Date: Wed, 4 Feb 2009 16:33:22 +0100
From: Ralf Liebenow <ralf@theco.de>
To: xfs@oss.sgi.com
Subject: Re: xfs_force_shutdown after Raid crash
Message-ID: <20090204153322.GC15454@theco.de>
Reply-To: ralf@theco.de
References: <498376CF.8020806@renderforce.de> <200902031140.08576@zmi.at> <20090203154907.GA21278@infradead.org> <200902040952.45440@zmi.at> <20090204122241.GL24173@disturbed>
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Hello !

Maybe this is a stupid question:

Should Battery backed RAID controllers not always set their discs cache off ?

As I see it (in case of a power failure):
  - the discs are connectet to the main power, so if there is a power
    failure they're offline at that moment in time and their (write) cache will
    be gone in that instance of time too 

  - if they are connected to a battery backed RAID cache, I assume that
    this cache will be written as soon as the system is online again
    (if the battery lasts that long)

  - if a RAID controller does not turn off the disks write cache, the controller
    cannot know if previous writes have made it to the disk. A good
    RAID Controller would also use its cache to re-organise the disc 
    writes to minimize seek times doing somthing like intelligent
    command queuing. This would also mean, that any order of writes
    to a disk could have been changed by the controller. This would
    ultimately break any filesystem which does not explicitly fsyncing
    consistent checkpoints to the disk, which would make battery backed
    RAID Systems pretty useless ... would it ?
  
    So .. a battery backed RAID controller should default to "no disk write cache"
    should it ? Otherwise why should anyone want to use such expensive
    controllers ... it just does not make sense to have a battery backed
    cache on the controller, when things get inconsistent at a power
    outage ... It wouldn't have any purpuse ... I hope developers of
    battery backed RAID controllers are aware of that implication ...

  Greets
    Ralf

> 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."
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
> 
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> 

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