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Date:   Wed, 16 Aug 2023 00:43:53 +0000
From:   "Dr. David Alan Gilbert" <dave@treblig.org>
To:     Carlos Carvalho <carlos@fisica.ufpr.br>
Cc:     linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: 6.5.0rc5 fs hang - ext4? raid?
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* Dr. David Alan Gilbert (dave@treblig.org) wrote:
> * Carlos Carvalho (carlos@fisica.ufpr.br) wrote:
> > Dr. David Alan Gilbert (dave@treblig.org) wrote on Mon, Aug 14, 2023 at 06:02:53PM -03:
> > >   I'm seeing a few hangs on a fs after upgrading to fedora 39's bleeding
> > > edge; which is running kernel 6.5.0-0.rc5.20230808git14f9643dc90a.37.fc39.x86_64
> > > It was always solid prior to that.  It seems to trigger on heavy IO
> > > on this fs.
> > 
> > Good news! No, I didn't forget the smiley... Maybe now the problem has become
> > sufficiently bad to be visible/solvable...
> > 
> > 6.4.* also doesn't run in one of our machines, which has heavy I/O load. The
> > first symptom is that rsync downloads hang and abort with timeout. 1 or 2
> > days later the amount of modified pages waiting to go to disk reaches several
> > GB, as reported by /proc/meminfo, but disks remain idle. Finally reading from
> > the arrays collapses.
> 
> I'm not sure this is a related fault - I mean it might be, but my
> failure is much more deterministic; it seems solid on 6.4.x to me, but
> just fails reliably somewhere in 6.5.

Actually, you know your description smells a lot closer to the one
Rishabh has just posted today:
   https://lore.kernel.org/lkml/153d081d-e738-b916-4f72-364b2c1cc36a@amazon.com/

Dave

> Dave
> 
> > This is just the worst case. Since early 5.* I/O performance has dropped
> > absurdly. In all our disk servers this is easy to see: just generate lots of
> > writes quickly (for example expanding a kernel tarball). Using top I see that
> > kworker starts using 100% cpu but disks stay idle (as seen by dstat or sar). If
> > you do a sync or umount it takes looooong to reach ~0 modified pages for the
> > sync or umount to return.
> > 
> > In the server I mentioned above where 6.4.* don't stand the load, which is one
> > of the largest free software mirrors of the world, even sometimes 6.1
> > collapses: I/O becomes so slow that service (apache) stops.
> > 
> > The problem gets progressively worse with time after booting. It's hardly
> > noticeable in the first hour after boot, and easily seen after ~3-4 days of
> > uptime. The higher the (write) I/O load the faster it appears.
> > 
> > All this is with ext4 and raid6 with >~ 14 disks in the arrays.
> > 
> > I don't have debug info because these are production machines and I only
> > compile in the kernel the bare minimum essential for operation. It's always
> > pure kernel.org releases; gcc versions vary, for 6.4* it's gcc-13, for 6.1*
> > gcc-12 is used, on Debian unstable updated more than 4 times/week.
> -- 
>  -----Open up your eyes, open up your mind, open up your code -------   
> / Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
> \        dave @ treblig.org |                               | In Hex /
>  \ _________________________|_____ http://www.treblig.org   |_______/
-- 
 -----Open up your eyes, open up your mind, open up your code -------   
/ Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
\        dave @ treblig.org |                               | In Hex /
 \ _________________________|_____ http://www.treblig.org   |_______/