[WBEL-users] WBEL, Clustering, GFS, and redundancy

[Kirby C. Bohling]

On Fri, Apr 22, 2005 at 06:29:53PM -0400, Luke Scharf wrote:
> Kirby C. Bohling wrote:
> 
> >I thought you needed a Multi-Path SAN, firewire, or a SCSI equipment
> >where you connected the physical drives themselves to multiple
> >machines, to have GFS work.  I didn't think it was "NFS-like" in
> >that it provided network transport on the backend.
> > 
> >
> Doesn't iSCSI provide a block device with a network transport on the 
> back end?

Yes.  I mentioned it in the paragraph after the quoted section.
iSCSI and nbd (a poor man's version of iSCSI from what I can tell),
would both probably work with GFS (as iSCSI and nbd appear as local
block devices).  iSCSI essentially talks the SCSI protocol over
TCP/IP, like you can talk SCSI over fibre instead of a standard SCSI
bus.  The only question is if they support enough of the locking
primatives that GFS uses (iSCSI should, nbd I have no idea if nbd
even supports having multiple people attached to the same device).

I didn't explicitly mention it, but GFS is really to be used in
situations where you want "NFS", but NFS has some limitations that
will cause you problems.  You use NFS where you want multiple
clients to have access to the same filesystem concurrently.  GFS is
really good at that.

A lot of applications can't be run over NFS, or have to be
configured special to work around problems with NFS.  Oracle can't
run over NFS unless it passes some fairly stringent testing.  Most
NFS server/clients don't.  If they are latency (stat over NFS/AFS is
brutally slow, it's why some applications, CVS and subversion come
to mind, run horribly over NFS), race conditions (things like each
client doing local caching), or that you generally can't mmap the
files (I'm not positive you can mmap GFS files, but I believe it
works).

I'd like to use GFS in some situations with Oracle, as it would be
feasible to both filesystems mounted at the same time.  It would
make me more comfortable with the fail over.  However, it's still
not shipping as a part of any Enterprise ready Linux distro I know
of.  (I don't think it's part of the supported portions of RHEL4
release, but I haven't looked into RHEL4 closely.  I know RedHat
didn't make big noises about it if it is.  I'm not ready to start
using RHEL4 as an Oracle Server yet anyways).

> 
> Of course, the devices on your SAN have to support iSCSI.

You really don't need a SAN to run iSCSI, but you'll definitely get
much better performance if you do.  You should be able to run it
over a stock switch and ethernet card.

    Kirby