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

[Vladimir Sementsov-Ogievskiy]

ping

06.02.2019 15:30, Vladimir Sementsov-Ogievskiy wrote:
> 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