Re: XFS and nobarrier with SSDs

From: Linda Walsh <xfs@tlinx.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: Linux Block mailing list <linux-block@vger.kernel.org>,
	Linux FS-Devel <linux-fsdevel@vger.kernel.org>,
	XFS mailing list <xfs@oss.sgi.com>
Subject: Re: XFS and nobarrier with SSDs
Date: Sat, 26 Dec 2015 15:44:55 -0800	[thread overview]
Message-ID: <567F2677.7040304@tlinx.org> (raw)
In-Reply-To: <20151214102750.GA29192@infradead.org>

Christoph Hellwig wrote:
> The rule of thumb is: if nobarrier makes your workload run faster you
> should not be using it, aka: don't use it. 
----
	So what is the purpose of the switch if it is to only
be used when it makes no difference?

I.e. My raid controller does write-through if it's internal
battery needs replacing, otherwise, it does write-back.

On top of that my system is on a UPS that is good for a hour or more
of running.  

So, I used to use nobarrier on "work" disks where there were likely
to be alot of "writes".  Those disks are also backed up daily via
xfsdump/restore.  I figured those would benefit most, and at worst
I could restore to previous morning's backup.

Eventually stopped using the option, as for the most part, I couldn't
really measure any reliable difference in performance (which means
I should use it?!?).

Hmmm...

The only times I have experienced disk corruption on a single
disk were either back before I ever tried the option, or when
I had several months to a year where I tried to use software
RAID5 (several-10+ years ago, before it was possible to use
multiple cores for doing some RAID operations).

I doubt I'm going to try it again soon, but being told that
it's only "ok" to use an option when it makes no difference
in performance *sounds* more than a little confusing.

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