[Qemu-devel] IO : Disk Sector Reads not aligned

[Qemu-devel] IO : Disk Sector Reads not aligned

[Saptarshi Sen]

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Hi,

   I am trying to monitor the disk access pattern from the guest.

   and i am using an IDE as my virtual hard drive. My observation is that

   the sector reads are not 512 bytes aligned.

   I am gathering the stats from hw/ide/core.c in the function

   ide_dma_cb. variable :sector_num

   Should not these disk reads be 512 bytes aligned.

   Please help.

Regards

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Re: [Qemu-devel] IO : Disk Sector Reads not aligned

[Alex Bligh]

--On 10 July 2013 01:04:35 +0800 Saptarshi Sen <saptarshi.mrg@gmail.com> 
wrote:

>    the sector reads are not 512 bytes aligned.
>
>    I am gathering the stats from hw/ide/core.c in the function
>
>    ide_dma_cb. variable :sector_num
>
>    Should not these disk reads be 512 bytes aligned.

As illustrated by the code below, sector_num is the number of the sector.
The byte offset is sector_num << 9. As sector_num is an integer, this makes
all the reads 512 byte aligned as far as I can see.

    n = s->io_buffer_size >> 9;
    sector_num = ide_get_sector(s);
    if (n > 0) {
        dma_buf_commit(s);
        sector_num += n;
        ide_set_sector(s, sector_num);
        s->nsector -= n;
    }

-- 
Alex Bligh