+folks from linux-mm thread for your suggestion On Wed, Nov 30, 2016 at 01:00:45PM -0500, Austin S. Hemmelgarn wrote: > > swraid5 < bcache < dmcrypt < btrfs > > > > Copying with btrfs send/receive causes massive hangs on the system. > > Please see this explanation from Linus on why the workaround was > > suggested: > > https://lkml.org/lkml/2016/11/29/667 > And Linux' assessment is absolutely correct (at least, the general > assessment is, I have no idea about btrfs_start_shared_extent, but I'm more > than willing to bet he's correct that that's the culprit). > > All of this mostly went away with Linus' suggestion: > > echo 2 > /proc/sys/vm/dirty_ratio > > echo 1 > /proc/sys/vm/dirty_background_ratio > > > > But that's hiding the symptom which I think is that btrfs is piling up too many I/O > > requests during btrfs send/receive and btrfs scrub (probably balance too) and not > > looking at resulting impact to system health. > I see pretty much identical behavior using any number of other storage > configurations on a USB 2.0 flash drive connected to a system with 16GB of > RAM with the default dirty ratios because it's trying to cache up to 3.2GB > of data for writeback. While BTRFS is doing highly sub-optimal things here, > the ancient default writeback ratios are just as much a culprit. I would > suggest that get changed to 200MB or 20% of RAM, whichever is smaller, which > would give overall almost identical behavior to x86-32, which in turn works > reasonably well for most cases. I sadly don't have the time, patience, or > expertise to write up such a patch myself though. Dear linux-mm folks, is that something you could consider (changing the dirty_ratio defaults) given that it affects at least bcache and btrfs (with or without bcache)? By the way, on the 200MB max suggestion, when I had 2 and 1% (or 480MB and 240MB on my 24GB system), this was enough to make btrfs behave sanely, but only if I had bcache turned off. With bcache enabled, those values were just enough so that bcache didn't crash my system, but not enough that prevent undesirable behaviour (things hanging, 100+ bcache kworkers piled up, and more). However, the copy did succeed, despite the relative impact on the system, so it's better than nothing :) But the impact from bcache probably goes beyond what btrfs is responsible for, so I have a separate thread on the bcache list: http://marc.info/?l=linux-bcache&m=148052441423532&w=2 http://marc.info/?l=linux-bcache&m=148052620524162&w=2 On the plus side, btrfs did ok with 0 visible impact to my system with those 480 and 240MB dirty ratio values. Thanks for your reply, Austin. Marc -- "A mouse is a device used to point at the xterm you want to type in" - A.S.R. Microsoft is to operating systems .... .... what McDonalds is to gourmet cooking Home page: http://marc.merlins.org/ | PGP 1024R/763BE901 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>