Re: [PATCH v5 4/9] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate

[Max Reitz]

On 23.04.20 15:25, Kevin Wolf wrote:
> Am 23.04.2020 um 12:53 hat Max Reitz geschrieben:
>> On 22.04.20 17:21, Kevin Wolf wrote:
>>> If BDRV_REQ_ZERO_WRITE is set and we're extending the image, calling
>>> qcow2_cluster_zeroize() with flags=0 does the right thing: It doesn't
>>> undo any previous preallocation, but just adds the zero flag to all
>>> relevant L2 entries. If an external data file is in use, a write_zeroes
>>> request to the data file is made instead.
>>>
>>> Signed-off-by: Kevin Wolf <address@hidden>
>>> ---
>>>  block/qcow2.c | 30 ++++++++++++++++++++++++++++++
>>>  1 file changed, 30 insertions(+)
>>>
>>> diff --git a/block/qcow2.c b/block/qcow2.c
>>> index 9cfbdfc939..bd632405d1 100644
>>> --- a/block/qcow2.c
>>> +++ b/block/qcow2.c
>>
>> [...]
>>
>>> @@ -4214,6 +4215,35 @@ static int coroutine_fn 
>>> qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
>>>          g_assert_not_reached();
>>>      }
>>>  
>>> +    if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
>>> +        uint64_t zero_start = QEMU_ALIGN_UP(old_length, s->cluster_size);
>>> +        uint64_t zero_end = QEMU_ALIGN_UP(offset, s->cluster_size);
>>> +
>>> +        /* Use zero clusters as much as we can */
>>> +        ret = qcow2_cluster_zeroize(bs, zero_start, zero_end - zero_start, 
>>> 0);
>>
>> It’s kind of a pity that this changes the cluster mappings that were
>> established when using falloc/full preallocation already (i.e., they
>> become preallocated zero clusters then, so when writing to them, we need
>> COW again).
>>
>> But falloc/full preallocation do not guarantee that the new data is
>> zero, so I suppose this is the only thing we can reasonably do.
> 
> If we really want, I guess we could make full preallocation first try
> passing BDRV_REQ_ZERO_WRITE to the protocol layer and if that succeeds,
> we could skip setting the zero cluster flag at the qcow2 level.

That might be nice.

> Feels like a separate patch, though.

Definitely, yes.

Max

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