On Fri, Apr 16, 2010 at 10:50:02AM +0100, Alan Cox wrote: > > No. If you are doing full disk seeks between random chunks, then you > > still lose a large amount of throughput. e.g. if the seek time is > > 10ms and your IO time is 10ms for each 4k page, then increasing the > > size ito 64k makes it 10ms seek and 12ms for the IO. We might increase > > throughput but we are still limited to 100 IOs per second. We've > > gone from 400kB/s to 6MB/s, but that's still an order of magnitude > > short of the 100MB/s full size IOs with little in way of seeks > > between them will acheive on the same spindle... > > The usual armwaving numbers for ops/sec for an ATA disk are in the 200 > ops/sec range so that seems horribly credible. Yeah, in my experience 7200rpm SATA will get you 200 ops/s when you are doing really small seeks as the typical minimum seek time is around 4-5ms. Average seek time, however, is usually in the range of 10ms, because full head sweep + spindle rotation seeks take in the order of 15ms. Hence small random IO tends to result in seek times nearer the average seek time than the minimum, so that's what i tend to use for determining the number of ops/s a disk will sustain. > But then I've never quite understood why our anonymous paging isn't > sorting stuff as best it can and then using the drive as a log structure > with in memory metadata so it can stream the pages onto disk. Read > performance is goig to be similar (maybe better if you have a log tidy > when idle), write ought to be far better. Sounds like a worthy project for someone to sink their teeth into. Lots of people would like to have a system that can page out at hundreds of megabytes a second.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
