On Wednesday 21 March 2007, Mark Hull-Richter wrote: > I have been chasing down the way the inode and page caches are > structured and handled, but there is a mystery I have not been able to > track down yet. How does the I/O queue that dirty inodes and pages are > put into when it's time to flush them out to disk get picked up? Also, > where are the sources for the I/O schedulers? I haven't been able to > locate them yet either. The actual I/O schedulers are in-kernel thingies that live in the $KERNELSRC/block directory. There are (normally) four: noop, as, deadline and cfq (cfq being the default in centos-4). However, it doesn't seem like to me that you're actually looking for the I/O schedulers but more for why and when the kernel decides to flush dirty pages (I've been known to be wrong though ;-). When a page has yet to be committed to disk it's called a dirty page (see eg. /proc/meminfo). Dirty pages are flushed to disk either explicitly (umount, sync, ..) or periodically (the kernel won't allow a dirty page to age forever). When the they are finally flushed, then the I/O schedulers for the involved block device(s) gets involved as they handle the queueing of all requests for their device. I hope some of that made sense :-) /Peter > I know this should not be too hard to locate, but it seems as though > there's some magic involved when the inode/page gets put in the i/o > queue and then <something happens here> and the i/o is scheduled and, > eventually, performed.
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