On Sun, 31 Jul 2011, Christoph Hellwig wrote: > On Tue, Jul 26, 2011 at 01:36:37PM -0700, Sage Weil wrote: > > Sage Weil (19): > > ceph: add flags field to file_info > > ceph: add F_SYNC file flag to force sync (non-O_DIRECT) io > > Can you document what the exact semantics of this flag are, and how > they different from using O_SYNC/O_DSYNC. I should document this... in the fs/ceph/ioctl.h? Not sure where this normally goes. The CEPH_F_SYNC flag forces synchronous reads/writes that bypass the page cache. It's similar to O_DIRECT, but makes a copy of the data before doing the IO and does not have the alignment restrictions. The real value is that it forces us to take a code path that's hard to trigger otherwise. When a single client writes to a file, we can do buffered IO. Even with O_SYNC/D_SYNC this just causes a flush after every write. When multiple clients open a file for read/write, though, ceph bypasses the page cache and does synchronous IO to the server. It's similar to the O_DIRECT path, but not quite, and doesn't get good test coverage without an easy way to use it. A ceph ioctl sets the CEPH_F_SYNC flag in the file private struct. It's similar to the CEPH_F_LAZYIO ioctl, but with opposite results; that one avoids sync io despite multi-client write sharing (for applications who know what they're doing). sage -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
