Dave Chinner <david@xxxxxxxxxxxxx> writes: > 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. Hmm, O_NONBLOCK on regular files, eh? That brings back memories: http://www.gossamer-threads.com/lists/linux/kernel/481855 I don't recall exactly how that ended, but I'm pretty sure the conclusion was that it was a bad idea. -Jeff -- 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
