On Thu, Jul 30, 2009 at 12:47:21PM -0400, Jeff Moyer wrote: > 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)? I would say both as long as they don't fail it. IOW a priority decision instead of a discard this request at will decision. > 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? This was a rhel bug with ext3 and (both dm and powerpath ) multipath, but in theory it could be triggered on regular drives. I don't think we ever managed to, but removing READA definitely fixed it. It was bug 213921 in the RH bugzilla, and I think it had been fixed in other ways in mainline by the time we found it. -chris -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
