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
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 ...
> 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
> xfs mailing list
HRB 78053, Amtsgericht Charlottenbg
Vorstand: Ralf Liebenow, Michael Oesterreich, Peter Witzel
Aufsichtsratsvorsitzender: Wolf von Jaduczynski
Oranienstr. 10-11, 10997 Berlin [×]
fon +49 30 617 897-0 fax -10