🚀 go-pugleaf

Steve Holly <steve@sbholly.com> wrote in
news:BBD14FB8.16%steve@sbholly.com: 

> I'm an IT admin starting to look at building a SAN for my company and
> I'm curious if anyone out there can explain how some of the popular
> RAID vendors (i.e. EMC, Chaparral, Infortrend) handle failover
> (resuming I/O with another RAID controller after one has failed)? I'm
> mostly interested in failover on the storage side (as opposed to the
> host side). 
> 
> Specifically I'm interested in knowing if failover is generally
> accomplished by a surviving controller taking over the failed
> controller's (or failed port's) AL_PA('s) or if surviving controllers
> actually alias failed controller's WWN's?
> 
> Or is this something that's generally handled at the switch level.
> 
> I'm trying to better understand how failover is accomplished
> transparent to the host. Many thanks for any input regarding this.
> 
> 

The EMC Symetrix storage units have duplicate everything... duplicate 
incomming fibre cards, duplicate bus to scsi cards, duplicate scsi cards, 
and duplicate scsi bus to disk arrays.

I believe the state of any redundant scsi cards is kept the same at any 
time, even if only one is actually writing.  The software is smart enough 
to know what is committed to disk and what is not, and so can recover from 
write errors.  Of course everything is battery backed up.