Re: [SOLVED] Re: system hangs due to qgroups

[Marc Joliet <marcec@gmx.de>]

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On Tuesday 06 December 2016 08:29:48 Qu Wenruo wrote:
> At 12/05/2016 10:43 PM, Marc Joliet wrote:
> > On Monday 05 December 2016 12:01:28 Marc Joliet wrote:
> >>> This seems to be a NULL pointer bug in qgroup relocation fix.
> >>> 
> >>> 
> >>> 
> >>> The latest fix (not merged yet) should address it.
> >>> 
> >>> 
> >>> 
> >>> You could try the for-next-20161125 branch from David to fix it:
> >>> https://github.com/kdave/btrfs-devel/tree/for-next-20161125
> >> 
> >> OK, I'll try that, thanks!  I just have to wait for it to finish
> >> cloning...
> > 
> > [...]
> > 
> >>> And for your recovery, I'd suggest to install an Archlinux into a USB
> >>> HDD or USB stick, and compile David's branch and install it into the USB
> >>> HDD.
> >>> 
> >>> 
> >>> 
> >>> Then use the USB storage as rescue tool to mount the fs, which should do
> >>> RW mount with or without skip_balance mount option.
> >>> So you could disable quota then.
> >> 
> >> OK, I'll try that, thanks!
> > 
> > Excellent, thank you, that worked!  My laptop is working normally again. 
> > I'll keep an eye on it, but so far two balance operations ran normally
> > (that is, they completed within a few minutes and without hanging the
> > system).
> > 
> > (Specifically, since I didn't find out how to get a different kernel onto
> > the Arch USB stick, I simply installed the kernel on my desktop, then did
> > everything from an initramfs emergency shell, then moved the SSD back
> > into the laptop.)
> > 
> > Thanks, everyone!
> 
> Glad that helped.
> 
> I just forgot that you're using gentoo, not archlinux, and kernel
> install script won't work for archlinux.
> 
> Anyway, I'm glad that works for you.
> 
> BTW, if you haven't yet disable quota, would you please give a report on
> how many qgroup you have?

I have disabled quotas already (first thing I did after mounting).  However, 
there were definitely 20-30, maybe more (enough for 2, maybe 3, console pages 
-- I don't know how many lines the initramfs rescue shell has, but based on 
that, you could estimate the number of qgroups).

> And how CPU is spinning for balancing with quota enabled?

All I can say is, based on past observations, that I would see a single 
process (usually btrfs-transaction, but often a user-space process, such as 
baloo_file_extractor) using a single CPU at 100% and blocking (almost) 
everything else, and either finish after a while if it was quick enough, or 
there would be intermittent time frames where other processes weren't blocked.  
With balancing the behaviour was the latter, only it was the btrfs process 
using 100% CPU.  Furthermore, metadata balances were worse than data balances.

> This would help us to evaluate how qgroup slows down the process if
> there are too many snapshots.

Again, sorry that I was so quick to disable quotas, but I was only willing to 
do so much debugging with this laptop.

> Thanks,
> Qu

Greetings
-- 
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup

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