On Thursday 25 January 2007 16:44, Phillip Susi wrote: > Denis Vlasenko wrote: > > I will still disagree on this point (on point "use O_DIRECT, it's faster"). > > There is no reason why O_DIRECT should be faster than "normal" read/write > > to large, aligned buffer. If O_DIRECT is faster on today's kernel, > > then Linux' read()/write() can be optimized more. > > Ahh but there IS a reason for it to be faster: the application knows > what data it will require, so it should tell the kernel rather than ask > it to guess. Even if you had the kernel playing vmsplice games to get > avoid the copy to user space ( which still has a fair amount of overhead > ), then you still have the problem of the kernel having to guess what > data the application will require next, and try to fetch it early. Then > when the application requests the data, if it is not already in memory, > the application blocks until it is, and blocking stalls the pipeline. > > > (I hoped that they can be made even *faster* than O_DIRECT, but as I said, > > you convinced me with your "error reporting" argument that reads must still > > block until entire buffer is read. Writes can avoid that - apps can do > > fdatasync/whatever to make sync writes & error checks if they want). > > > fdatasync() is not acceptable either because it flushes the entire file.
If you opened a file and are doing only O_DIRECT writes, you *always* have your written data flushed, by each write(). How is it different from writes done using "normal" write() + fdatasync() pairs? > This does not allow the application to control the ordering of various > writes unless it limits itself to a single write/fdatasync pair at a > time. Further, fdatasync again blocks the application. Ahhh shit, are you saying that fdatasync will wait until writes *by all other processes* to thios file will hit the disk? Is that thue? -- 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/
