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    SubjectRe: Should a block device enforce block atomicity?
    On Mon, Jun 30, 2008 at 9:55 AM, Jens Axboe <> 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).


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