Re: [Qemu-block] [PATCH v10 8/9] qcow2: skip writing zero buffers to empty COW areas

[Anton Nefedov]

On 5/12/2018 5:01 PM, Vladimir Sementsov-Ogievskiy wrote:
> 03.12.2018 13:14, Anton Nefedov wrote:
>> If COW areas of the newly allocated clusters are zeroes on the backing image,
>> efficient bdrv_write_zeroes(flags=BDRV_REQ_ALLOCATE) can be used on the whole
>> cluster instead of writing explicit zero buffers later in perform_cow().
>>
>> iotest 060:
>> write to the discarded cluster does not trigger COW anymore.
>> Use a backing image instead.
>>
>> Signed-off-by: Anton Nefedov <address@hidden>
>> Reviewed-by: Alberto Garcia <address@hidden>
>> ---
>>    qapi/block-core.json       |  4 +-
>>    block/qcow2.h              |  6 +++
>>    block/qcow2-cluster.c      |  2 +-
>>    block/qcow2.c              | 80 +++++++++++++++++++++++++++++++++++++-
>>    block/trace-events         |  1 +
>>    tests/qemu-iotests/060     | 26 ++++++++-----
>>    tests/qemu-iotests/060.out |  5 ++-
>>    7 files changed, 109 insertions(+), 15 deletions(-)
>>
>> diff --git a/qapi/block-core.json b/qapi/block-core.json
>> index d4fe710836..50598aa8fe 100644
>> --- a/qapi/block-core.json
>> +++ b/qapi/block-core.json
>> @@ -3004,6 +3004,8 @@
>>    #
>>    # @cor_write: a write due to copy-on-read (since 2.11)
>>    #
>> +# @cluster_alloc_space: an allocation of file space for a cluster (since 
>> 4.0)
>> +#
>>    # Since: 2.9
>>    ##
>>    { 'enum': 'BlkdebugEvent', 'prefix': 'BLKDBG',
>> @@ -3022,7 +3024,7 @@
>>                'pwritev_rmw_tail', 'pwritev_rmw_after_tail', 'pwritev',
>>                'pwritev_zero', 'pwritev_done', 'empty_image_prepare',
>>                'l1_shrink_write_table', 'l1_shrink_free_l2_clusters',
>> -            'cor_write'] }
>> +            'cor_write', 'cluster_alloc_space'] }
>>    
>>    ##
>>    # @BlkdebugInjectErrorOptions:
>> diff --git a/block/qcow2.h b/block/qcow2.h
>> index 8662b68575..8a64077897 100644
>> --- a/block/qcow2.h
>> +++ b/block/qcow2.h
>> @@ -389,6 +389,12 @@ typedef struct QCowL2Meta
>>         */
>>        Qcow2COWRegion cow_end;
>>    
>> +    /**
>> +     * Indicates that COW regions are already handled and do not require
>> +     * any more processing.
>> +     */
>> +    bool skip_cow;
>> +
>>        /**
>>         * The I/O vector with the data from the actual guest write request.
>>         * If non-NULL, this is meant to be merged together with the data
>> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
>> index d37fe08b3d..3685c5f67e 100644
>> --- a/block/qcow2-cluster.c
>> +++ b/block/qcow2-cluster.c
>> @@ -806,7 +806,7 @@ static int perform_cow(BlockDriverState *bs, QCowL2Meta 
>> *m)
>>        assert(start->offset + start->nb_bytes <= end->offset);
>>        assert(!m->data_qiov || m->data_qiov->size == data_bytes);
>>    
>> -    if (start->nb_bytes == 0 && end->nb_bytes == 0) {
>> +    if ((start->nb_bytes == 0 && end->nb_bytes == 0) || m->skip_cow) {
>>            return 0;
>>        }
>>    
>> diff --git a/block/qcow2.c b/block/qcow2.c
>> index 991d6ac91b..027188a1a3 100644
>> --- a/block/qcow2.c
>> +++ b/block/qcow2.c
>> @@ -2015,6 +2015,11 @@ static bool merge_cow(uint64_t offset, unsigned bytes,
>>                continue;
>>            }
>>    
>> +        /* If COW regions are handled already, skip this too */
>> +        if (m->skip_cow) {
>> +            continue;
>> +        }
>> +
>>            /* The data (middle) region must be immediately after the
>>             * start region */
>>            if (l2meta_cow_start(m) + m->cow_start.nb_bytes != offset) {
>> @@ -2040,6 +2045,68 @@ static bool merge_cow(uint64_t offset, unsigned bytes,
>>        return false;
>>    }
>>    
>> +static bool is_unallocated(BlockDriverState *bs, int64_t offset, int64_t 
>> bytes)
>> +{
>> +    int64_t nr;
>> +    return !bytes ||
>> +        (!bdrv_is_allocated_above(bs, NULL, offset, bytes, &nr) && nr == 
>> bytes);
> 
> bdrv_is_allocated_above may return error < 0
> 

