[ale] Sportin' SCSI/RAID

[Eric Z. Ayers]

If I had a dime for every time I heard the guys familiar with VMS at
our office bemoan the shortcomings of UNIX clustering!

It is likely that the SCSI reservation feature is optional...

One difference is, that in VMS there is not a large filesystem cache
like there is in UNIX, which would make it almost impossible to keep
the filesystem consistent if mounted on multiple machines.  We have
installaons with many hundreds of megabytes on outboard RAID
controllers.  Compaq is introducing a new version of their clustering
with Tru64Unix (was Digital Unix) version 5.0A. But my understanding
is that the way they provide access is through a cluster interconnect
(they call it memory channel) - still only one machine actually mounts
the drive. 

-Eric.

Jeff Hubbs writes:
 > Eric -
 > 
 > My only real clustering experience is under VMS, in which case the systems do all
 > mount the same volumes and in some cases do it over a SCSI channel.  The DEC
 > AlphaServers I worked with most recently had outboard RAID controllers and shared a
 > set of disks between them, cut up into several RAID arrays.  I'm not trying to
 > contradict you, though - I have every expectation that there was some mechanism that
 > made one controller or the other "lay out" while one worked the drives.  At the time
 > I got laid off, we were working on an Ultra-SCSI-2 SAN connecting three nodes.  But,
 > yes, it's looking like in Linux at least, what I'm
 > finding is that there's not a similar mechanism, at least not yet.
 > 
 > Actually, I was not looking at this in terms of a cluster per se - I was trying to
 > work out a possible arrangement of machines for a public/private Internet Web server
 > in which one machine ran Apache and MySQL (but only reading a database) and the other
 > ran Apache and MySQL (read/write).
 > 
 > - jeff
 > 
 > 
 > "Eric Z. Ayers" wrote:
 > 
 > > Hello Jeff,
 > >
 > > Most UNIX clustering solutions don't allow both machines to mount the
 > > same SCSI target,LUN at the same time.  In fact, the SCSI protocol has
 > > this concept called a reservation which explicitly prohibits two host
 > > adapters from talking to the same device simultaneously.
 > >
 > > -Eric.
 > >
 > > Jeff Hubbs writes:
 > >  > Does anyone here have any experience with sharing a RAID array between two
 > >  > Linux systems over a common SCSI bus?  What is required?  What are the
 > >  > gotchas?  Is it less of a problem if one of the two machines only reads from
 > >  > the RAID volume?
 > >  >
 > >  > If the shared RAID array in question held, say, a MySQL database, how would
 > >  > both machines access it (again, presuming one machine only reads)?  An
 > >  > instance of MySQL on each system?
 > >  >
 > >  > - Jeff
 > >  >
 > >  > ** Happy Holidays to the ALE! **
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