Re: High Availability

Here is my perception, from working in a wide variety of environments.
1. Lowest form of protection - RAID. Lowest cost, but you can survive the 
failure
of a drive. I just replaced the
fourth of 236 17G 10Krpm drives installed across two 820 DSD machines.
THAT's 4 DRIVES IN LESS THAN 3 MONTHS!!
Neither box ever crashed, but once one drive fails, you no longer have ANY
protection
until the failed unit is replaced.

2. Mirroring provides the next level of protection. You can mirror at the drive
level, or all the way to the buss level.
You can suffer multiple drive failures, yet the system can continue to run. We
have 4 machines  in this configuration.
You may get increased performance in a read intensive environment, due to the
larger number of arms present.

3. System Mirroring/replication - Even Higher level of availability. If properly
configured, a remote system that is
a real-time mirror provides high availability, but not non-stop. Most systems
would require a reconnect after
failure of the primary system, the inconvenience of this depends on the
applications supported.

Now the variations:
Raid OR Mirrored DASD in Mirrored/replicated systems. If drive(s) fail, primary
system continues non-stop, no switch over to mirrored system required. 
Typically,
disk failures seem more prevelant than other components, giving disk level
protection a greater chance of preventing problems. The systems I have that are
RAID protected are clustered and replicated by Domino, allowing very high fault
tolerance.

Loss of the physical facilities can stop your systems also. Fire, Flood, Ice
storms, Earthquake, Vandalism, Sabatoge, etc... can stop your systems cold. If
you have replicated system located elsewhere, IT can provide it's part of the
bsuiness. But can the rest of the business function? If a factory is flooded, do
they need their manufacturing system working for that plant? This needs to be
factored in.

Also to be considered is use of off-site recovery facilities where system can be
restored using back-up that are stored off-site. This is an alternative for
single location shops using established recovery centers. In a multi location
environment,
proper sizing of systems would allow in-house recovery at a different location.
As an added benefit, the systems for the recovery would be able to use the
"recovery" capacity when t a recovery was not in progress.

IBM has several pubs/redbooks on High Availability. All of the system mirroring
folks will be glad to talk with you. A word of advice on the system mirroring.
Each vendor has strengths and weaknesses. If a particular vendor's package
appeals to you, customer references would be in order. The key is to make sure
that these references include systems running the same or similar applications.
This would make sure that your environment doesn't tread in one of the vendor's
weaker areas.

Bob

DanDaFourOOMan wrote:

> I am needing to consider high availability options for our new
> AS/400s, which I am responsible for configuring.  A recent
> thread discussed RAID vs. mirroring.  Another thread mentioned a
> product named Mimix, which I've seen advertised.
>
> The new AS/400s are replacing old ones and we have one in each
> of our five plants spread across the U.S., one in Mexico, and
> one in England.  Needless to say, I need to be able to manage
> these systems and diagnose and solve problems from our corporate
> office.  Management appears to be opening the wallet to ensure
> HA.  Some critical considerations: Unplanned outages of 8 or
> more hours would hurt us.  Greater than 24 hours and I'll be
> passing out resumes.
>
> Where can I go to read up on the issues I need to consider for
> HA?
>
> Any advice on which way to go on RAID vs. mirroring?
>
> Any other advice?
>
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