On Sun, Jan 04, 2009 at 03:19:27PM -0800, Andrew Morton wrote: > > Still, if we are > > submitting I/O which we are going to end up waiting on, we really > > should submit it with WRITE_SYNC, and this patch should optimize > > writes in other situations; for example, if we fsync() a file, we will > > also end up calling block_write_full_page(), and so supplying the > > WRITE_SYNC hint to the block layer would be a Good Thing. > > Is it? WRITE_SYNC means "unplug the queue after this bh/BIO". By setting > it against every bh, don't we risk the generation of more BIOs and > the loss of merging opportunities? Good point, yeah, that's a problem. Some of IO schedulers also use REQ_RW_SYNC to prioritize the I/O's above non-sync I/O's. That's an orthognal issue to unplugging the queue; it would be useful to be able to mark an I/O as "this is bio is one that we will eventually end up waiting to complete", separately from "please unplug the the queue after this bio submitted". BTW, I notice that the CFQ io scheduler prioritizes REQ_RW_META bio's behind REQ_RW_SYNC bio's, but ahead of normal bio requeuss. But as far as I can tell nothing is actually marking requests REQ_RW_META. What is the intended use for this, and are there plans to make other I/O schedulers honor REQ_RW_META? - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
