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Re: xfs_force_shutdown after Raid crash

To: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Subject: Re: xfs_force_shutdown after Raid crash
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 4 Feb 2009 23:22:41 +1100
Cc: xfs@xxxxxxxxxxx
In-reply-to: <200902040952.45440@xxxxxx>
Mail-followup-to: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
References: <498376CF.8020806@xxxxxxxxxxxxxx> <200902031140.08576@xxxxxx> <20090203154907.GA21278@xxxxxxxxxxxxx> <200902040952.45440@xxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
On Wed, Feb 04, 2009 at 09:52:45AM +0100, Michael Monnerie wrote:
> On Dienstag 03 Februar 2009 Christoph Hellwig wrote:
> > Yeah, that sounds correct.  Do you volunteer for the FAQ entry?
> >  xfs.org is a wiki so you could add it.  I'm happy to proof-read it
> > if you want.
> I don't know if it's good and correct, I just put this in the wiki, and 
> additionally changed 2 sections, please check the wiki log if it's 
> correct:
> == Q. What about the hard disk write cache? ==
> The problem with hard disk write caches is that their contents are lost 
> in case of a power outage. With hard disk cache sizes of currently up to 
> 32MB that can be a lot of valuable information.
> With a single hard disk and barriers turned on (on=default), a powerfail 
> "only" looses data in the cache but at least does not destroy the 
> filesystem.

I'd drop this paragraph - powerfail can destroy filesystems even on
a single disk (e.g. root directory gets corrupted).

> With a RAID controller with battery backed cache, you should turn off 
> barriers, as recommended above. But then you *must* disable the hard 
> disk write cache in order to ensure to keep the filesystem intact after 
> a power failure.

I'd change this to say "*must* disable the individual hard disk
write caches" to make it clear that it is referencing the disks
behind the raid controller. I'd also say "The method for doing this
is different for each RAID controller. Please consult your RAID
controller documentation to determine how to change these settings."


Dave Chinner

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