On Mon, 2013-01-21 at 17:12 +0000, Alex Bligh wrote: > Trond, > > --On 21 January 2013 15:50:48 +0000 "Myklebust, Trond" > <Trond.Myklebust@xxxxxxxxxx> wrote: > > >> I don't think QEMU is actually using O_DIRECT unless I set cache=none > >> on the drive. That causes a different interesting failure which isn't > >> my focus just now! > > > > Then your reference to Ian's bug is a red herring. > > > > If the application is using buffered writes, then the data is > > immediately copied from userspace to the page cache. Once the copy to > > the page cache is done, userspace can do whatever it wants with the > > original buffer, because only the page cache pages are used in the RPC > > calls. > > > > aio doesn't change any of this... > > So, just to be clear, if a process is using NFS and AIO with O_DSYNC > (but not O_DIRECT) - which is I think what QEMU is meant to be doing - > then it should *never* be zero copy (even if writes happen to be > appropriately aligned). Is that correct? If so, I can strace the > process and see exactly what flags it is using. > That is correct. If you want zero-copy, then O_DIRECT is your thing (with or without aio). Otherwise, the kernel will always write to disk by copying through the page cache. -- Trond Myklebust Linux NFS client maintainer NetApp Trond.Myklebust@xxxxxxxxxx www.netapp.com -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
