[dm-devel] Newbie device mapper questions

[Minfei Huang]

On Tue, Jun 16, 2015 at 6:20 AM, Vivek Goyal <vgoyal at redhat.com> wrote:
> On Mon, Jun 15, 2015 at 12:52:57PM -0700, Doug Dumitru wrote:
>> >
>> > - Is i_size_read(lc->src1dev->bdev->bd_inode) the correct way of
>> > determining the size of the underlying block device? If not, which
>> > function is?
>> >
>
> I believe that's correct. Look at block/blk-core.c
>
> handle_bad_sector(struct bio *bio)
> {
> }
>

Kernel will call i_size_write to assign the device capacity, when the
device try to be registered(register_disk). During the period of
register, the function blkdev_get will call bd_set_size to set the
capacity.

So it is correct that you can call i_size_read to determine the device's
capacity.

>>
>> ... I am happy to leave out answers that I don't know ...
>>
>>
>> >
>> > - Can I safely assume the logical sector size is fixed to be 512 bytes
>> > in all cases?
>> >
>>
>> Probably not, but maybe.  You are in control of the hardware.
>
> You mean block size or sector size? I think sector sizes can vary and
> targets can allow user space to specify one. Typically metadata is per
> block can lead to smaller metadata foot print.
>

For now, the minimum align unit is 512 bytes. But as I know, a lot of
devices start to support 4KB align unit.

Please correct me if there is something wrong in the comment.

Thanks
Minfei