On Fri, Dec 14, 2012 at 10:44:34AM +0100, Bart Van Assche wrote: > On 12/13/12 17:49, Christoph Hellwig wrote: > >On Thu, Dec 13, 2012 at 05:47:14PM +0100, Bart Van Assche wrote: > >> From my experience with block and SCSI drivers option (1) doesn't > >>look attractive from a performance point of view. From what I have > >>seen performance with QD=1 is several times lower than performance > >>with QD > 1. But maybe I overlooked something ? > > > >What you might be missing is that at least on Linux no one who cares > >about performance uses the Posix AIO inferface anyway, as the > >implementation in glibc always has been horrible. The Linux-native > >aio interface or the various thread pool implementations don't imply > >useless ordering and thus can be used to fill up large queues. > > Some applications need write ordering without having a need for > enforcing durability as fsync() does [1]. What I'm wondering about is > whether an operating system kernel like the Linux kernel should penalize > application performance when using block drivers and storage hardware > that preserve the order of write requests because there exist other > drivers and storage devices that do not preserve the order of write > requests ? Which devices don't re-order requests? So far as I know every single disk drive ever made that is capable of handling multiple requests will also re-order requests as it sees fit. I expect the flash devices re-order requests as well, simply because to feed requests to the things at a sufficient rate, you have to pump requests into them concurrently on multiple hardware queues -- a single cpu jamming requests into them as fast as it can is still not fast enough to keep them busy. Consequently, they *can't* care about ordering, as the relative order requests on different hardware queues are submitted into them is not even really controlled, so the OS *can't* count on concurrent requests not to be essentially "re-ordered", just because of the nature of the way requests get into the device. So I think the property that devices and drivers are free to reorder concurrent requests is not going away. -- 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
