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Message-ID: <20100219210517.GF28392@discord.disaster> Date: Sat, 20 Feb 2010 08:05:17 +1100 From: Dave Chinner <david@...morbit.com> To: Michael Breuer <mbreuer@...jas.com> Cc: Jan Kara <jack@...e.cz>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: Hung task - sync - 2.6.33-rc7 w/md6 multicore rebuild in process On Fri, Feb 19, 2010 at 12:31:13AM -0500, Michael Breuer wrote: > On 2/18/2010 11:02 PM, Dave Chinner wrote: >> On Thu, Feb 18, 2010 at 09:31:41PM -0500, Michael Breuer wrote: >> >>> On 2/18/2010 8:43 PM, Dave Chinner wrote: >>> >>>> >>>> This is probably where the barrier IOs are coming from. With a RAID >>>> resync going on (so all IO is going to be slow to begin with) and >>>> writeback is causing barriers to be issued (which are really slow on >>>> software RAID5/6), having sync take so long is not out of the >>>> question if you have lots of dirty inodes to write back. A kernel >>>> compile will generate lots of dirty inodes. >>>> >>>> Even taking the barrier IOs out of the question, I've seen reports >>>> of sync or unmount taking over 10 hours to complete on software >>>> RAID5 because there were hundreds of thousands of dirty inodes to >>>> write back and each inode being written back caused a synchronous >>>> RAID5 RMW cycle to occur. Hence writeback could only clean 50 >>>> inodes/sec because as soon as RMW cycles RAID5/6 devices start >>>> they go slower than single spindle devices. This sounds very >>>> similar to what you are seeing here, >>>> >>>> i.e. The reports don't indicate to me that there is a bug in the >>>> writeback code, just your disk subsystem has very, very low >>>> throughput in these conditions.... >>>> >>> Probably true... and the system does recover. The only thing I'd point >>> out is that the subsystem isn't (or perhaps shouldn't) be this sluggish. >>> I hypothesize that the low throughput under these condition is a result >>> of: >>> 1) multicore raid support (pushing the resync at higher rates) >>> >> Possibly, though barrier support for RAID5/6 is shiny new as well. >> >> >>> 2) time spent in fs cache reclaim. The sync slowdown only occurs when fs >>> cache is in heavy (10Gb) use. >>> >> Not surprising ;) >> >> >>> I actually could not recreate the issue until I did a grep -R foo /usr/ >>> >>>> /dev/null to force high fs cache utilization. For what it's worth, two >>>> >>> kernel rebuilds (many dirty inodes) and then a sync with about 12Mb >>> dirty (/proc/meminfo) didn't cause an issue. The issue only happens when >>> fs cache is heavily used. I also never saw this before enabling >>> multicore raid. >>> >> "grep -R foo /usr/" will dirty every inode that touchs (atime) and >> they have to be written back out. That's almost certainly creating >> more dirty inodes than a kernel build - there are about 400,000 >> inodes under /usr on my system. That would be enough to trigger very >> long sync times if inode writeback is slow. > > My filesystems are mounted relatime. If the inode atime is older than a day, then they will still have atime updated (i.e. be dirtied) and need writing out. Relatime only reduces the number of atime updates; it doesn't prevent them entirely like noatime does. Cheers, Dave. -- Dave Chinner david@...morbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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