Re: Should a block device enforce block atomicity?

["Erez Zilber" <erezzi.list@gmail.com>]

On Mon, Jun 30, 2008 at 9:55 AM, Jens Axboe <jens.axboe@oracle.com> wrote:
> On Mon, Jun 30 2008, Erez Zilber wrote:
>> Hi,
>>
>> I have a question about block devices and whether they are required to
>> enforce block atomicity:
>>
>> I read the code of drivers/block/brd.c, and I didn't see any locking
>> when blocks are read/written. I also looked at the block layer code
>> that calls brd and didn't find any locking there. Does it mean that
>> there's no block atomicity (i.e. multiple threads can write a single
>> block at the same time)? Is there any hidden assumption here? Is this
>> the responsibility of the application to do that (e.g. not start a
>> WRITE request before other READ/WRITE requests to the same block were
>> completed)?
>
> The block layer doesn't give such guarentees, not for "regular" block
> devices either. If the IO goes through the page cache then that will
> serialize IO to a given page, but with eg O_DIRECT IO, you could have
> the same block in flight several times. So if you are doing raw IO, the
> application has to ensure ordering of the same block.
>

So, do you say that people that write applications need to take care
of I/O serialization, and block devices (and the block layer itself)
don't need to care about this problem? I thought that standard disks
guarantee block atomicity (i.e. they don't count on the layers above
them to do that).

Erez