Chris Mason <chris.mason@xxxxxxxxxx> writes: > On Thu, Jul 30, 2009 at 08:06:49AM +0200, Jens Axboe wrote: >> 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. > > I dug through my old email and found this fun bug w/buffer heads and > reada. > > 1) submit reada ll_rw_block on ext3 directory block > 2) decide that we really really need to wait on this block > 3) wait_on_buffer(bh) ; check up to date bit when done > > The problem in the bugzilla was that reada was returning EAGAIN or > EWOULDBLOCK, and the whole filesystem world expects that if we > wait_on_buffer and don't find the buffer up to date, its time > set things read only and run around screaming. > > The expectations in the code at the time were that the caller needs to > be aware the request may fail with EAGAIN/EWOULDBLOCK, but the reality > was that everyone who found that locked buffer also needed to be able to > check for it. This one bugzilla had a teeny window where the reada > buffer head was leaked to the world. > > So, I think we can start using it again if it is just a hint to the > elevator about what to do with the IO, and we never actually turn the > READA into a transient failure (which I think is mostly true today, there > weren't many READA tests in the code I could see). Well, is it a hint to the elevator or to the driver (or both)? The one bug I remember regarding READA failing was due to the FAILFAST bit getting set for READA I/O, and the powerpath driver returning a failure. Is that the bug to which you are referring? Cheers, Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
