On Wed, Jul 29 2009, Chris Mason wrote: > On Wed, Jul 29, 2009 at 11:18:45PM +0200, Jens Axboe wrote: > > On Wed, Jul 29 2009, Lars Ellenberg wrote: > > > I naively assumed, from the "readahead" in the name, that readahead > > > would be submitting READA bios. It does not. > > > > > > I recently did some statistics on how many READ and READA requests > > > we actually see on the block device level. > > > I was suprised that READA is basically only used for file system > > > internal meta data (and not even for all file systems), > > > but _never_ for file data. > > > > > > A simple > > > dd if=bigfile of=/dev/null bs=4k count=1 > > > will absolutely cause readahead of the configured amount, no problem. > > > But on the block device level, these are READ requests, where I'd > > > expected them to be READA requests, based on the name. > > > > > > This is because __do_page_cache_readahead() calls read_pages(), > > > which in turn is mapping->a_ops->readpages(), or, as fallback, > > > mapping->a_ops->readpage(). > > > > > > On that level, all variants end up submitting as READ. > > > > > > This may even be intentional. > > > But if so, I'd like to understand that. > > > > I don't think it's intentional, and if memory serves, we used to use > > READA when submitting read-ahead. Not sure how best to improve the > > situation, since (as you describe), we lose the read-ahead vs normal > > read at that level. I did some experimentation some time ago for > > flagging this, see: > > > > http://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff;h=16cfe64e3568cda412b3cf6b7b891331946b595e > > > > which should pass down READA properly. > > One of the problems in the past was that reada would fail if there > wasn't a free request when we actually wanted it to go ahead and wait. > Or something. We've switched it around a few times I think. Yes, we did used to do that, whether it was 2.2 or 2.4 I don't recall :-) It should be safe to enable know, whether there's a prettier way than the above, I don't know. It works by detecting the read-ahead marker, but it's a bit of a fragile design. -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
