On Sunday 28 January 2007 16:18, Bill Davidsen wrote: > Denis Vlasenko wrote: > > On Friday 26 January 2007 19:23, Bill Davidsen wrote: > >> Denis Vlasenko wrote: > >>> On Thursday 25 January 2007 21:45, Michael Tokarev wrote: > >>>> Phillip Susi wrote: > > [...] > > >>>> But even single-threaded I/O but in large quantities benefits from > >>>> O_DIRECT > >>>> significantly, and I pointed this out before. > >>> Which shouldn't be true. There is no fundamental reason why > >>> ordinary writes should be slower than O_DIRECT. > >>> > >> Other than the copy to buffer taking CPU and memory resources. > > > > It is not required by any standard that I know. Kernel can be smarter > > and avoid that if it can. > > Actually, no, the whole idea of page cache is that overall system i/o > can be faster if data sit in the page cache for a while. But the real > problem is that the application write is now disconnected from the > physical write, both in time and order.
Not in O_SYNC case. > No standard says the kernel couldn't do direct DMA, but since having > that required is needed to guarantee write order and error status linked > to the actual application i/o, what a kernel "might do" is irrelevant. > > It's much easier to do O_DIRECT by actually doing the direct i/o than to > try to catch all the corner cases which arise in faking it. I still don't see much difference between O_SYNC and O_DIRECT write semantic. -- vda - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
