Re: [Qemu-devel] [RFC 4/6] block: request overlap detection

[Stefan Hajnoczi]

On Mon, Nov 7, 2011 at 11:49 AM, Zhi Yong Wu <address@hidden> wrote:
> On Mon, Oct 17, 2011 at 11:47 PM, Stefan Hajnoczi
> <address@hidden> wrote:
>> Detect overlapping requests and remember to align to cluster boundaries
>> if the image format uses them.  This assumes that allocating I/O is
>> performed in cluster granularity - which is true for qcow2, qed, etc.
>>
>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>> ---
>>  block.c |   39 +++++++++++++++++++++++++++++++++++++--
>>  1 files changed, 37 insertions(+), 2 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index cc3202c..0c22741 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -1052,21 +1052,56 @@ static BdrvTrackedRequest 
>> *tracked_request_add(BlockDriverState *bs,
>>     return req;
>>  }
>>
>> +/**
>> + * Round a region to cluster boundaries
>> + */
>> +static void round_to_clusters(BlockDriverState *bs,
>> +                              int64_t sector_num, int nb_sectors,
>> +                              int64_t *cluster_sector_num,
>> +                              int *cluster_nb_sectors)
>> +{
>> +    BlockDriverInfo bdi;
>> +
>> +    if (bdrv_get_info(bs, &bdi) < 0 || bdi.cluster_size == 0) {
>> +        *cluster_sector_num = sector_num;
>> +        *cluster_nb_sectors = nb_sectors;
>> +    } else {
>> +        int64_t c = bdi.cluster_size / BDRV_SECTOR_SIZE;
>> +        *cluster_sector_num = (sector_num / c) * c;
>             I can understand the above formula, but the one below is
> very magic. :) and can not be understood by me.
>> +        *cluster_nb_sectors = ((sector_num % c) + nb_sectors + c - 1) / c * 
>> c;

I agree this is ugly.  Here is what is going on:

c = number of sectors per cluster
cluster_sector_num = sector number rounded *down* to cluster boundary
cluster_nb_sectors = number of sectors from cluster_sector_num to
rounded up sector_num+nb_sectors

So the magic expression is takes the original sector_num to
sector_num+nb_sectors region:

|---XXX|XXX---|

Where |-----| is a cluster and XXXX is the region from sector_num to
sector_num+nb_sectors, then the output should be:

|RRRRRR|RRRRRR|

Everything has been rounded to clusters.  So here is the expression broken down:

*cluster_nb_sectors = ((sector_num % c) + nb_sectors + c - 1) / c * c;
                        AAAAAAAAAAAAAA    XXXXXXXXXX   BBBBBBBBBBBBBB

|AAAXXX|XXXBBB|

A is actually equivalent to sector_num - cluster_sector_num.

X is the original unrounded region.

B is the rounding up to the next cluster bounary.

Another way of writing this:

*cluster_nb_sectors = ROUND_UP((sector_num - cluster_sector_num) +
nb_sectors, c);

I'll try to improve the code in the next revision.

Stefan