Probably I just took is_zero() as an example.
But somewhere there's even a rationale (bdrv_co_do_copy_on_readv):

         ret = bdrv_is_allocated(bs, cluster_offset,
                                 MIN(cluster_bytes, max_transfer), &pnum);
         if (ret < 0) {
             /* Safe to treat errors in querying allocation as if
              * unallocated; we'll probably fail again soon on the
              * read, but at least that will set a decent errno.
              */
             pnum = MIN(cluster_bytes, max_transfer);
         }

>> +}
>> +
>> +static bool is_zero_cow(BlockDriverState *bs, QCowL2Meta *m)
>> +{
>> +    /* This check is designed for optimization shortcut so it must be
>> +     * efficient.
>> +     * Instead of is_zero(), use is_unallocated() as it is faster (but not
>> +     * as accurate and can result in false negatives). */
>> +    return is_unallocated(bs, m->offset + m->cow_start.offset,
>> +                          m->cow_start.nb_bytes) &&
>> +           is_unallocated(bs, m->offset + m->cow_end.offset,
>> +                          m->cow_end.nb_bytes);
>> +}
>> +
>> +static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
>> +{
>> +    BDRVQcow2State *s = bs->opaque;
>> +    QCowL2Meta *m;
>> +
>> +    if (!(bs->file->bs->supported_zero_flags & BDRV_REQ_ALLOCATE)) {
>> +        return 0;
>> +    }
>> +
>> +    if (bs->encrypted) {
>> +        return 0;
>> +    }
>> +
>> +    for (m = l2meta; m != NULL; m = m->next) {
>> +        int ret;
>> +
>> +        if (!m->cow_start.nb_bytes && !m->cow_end.nb_bytes) {
>> +            continue;
>> +        }
>> +
>> +        if (!is_zero_cow(bs, m)) {
>> +            continue;
>> +        }
>> +
>> +        BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE);
>> +        /* instead of writing zero COW buffers,
>> +           efficiently zero out the whole clusters */
>> +        ret = bdrv_co_pwrite_zeroes(bs->file, m->alloc_offset,
>> +                                    m->nb_clusters * s->cluster_size,
>> +                                    BDRV_REQ_ALLOCATE);
>> +        if (ret < 0) {
>> +            if (ret != -ENOTSUP && ret != -EAGAIN) {
> 
> why do you ignore ENOTSUP?
> 

This might particularly happen after first pwrite_zeroes() to
file-posix.
It generally reports BDRV_REQ_ALLOCATE on open, but then it might reset
the flag if fallocate() returns ENOTSUP.

Nevermind file-posix; with any file driver what can we possibly do here:
this operation is non-mandatory and the error is not critical.
Never trying again doesn't look quite right, the file driver can reset
BDRV_REQ_ALLOCATE if it likes and we won't bother it anymore.


> And also, hmm, should not we have problems with serializing? If already 
> started unaligned request,
> it is serializing.. And now, we are going to start nested serializing 
> request, which will definitely
> return EAGAIN..
> 

The request we're processing here is on qcow2 level yet, it won't affect
the bs->file serialization queue.

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