On Thu, Jul 18, 2013 at 10:59:34PM -0400, Theodore Ts'o wrote: > On Thu, Jul 18, 2013 at 07:56:45PM -0500, Eric Sandeen wrote: > > > The problem is we don't know that we're doing AIO until we see the > > > first io_submit(2) call. With this patch series, we'll pull the > > > contents of the entire leaf tree block into extent cache, but if the > > > extent tree is larger than that, if we read in the entire extent tree > > > on the first AIO request, then that first request will delayed even > > > more, and it's not clear that's a good thing. > > > > Is blocking on a pre-AIO ioctl better than blocking on the > > first AIO? > > The precache ioctl is something which the application is expecting to > block. The question is, if we have a heavily fragmented extent tree, > is it better for the first AIO to block long enough to read in one > metadata block --- and then never block again, or to have that first > AIO request take a long, LONG time? Especially if the application > isn't expecting it? > > Also there are use cases for the precache ioctl even if you are not > using AIO. If you've taken care to make sure the file is as > contiguous as possible, having the extents be cached will save a lot > of memory compared to if the buffer heads are always entering the > buffer cache. So reading in all of the metadata can be a good thing > to do, especially if you can do this *before* you declare that the > server is healthy and is ready to start receiving traffic. An ioctl is kinda silly for this. Just use O_NONBLOCK when calling open() and do the prefetch right in the open call. The open() can block, anyway, and what you are trying to do is non-blocking IO with AIO, so it seems like we've already got a sensible, generic interface for triggering this sort of prefetch operation. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
