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Re: XFS and net devices, any pros or cons ?

To: Steve Lord <lord@xxxxxxx>
Subject: Re: XFS and net devices, any pros or cons ?
From: Simon Matter <simon.matter@xxxxxxxxxxxxxxxx>
Date: Thu, 15 Aug 2002 09:21:58 +0200
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Cc: Greg Freemyer <freemyer@xxxxxxxxxxxxxxxxx>, Blizbor <tb670725@xxxxxx>, linux-xfs@xxxxxxxxxxx
Organization: Sauter AG, Basel
References: <20020814171123.FZMF1197.imf05bis.bellsouth.net@TAZ2> <1029345349.15708.98.camel@jen.americas.sgi.com>
Sender: owner-linux-xfs@xxxxxxxxxxx
Steve Lord schrieb:
> 
> On Wed, 2002-08-14 at 12:08, Greg Freemyer wrote:
> >  >>  The normal way we setup something like this is to use a shared scsi
> >  >>  or fiberchannel disk setup, that way you no longer have a singe
> >  >>  point of failure. If you are putting the disks into one of the
> >  >>  two computers then if that system goes down you are dead in the
> >  >>  water. The other aspect of the above setup is using something
> >  >>  like failsafe, the two nodes monitor each other, and can (if
> >  >>  setup correctly) shoot the other node down and take over the
> >  >>  filesystem if it detects problems. failsafe itself is opensource,
> >  >>  not sure if the components of it which fail over the filesystem
> >  >>  to the other node are. Of course all this assumes using the fs
> >  >>  via NFS - the second node takes over the ip address of the failed
> >  >>  node.
> >
> >  >>  A quick look at drbd on the net seems to show that it is designed
> >  >>  for these sorts of setups, and it has links to all the high
> >  >>  availability stuff for linux.
> >
> >  >>  That aside, there should be no problem doing this, provided you
> >  >>  make sure the unmount from one system is complete before attempting
> >  >>  to mount on the other system. Mounting a filesystem which is
> >  >>  already mounted elsewhere is not a good thing, the second system
> >  >>  will think the fs needs recovery running on it.
> >
> >  >>  Steve
> >
> > I don't think drbd is designed for use in a shared SCSI environment.
> 
> I did not intend to imply it was, just presenting a different way
> to build a resilient configuration.
> 
> >
> > It is more of a RAID 1 driver where the 2 halves of the mirror are on the 2 
> > different servers using internal disks.
> >
> > I believe it has support for ordered writes, but I for one would not simply 
> > assume XFS and drbd are compatible.
> >
> 
> Ah, I should have read more.... if the fs is mirrored between the two
> hosts then there is a chance it will work OK. However, the interesting
> part of XFS is write ordering - there are certain writes which we need
> to know have made it down to disk and will survive a crash. In this
> sort of setup I really do not know where the data will be once drbd
> says it is written. Probably still in cache on the remote box for a
> start. There may well be circumstances where loss of both machines
> will cause filesystem corruption.
> 
> you are right, testing such a setup before going live with it is
> important, please do not take my comments as meaning it will definitely
> work.

If I'm not completely wrong here drbd is similar to nbd, isn't it.
Long time ago I have successfully built a software RAID5 over three
server using nbd, of course running XFS as filesystem.

Simon

> 
> Steve
> 
> > Greg Freemyer
> > Internet Engineer
> > Deployment and Integration Specialist
> > Compaq ASE - Tru64 v4, v5
> > Compaq Master ASE - SAN Architect
> > The Norcross Group
> > www.NorcrossGroup.com
> --
> 
> Steve Lord                                      voice: +1-651-683-3511
> Principal Engineer, Filesystem Software         email: lord@xxxxxxx



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