[olug] SAN Information

Fri Sep 27 20:02:27 UTC 2002

This is an interesting idea, but (I think) it kind of boils down to a
remote RAID attached via FC.   There's no processing unit, right?  While
this gives you some of the benefits of a SAN appliance it doesn't give you
a lot of the really useful features I've seen (at least I don't see how it
does, please correct me if I'm wrong).  I think this is referred to as a
JBOD.  (Just a bunch of disks)

For instance, using the software with a Magnitude I could create a new
virtual disk (vdisk) from unused space.  I could then  mirror it to an
existing disk.  Once it was mirrored I could break the mirror and I have a
perfect copy of that data.  I could back it up, assign it to another
server, upgrade it, or do anything else I wanted to it, and my original
data is unaffected, and the server OS never even knows anything about this.
Let's say I make a mirror, then upgrade the mirror copy to a newer version
of software and test it.  If it works, I can then SWAP the two disks with
my production server.  If it doesn't, no biggy, delete the vdisk and start
over.

Oh, one thing I forgot to mention that's unique to the Xiotech Magnitude is
the ability to change RAID levels on the fly.  You can just select a vdisk
and tell to to change from a RAID 5 to a RAID 10, or a RAID 5/parity 3 to a
RAID 5/parity 9, etc.

As pretty much everyone has said, it all boils down to what you need.  :-)
The more you need, the more money you'd better have.

Bill

                      "Rogers, John C NWD02"                                                                                 
                      <John.C.Rogers at nwd02.usac        To:       "'olug at olug.org'" <olug at olug.org>                           
                      e.army.mil>                      cc:                                                                   
                      Sent by:                         Subject:  RE: [olug] SAN Information                                  
                      olug-admin at olug.org                                                                                    

                      09/27/2002 02:09 PM                                                                                    
                      Please respond to olug                                                                                 

The software depends on how fancy you want to be.  If you use LUN masking
etc. in the controller or switch then each host can only see the LUN the
controller lets it see via hardware addressing on the loop.  Now some say
that is not a true SAN but I use the definition that multiple hosts are
using one physical disk array so I call it a SAN.  Now if you add software
to the mix then a whole bucket of possibilities opens up.  You can choose
Veritas or the software from the SAN vendor.  In this case the SAN is more
like another host on the loop but it's job is to store and retrieve data
to/from the disks.  These more advanced SANs allow you to do all sorts of
cool things like have been discussed (volume management, filesystem
resizing, cache stripe optimization, disk block size masking and the like).
In that environment the host never really "owns" the data is how I look at
it.  The data belongs to the SAN and it is served to the host as it
requests it.

In my environment the host really owns the data because it probes the loop
for the LUNs and then attaches to them and will get really mad if it does
not see them (it is not a virtual volume).  What is cool is that if you use
fibre channel drives and fibre controllers they are all dual attached by
design.  So you can build a completely separate data path to the disks for
redundancy if you want to.  Again the more you add the more you pay.  In
Sun's case they have redundant interface software that watches for hardware
failure and can switch to the secondary path if needed.  This is basically
Veritas software under the covers.  The really big arrays spend a lot of
effort to make them fast.  Some big Hitachi units have over 10GB of disk
cache (ram) that the CPU manages to allocate the writes in the most
efficient manner.  There is a world of difference between what I use and
those units.  We wanted something reasonable in cost, expandable, reliable
and vendor neutral for upgrading in the future.  So we built our system.

John

-----Original Message-----
From: roger schmeits [mailto:schmeits at clarksoncollege.edu]
Sent: Friday, September 27, 2002 1:25 PM
To: olug at olug.org
Subject: Re: [olug] SAN Information

Ok. lets say we build the SANs. Buy all the hardware ans so forth. Dont
you need software to interface with the different o/s?

That where it gets pricey right?  I understand the hardware part but I
thought there had to something in between the servers.

Please correct me if I am wrong.

Congrads on building your own SANs ..impressive..

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