[dm-devel] failover vs multibus

[Tore Anderson]

* Scott Moseman

> Re: default_path_grouping_policy in multipath.conf
> 
> failover = 1 path per priority group
> multibus = all valid paths in 1 priority group
> 
> Does this mean that if I'm using failover I'm not going to get
> multiple path throughput?  And, on the flip side, if I'm sending data
> through multiple paths I'm not going to get failover support?

Correct, and incorrect.  With "failover" topology only one path will be
used at a time.  With "multibus" all of them will be - but failing paths
_will not_ be used.  So if you have eight paths to your storage and are
using multibus topology, load will be balanced over all eight paths.  If
one fails, load will be balanced over the remaining seven.  And if you
fix the failed path I/O will be balanced over all eight again.

There's also the group_by_prio or group_by_serial topologies which is
normally used in setups with an active/passive controller pair (most
midrange gear are built in this way).  In this case I/O is load balanced
over all the paths to the primary controller of a volume only, while the
remaining paths (usually to a standby controller) will only be used if
all (or enough) of the primary paths fail.

Regards
-- 
Tore Anderson