[PATCH v11 05/13] copy-on-read: limit COR operations to base in COR driver

[Andrey Shinkevich]

On 15.10.2020 18:56, Max Reitz wrote:
> On 14.10.20 20:57, Andrey Shinkevich wrote:
>> On 14.10.2020 15:01, Max Reitz wrote:
>>> On 12.10.20 19:43, Andrey Shinkevich wrote:
>>>> Limit COR operations by the base node in the backing chain when the
>>>> overlay base node name is given. It will be useful for a block stream
>>>> job when the COR-filter is applied. The overlay base node is passed as
>>>> the base itself may change due to concurrent commit jobs on the same
>>>> backing chain.
>>>>
>>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich at virtuozzo.com>
>>>> ---
>>>>    block/copy-on-read.c | 39 +++++++++++++++++++++++++++++++++++++--
>>>>    1 file changed, 37 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
>>>> index c578b1b..dfbd6ad 100644
>>>> --- a/block/copy-on-read.c
>>>> +++ b/block/copy-on-read.c
>>>> @@ -122,8 +122,43 @@ static int coroutine_fn
>>>> cor_co_preadv_part(BlockDriverState *bs,
>>>>                                               size_t qiov_offset,
>>>>                                               int flags)
>>>>    {
>>
>> [...]
>>
>>>> +            ret = bdrv_is_allocated_above(bdrv_cow_bs(bs->file->bs),
>>>> +                                          state->base_overlay, true,
>>>> offset,
>>>> +                                          n, &n);
>>>> +            if (ret) {
>>>> +                local_flags |= BDRV_REQ_COPY_ON_READ;
>>>> +            }
>>>> +        }
>>>
>>> Furthermore, I just noticed – can the is_allocated functions not return
>>> 0 in @n, when @offset is a the EOF?  Is that something to look out for?
>>>    (I’m not sure.)
>>>
>>> Max
>>>
>>
>> The check for EOF is managed earlier in the stream_run() for a
>> block-stream job. For other cases of using the COR-filter, the check for
>> EOF can be added to the cor_co_preadv_part().
>> I would be more than happy if we can escape the duplicated checking for
>> is_allocated in the block-stream. But how the stream_run() can stop
>> calling the blk_co_preadv() when EOF is reached if is_allocated removed
>> from it?
> 
> True.  Is it that bad to lose that optimization, though?  (And I would
> expect the case of a short backing file to be rather rare, too.)
> 
>> May the cor_co_preadv_part() return EOF (or other error code)
>> to be handled by a caller if (ret == 0 && n == 0 && (flags &
>> BDRV_REQ_PREFETCH)?
> 
> That sounds like a bad hack.  I’d rather keep the double is_allocated().
> 
> But what would be the problem with losing the short backing file
> optimization?  Just performance?  Or would we end up writing actual
> zeroes into the overlay past the end of the backing file?  Hm, probably
> not, if the COR filter would detect that case and handle it like stream
> does.
> 
> So it seems only a question of performance to me, and I don’t think it
> would be that bad to in this rather rare case to have a bunch of useless
> is_allocated and is_allocated_above calls past the backing file’s EOF.
> (Maybe I’m wrong, though.)
> 
> Max
> 

Thank you, Max, for sharing your thoughts on this subject.
The double check for the is_allocated in the stream_run() is a 
performance degradation also.
And we will make a check for the EOF in the cor_co_preadv_part() in 
either case, won't we?

Andrey