[Linux-cluster] GFS, iSCSI, multipaths and RAID

Ross Vandegrift ross at kallisti.us
Wed May 28 21:37:13 UTC 2008

On Wed, May 28, 2008 at 02:18:39PM -0700, Alex Kompel wrote:
> I would not use multipath I/O with iSCSI unless you have specific
> reasons for doing so. iSCSI is only as highly-available as you network
> infrastructure allows it to be. If you have a full failover within the
> network then you don't need multipath. That simplifies configuration a
> lot. Provided your network core is fully redundant (both link and
> routing layers), you can connect 2 NICs on each server to separate
> switches and bond them (google for "channel bonding"). Once you have
> redundant network connection you can use the setup from the article I
> posted earlier. This will give you iSCSI endpoint failover.

This depends on a lot of things.  In all of the iSCSI storage systems
I'm familiar with, the same target is provided redundantly via
different portal IPs.  This provides failover in the case of an iscsi
controller failing on the storage system.  The network can be as
redundant as you like, but without multipath, you won't survive a
portal failure.

If you bond between two different switches, you'll only be able to do
failover between the NICs.  If you use multipath, you can round-robin
between them to provide a greater bandwidth overhead.

I'd suggest using multipath.  Check the open-iscsi documentation and
mailing list archives for tips on tuning the timing for those pieces.

Ross Vandegrift
ross at kallisti.us

"The good Christian should beware of mathematicians, and all those who
make empty prophecies. The danger already exists that the mathematicians
have made a covenant with the devil to darken the spirit and to confine
man in the bonds of Hell."
	--St. Augustine, De Genesi ad Litteram, Book II, xviii, 37

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