linux-ext4 - Re: parity raid and ext4 get stuck in writes

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Date: Sun, 24 Dec 2023 23:39:05 -0800
From: Daniel Dawson <danielcdawson@...il.com>
To: Carlos Carvalho <carlos@...ica.ufpr.br>, linux-ext4@...r.kernel.org,
 linux-raid@...r.kernel.org
Subject: Re: parity raid and ext4 get stuck in writes

On 12/22/23 12:48 PM, Carlos Carvalho wrote:
> This is finally a summary of a long standing problem. When lots of writes to
> many files are sent in a short time the kernel gets stuck and stops sending
> write requests to the disks. Sometimes it recovers and finally sends the
> modified pages to permanent storage, sometimes not and eventually other
> functions degrade and the machine crashes.
>
> A simple way to reproduce: expand a kernel source tree, like
> xzcat linux-6.5.tar.xz | tar x -f -
This sounds almost exactly like a problem I was having, right down to 
triggering it by writing the files of a kernel tree, though the details 
in my case are slightly different. I wanted to report it, but wanted to 
get a better handle on it and never managed it, and now I've changed my 
setup such that it doesn't happen anymore.
> - it happens only with ext4 on a parity raid array

This is where it differs for me. I experienced it only with btrfs. But I 
had two arrays with it, one on SSDs and one on HDDs. The HDD array 
exhibited the problem almost exclusively (the SSDs, I think, exhibited 
it once in several months, while the HDDs did pretty much every time I 
tried to compile a new kernel (until I started working around it), and 
even from some other things, which was a couple of times a week). I 
imagine because HDDs much slower and therefore allow more data to get 
cached.

Now that I've switched the HDD array to ext4, I haven't experienced the 
issue even once. But the setup has better performance, so maybe it's 
just because it flushes its writes faster.

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