                if (ret < 0) {
>> +                    error_report("error while reading image metadata: %s",
>> +                                 strerror(-ret));
>> +                    goto out;
>> +                }
>> +                if (!ret) {
>> +                    continue;
>> +                }
>> +            }
>> +
>>             /*
>>              * Read old and new backing file and take into consideration that
>>              * backing files may be smaller than the COW image.