[Bug 71641] Unreasonable performance degradation in ext4 with full data journaling

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https://bugzilla.kernel.org/show_bug.cgi?id=71641

--- Comment #2 from Chia-Hung Chang <fredchang.tc@gmail.com> ---

> 
> How big was the ramdisk?  Since all of the blocks are going through
> the journal, even if it is on the journal, it requires more commits
> and thus more checkpoint operations, which means more updates to the
> disk.  A bigger journal will help minimize this issue.
> 
> Would you be willing to grab block traces for both the disk and the
> external journal device?
> 
> I will add that the workload of "dd if=/dev/zero of=file" is probably
> the worst case for data=journal, and if that's really what you are
> doing, it's a case of "doctor, doctor, it hurts when I do that".  All
> file systems modes will have strengths and weaknesses, and your use
> case one where I would simply tell people, "don't use that mode".
> 
> If you want to work on improving it, that's great.  Gather data, and
> if we can figure out an easy way to improve things, great.  But I'll
> warn you ahead of time this is not necessarily something I view as
> "unreasonable", nor is it something that I would consider a high
> priority thing to fix.
> 
                      - Ted
I use two sizes of ramdisk, 128MB and 1024MB. With 1024MB journal area, the
performance is slight improved. But performance degradation is still
significant. 

I am willing to grab block traces. Please tell me how to get the traces you
want.

As you can see that the performance degradation of applying data=journal in
raid5 is 80%, which makes it hard to use. If I know where the problem is, I
will try to improve it.

Thanks for your help.

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