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Re: [PATCH] disable queue flag test in barrier check

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: [PATCH] disable queue flag test in barrier check
From: Timothy Shimmin <tes@xxxxxxx>
Date: Fri, 27 Jun 2008 14:51:41 +1000
Cc: xfs-oss <xfs@xxxxxxxxxxx>, LinuxRaid <linux-raid@xxxxxxxxxxxxxxx>, NeilBrown <neilb@xxxxxxx>, jeremy@xxxxxxxxx
In-reply-to: <486398B7.50306@xxxxxxxxxxx>
References: <486307EA.7080007@xxxxxxxxxxx> <48635284.3060001@xxxxxxx> <486398B7.50306@xxxxxxxxxxx>
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Eric Sandeen wrote:
> Timothy Shimmin wrote:
> 
>> Also from memory, I believe Neil checked this removal into the SLES10sp1 tree
>> and some sgi boxes started having slow downs
>> (looking at Dave's email below - we were not wanting to tell them
>> to use nobarrier but needed it to work by default - I forget now).
> 
> But that's an admin issue.
> 
> The way it is now, for example a home user of md raid1 (me!) can't run
> barriers even if they wanted to.
> 
I understand what you are saying. I agree. And I agreed
when it went out last time.

But as it has:
-> gone in
<- gone out
-> gone in
I want to make sure that everyone is happy for it to go
back out again. (Cut the string of the yoyo :-)


> Until there is a way to know if a write cache is non-volatile the only
> safe option is to enable barriers when possible.
> 
>> 6.
>>> Date: Wed, 25 Jun 2008 08:57:24 +1000
>>> From: Dave Chinner <david@xxxxxxxxxxxxx>
>>> To: Eric Sandeen <sandeen@xxxxxxxxxxx>
>>> Cc: LinuxRaid <linux-raid@xxxxxxxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
>>> Subject: Re: md raid1 passes barriers, but xfs doesn't use them?
>>>
>>> Yeah, the problem was that last time this check was removed was
>>> that a bunch of existing hardware had barriers enabled on them when
>>> not necessary (e.g. had NVRAM) and they went 5x slower on MD raid1
>>> devices. Having to change the root drive config on a wide install
>>> base was considered much more of support pain than leaving the
>>> check there. I guess that was more of a distro upgrade issue than
>>> a mainline problem, but that's the history. Hence I think we
>>> should probably do whatever everyone else is doing here....
>>>
>>> Cheers,
>>>
>>> Dave.
>> So I guess my question is whether there are cases where we are
>> going to be in trouble again.
>> Jeremy, do you see some problems?
> 
> FWIW, the problem *I* foresee is that some people are going to slow down
> when using the defaults, yes, because barriers will start working again.
>  But I don't see any other safe way around it.
> 
> Education would be in order, I suppose.  :)
> 
Well that's an ongoing problem.
(Tell 'em to use laptop drives ;-))

--Tim


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