Re: [Qemu-devel] I have some questions in block , can anyone help me, thank you!

[Zhi Hui Li <zhihuili@linux.vnet.ibm.com>]

On 2011年11月14日 18:00, Stefan Hajnoczi wrote:
> On Mon, Nov 14, 2011 at 01:47:41PM +0800, Zhi Hui Li wrote:
>> 1) In qcow2.c, in function: qcow2_co_readv
>>     In qcow2.h, in struct BDRVQcowState
>> I want to know the relations between sector_num in function
>> qcow2_co_readv and cluster_sectors in struct BDRVQcowState ?
>
> sector_num is the starting offset of the I/O request.  For example,
> sector_num=10 means that the read begins at 10 * 512 = 5120 bytes.
>
> cluster_sectors is the number of sectors in a qcow2 cluster.  (The qcow2
> format manages space in "clusters" instead of sectors.  They are
> typically many sectors large, e.g. 128.)
>
>> 2) In qcow2.c, in function; qcow2_co_writev
>> at line 547:
>>
>> index_in_cluster = sector_num&  (s->cluster_sectors - 1);
>> How to understand it ?
>
> cluster_sectors is a power of 2, e.g. 1024, 2048, 4096, and so on.  So
> this expression is the same as:
>
> index_in_cluster = sector_num % cluster_sectors
>
> It calculates the offset from the start of the cluster.  For example:
>
> sector_num = 130
> cluster_sectors = 128
>
> cluster = sector_num / cluster_sectors = 1
> index_in_cluster = sector_num % cluster_sectors = 2
>
> We can get back to the original sector_num value like this:
>
> sector_num = cluster * cluster_sectors + index_in_cluster
>             = 1 * 128 + 2
> 	   = 130
>
> So this is about managing space in "clusters".  It's similar to how
> memory is managed in pages instead of bytes by the memory management
> unit.
>
>> 3) In qcow2.c,  in the function  : qcow2_co_readv and qcow2_co_writev
>> I want to know the least unit that it was read by the function.
>> for example:
>> BDRVQcowState s;
>> Is s->cluster_size or 512 ?
>
> The block layer minimum I/O size is BDRV_SECTOR_SIZE.  For convenience
> there is the bdrv_pread()/bdrv_pwrite() interface which allows
> byte-granularity access but uses bounce buffers underneath.
>
> s->cluster_size is the number of bytes per cluster.  A cluster is
> typically 64 KB but the value can be set in the image file.  Qcow2
> internally manages space in cluster but the I/O granularity is
> BDRV_SECTOR_SIZE (512).
>
> Stefan
>
>


Thank you very much!