Re: [Qemu-devel] [PATCH] qcow2: avoid lseek on block_status if possible

[Vladimir Sementsov-Ogievskiy]

ping.

Finally, what about this?

25.01.2019 17:21, Vladimir Sementsov-Ogievskiy wrote:
> drv_co_block_status digs bs->file for additional, more accurate search
> for hole inside region, reported as DATA by bs since 5daa74a6ebc.
> 
> This accuracy is not free: assume we have qcow2 disk. Actually, qcow2
> knows, where are holes and where is data. But every block_status
> request calls lseek additionally. Assume a big disk, full of
> data, in any iterative copying block job (or img convert) we'll call
> lseek(HOLE) on every iteration, and each of these lseeks will have to
> iterate through all metadata up to the end of file. It's obviously
> ineffective behavior. And for many scenarios we don't need this lseek
> at all.
> 
> However, lseek is needed when we have metadata-preallocated image.
> 
> So, let's detect metadata-preallocation case and don't dig qcow2's
> protocol file in other cases.
> 
> The idea is to compare allocation size in POV of filesystem with
> allocations size in POV of Qcow2 (by refcounts). If allocation in fs is
> significantly lower, consider it as metadata-preallocation case.
> 
> Suggested-by: Denis V. Lunev <address@hidden>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
> 
> Hi!
> 
> So, to continue talk about lseek/no lseek when qcow2 block_status reports
> DATA.
> 
> Results on tmpfs:
> cached is lseek cache by Kevin
> detect is this patch
> no lseek is just remove block_status query on bs->file->bs in
>           bdrv_co_block_status
> 
>      +---------------------+--------+--------+--------+----------+
>      |                     | master | cached | detect | no lseek |
>      +---------------------+--------+--------+--------+----------+
>      | test.qcow2          | 80     | 40     | 0.169  | 0.162    |
>      +---------------------+--------+--------+--------+----------+
>      | test_forward.qcow2  | 79     | 0.171  | 0.169  | 0.163    |
>      +---------------------+--------+--------+--------+----------+
>      | test_prealloc.qcow2 | 0.054  | 0.053  | 0.055  | 0.263    |
>      +---------------------+--------+--------+--------+----------+
> 
>   block/qcow2.h             |  1 +
>   include/block/block_int.h |  7 +++++++
>   block/io.c                |  3 ++-
>   block/qcow2-refcount.c    | 36 ++++++++++++++++++++++++++++++++++++
>   block/qcow2.c             |  7 +++++++
>   5 files changed, 53 insertions(+), 1 deletion(-)
> 
> diff --git a/block/qcow2.h b/block/qcow2.h
> index 438a1dee9e..d7113ed44c 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -610,6 +610,7 @@ int qcow2_change_refcount_order(BlockDriverState *bs, int 
> refcount_order,
>                                   void *cb_opaque, Error **errp);
>   int qcow2_shrink_reftable(BlockDriverState *bs);
>   int64_t qcow2_get_last_cluster(BlockDriverState *bs, int64_t size);
> +int qcow2_detect_metadata_preallocation(BlockDriverState *bs);
>   
>   /* qcow2-cluster.c functions */
>   int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index f605622216..c895ca7169 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -59,6 +59,12 @@
>   
>   #define BLOCK_PROBE_BUF_SIZE        512
>   
> +typedef enum BdrvYesNoUnknown {
> +    BDRV_UNKNOWN = 0,
> +    BDRV_NO,
> +    BDRV_YES,
> +} BdrvYesNoUnknown;
> +
>   enum BdrvTrackedRequestType {
>       BDRV_TRACKED_READ,
>       BDRV_TRACKED_WRITE,
> @@ -682,6 +688,7 @@ struct BlockDriverState {
>       bool probed;    /* if true, format was probed rather than specified */
>       bool force_share; /* if true, always allow all shared permissions */
>       bool implicit;  /* if true, this filter node was automatically inserted 
> */
> +    BdrvYesNoUnknown metadata_preallocation;
>   
>       BlockDriver *drv; /* NULL means no media */
>       void *opaque;
> diff --git a/block/io.c b/block/io.c
> index bd9d688f8b..815661750a 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -2186,7 +2186,8 @@ static int coroutine_fn 
> bdrv_co_block_status(BlockDriverState *bs,
>           }
>       }
>   
> -    if (want_zero && local_file && local_file != bs &&
> +    if (want_zero && bs->metadata_preallocation != BDRV_NO &&
> +        local_file && local_file != bs &&
>           (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) &&
>           (ret & BDRV_BLOCK_OFFSET_VALID)) {
>           int64_t file_pnum;
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 1c63ac244a..008196d849 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -3379,3 +3379,39 @@ int64_t qcow2_get_last_cluster(BlockDriverState *bs, 
> int64_t size)
>                               "There are no references in the refcount 
> table.");
>       return -EIO;
>   }
> +
> +int qcow2_detect_metadata_preallocation(BlockDriverState *bs)
> +{
> +    BDRVQcow2State *s = bs->opaque;
> +    int64_t i, end_cluster, cluster_count = 0;
> +    int64_t file_length, real_allocation, metadata_allocation, file_tail;
> +    uint64_t refcount;
> +
> +    file_length = bdrv_getlength(bs->file->bs);
> +    if (file_length < 0) {
> +        return file_length;
> +    }
> +    file_tail = offset_into_cluster(s, file_length);
> +
> +    real_allocation = bdrv_get_allocated_file_size(bs->file->bs);
> +    if (real_allocation < 0) {
> +        return real_allocation;
> +    }
> +
> +    end_cluster = size_to_clusters(s, file_length);
> +    for (i = 0; i < end_cluster; i++) {
> +        int ret = qcow2_get_refcount(bs, i, &refcount);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +        cluster_count += !!refcount;
> +    }
> +
> +    metadata_allocation = cluster_count * s->cluster_size;
> +    if (!!refcount && file_tail) {
> +        metadata_allocation -= s->cluster_size - file_tail;
> +    }
> +
> +    return real_allocation < 0.9 * metadata_allocation &&
> +        real_allocation + s->cluster_size < metadata_allocation;
> +}
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 4897abae5e..adc9cdcb27 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1800,6 +1800,13 @@ static int coroutine_fn 
> qcow2_co_block_status(BlockDriverState *bs,
>       unsigned int bytes;
>       int status = 0;
>   
> +    if (bs->metadata_preallocation == BDRV_UNKNOWN) {
> +        ret = qcow2_detect_metadata_preallocation(bs);
> +        if (ret >= 0) {
> +            bs->metadata_preallocation = ret ? BDRV_YES : BDRV_NO;
> +        }
> +    }
> +
>       bytes = MIN(INT_MAX, count);
>       qemu_co_mutex_lock(&s->lock);
>       ret = qcow2_get_cluster_offset(bs, offset, &bytes, &cluster_offset);
> 


-- 
Best regards,
Vladimir