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. -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
