On Thu, Dec 13, 2012 at 04:22:33PM +0100, Bart Van Assche wrote: > On 12/11/12 01:00, scameron@xxxxxxxxxxxxxxxxxx wrote: > >The driver, like nvme, has a submit and reply queue per cpu. > > This is interesting. If my interpretation of the POSIX spec is correct > then aio_write() allows to queue overlapping writes and all writes > submitted by the same thread have to be performed in the order they were > submitted by that thread. What if a thread submits a first write via > aio_write(), gets rescheduled on another CPU and submits a second > overlapping write also via aio_write() ? If a block driver uses one > queue per CPU, does that mean that such writes that were issued in order > can be executed in a different order by the driver and/or hardware than > the order in which the writes were submitted ? > > See also the aio_write() man page, The Open Group Base Specifications > Issue 7, IEEE Std 1003.1-2008 > (http://pubs.opengroup.org/onlinepubs/9699919799/functions/aio_write.html). It is my understanding that the low level driver is free to re-order the i/o's any way it wants, as is the hardware. It is up to the layers above to enforce any ordering requirements. For a long time there was a bug in the cciss driver that all i/o's submitted to the driver got reversed in order -- adding to head of a list instead of to the tail, or vice versa, I forget which -- and it caused no real problems (apart from some slight performance issues that were mostly masked by the Smart Array's cache. It was caught by firmware guys noticing LBAs coming in in weird orders for supposedly sequential workloads. So in your scenario, I think the overlapping writes should not be submitted by the block layer to the low level driver concurrently, as the block layer is aware that the lld is free to re-order things. (I am very certain that this is the case for scsi low level drivers and block drivers using a request_fn interface -- less certain about block drivers using the make_request interface to submit i/o's, as this interface is pretty new to me. If I am wrong about any of that, that would be very interesting to know. -- steve -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
