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Re: XFS filesystem corruption

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: XFS filesystem corruption
From: Julien FERRERO <jferrero06@xxxxxxxxx>
Date: Fri, 8 Mar 2013 11:16:27 +0100
Cc: Ric Wheeler <rwheeler@xxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
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>
> Yes they were. XFS had barrier support added in 2.6.15.
>

The XFS was running over a software RAID0 or software RAID5 depending
on the product. In both case, my understanding is that either software
RAID0 or software RAID5 did not support barrier in 2.6.18.

> Yes, that would have been me that said that. I started seeing lots
> of boy-racer "tweak your filesystem to go faster" blogs recommending
> that unwritten extents should be turned off high up in google
> results, with numbers to prove that it improved performance.
>
> There were two common things wrong with these blogs:
>
>         1. None of them mentioned that turning off unwritten extents
>         exposes stale data to users. i.e. a whopping great big
>         security hole.
>
>         2. they reported significant performance improvements for
>         workloads that *didn't use unwritten extents* when they set
>         this flag. i.e. they mistook run-to-run variablity of the
>         benchmark for a performance improvement. i.e. Benchmarking
>         101 Fail.
>
> When you get people who do not understand what they are doing and
> giving bad advice as the first 10 hits for a google search about
> optimising/tuning XFS filesystems, it's a major concern, and so I
> took steps to ensure you can't turn off unwritten extents with
> mkfs...

Thanks for the explanation.

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