http://www.MSExchange.org/
A cluster is a high-availability solution more than a load-balancing
solution. It tends to be more expensive (capital costs) than a
stand-alone
system, and offers the higher-availability in terms of hardware failure.
For your efforts, you basically get an automated "computer restarter"
should
hardware fail.
To achieve this, Microsoft recommends having at least one passive node in
the cluster. For example, if you deploy a 4 node cluster, you should
deploy
A/A/A/P, if 8 way, there's some thoughts that you should have a 6x2
cluster.
All active nodes, brings the PM's thoughts to mind: "For the love of God,
don't do A/A clustering with Exchange on 32-bit platforms!!" Why?
Plenty
of reasons, such as memory limitations (32-bit platform limitations etc).
Suffice to say, they have good reason to recommend this configuration.
Additional downtime and upgrades can be avoided with clusters, since as
you
mentioned, you would have the option of adding an upgraded node, evicting
a
node etc when upgrades are needed. A so-called "rolling upgrade" can be
done resulting in little downtime. Of course, if that's what I wanted, I
could just move mailboxes over to a new server with little impact as well.
Depends on what you need to accomplish.
From a diminishing returns perspective, this would mean instead of a
single
4-way machine, you'd deploy 3 2-way machines and one of them would be
"dark"
at all times waiting for failure. 6 processors, with 4 of them active at
any
given moment.
Given the added expense of hardware (you'll need particular hardware and
storage to make this work) and the added expense of cluster aware software
(third-party cluster-aware apps), plus the learning curve associated,
disaster recovery differences, etc, I'd say that 4-way looks pretty cheap
in
this scenario.
If you need geographical availability, it gets much more complex and
expensive.
Clustering in Microsoft technology is for high-availability, and also does
some load-balancing (you have to load balance it yourself, it's not
automated in MSCS vs. Web load-balancing with NLB type of balancing).
My thoughts anyway.
Al
-----Original Message-----
From: Paul_Lemonidis@xxxxxxxxxxx [mailto:Paul_Lemonidis@xxxxxxxxxxx]
Sent: Friday, January 28, 2005 1:15 PM
To: [ExchangeList]
Subject: [exchangelist] Re: Why 4 Procs
http://www.MSExchange.org/
Hi all
Considering the likely cost of a 4 way box (the law of diminishing returns
sets in fast once you go beyond 2 processors on a single box from I can
discern?) and the disk space that would be required would a cluster of two
two way boxes not be more cost effective?
A cluster has the advantage of spreading the load as well as redundancy.
Also many changes can be made to a cluster installation with no minimal or
no downtime to users. I am thinking primarily of hardware maintenance, O/S
updates (such as new software installation, hotfixes, Windows updates and
Service Packs etc.)
I would be interested to know people's thoughts? Many thanks in advance.
Regards,
Paul Lemonidis.
----- Original Message -----
From: "Mulnick, Al" <Al.Mulnick@xxxxxxxxxx>
To: "[ExchangeList]" <exchangelist@xxxxxxxxxxxxx>
Sent: Friday, January 28, 2005 3:47 PM
Subject: [exchangelist] Re: Why 4 Procs
http://www.MSExchange.org/
Mike, you bring highlight a great point. For a small implementation, a
single proc, if you can find them in a server chassis is likely fine for
just Exchange deployments. 100 users would likely be fine on a laptop if
not for the power-save functions :)
Each deployment will differ greatly. For example, some will have 100
users
per server and of that, the 80-10-10 rule will apply for usage as well as
the 75/25 concept. 10% of the users will reply, "email? I have email? I
didn't know" 10% will be taking 80% of the resource utilization and the
other 80% of the user density will use the server on a normal basis
similar
to the benchmark specs.
After that, you'll have to consider that not everybody uses a server at
the
same instant, so you might expect that 75% of the users would be active
(consuming resources) at a given time while 25% are making the company
money
in other ways. 75% is likely high, but I like to include the incoming
traffic that occurs when they do nothing.
On a 100 user machine, a single proc would be fine most likely. A PDA
might
be enough if not for the storage requirements. On a 1000 user machine
you're odds of seeing it more heavily utilized with larger db's is
higher.
On a 10000 user machine, your odds are even greater.
There's another angle to consider. Are all your users MAPI users? Or
are
some of them using internet protocols? If mixed, your resource
requirements
change yet again. It all needs to be considered.
So you highlight a great point about the sizing of Exchange servers: it
depends. (sounds like something a consultant might say, doesn't it?)
I believe the original poster mentioned 7000 users across 2-4 machines
(or
was it 5000 users?). That would be a density of about 3500 - 1750 per
machine depending on the final design decision. At 3500 user density I
can
tell that in most cases you won't want a dual-proc machine. It might
work
if you have a light or highly geographically dispersed user population
consuming the services and no other apps that suck the life out of the
procs
(like AV solutions tend to do). If you go 1750 per server, you're much
closer to border line. You may want to deploy with a 2 proc solution and
if
that doesn't work, upgrade to 4 way machines if the needs show you
require
it.
Keep in mind what happens if you take Exchange to a sustained proc over
75%.
It doesn't behave as well as you'd like, and any hiccup will result in
even
longer recovery times. Is that important? I think so, because what's
the
point of having email if you can't use it for days at a time? It needs to
be
as reliable as the door systems else it may as well go away.
DR/BC requirements play a part in the decision process, since you may at
some point want to use RSG's to put mail back for some bozo that lost it
and
has to have it.
On a 100 user system, you can likely tell them they'll be without mail
for
a
little while while you do the restore and the processor takes their
resources. Maybe during the lunch hour? On a 3500 user system, you have
much more utilization around the clock in most cases.
Al
-----Original Message-----
From: A. M. Salim [mailto:msalim@xxxxxxxxxxxx]
Sent: Friday, January 28, 2005 9:33 AM
To: [ExchangeList]
Subject: [exchangelist] Re: Why 4 Procs
http://www.MSExchange.org/
Hi,
Bottom line: you get better performance when scaling Exchange with
four processor machines. Fact. You may get acceptable performance on
a two-way machine. If you're really a small shop and can find a
single processor server class machine (I'm sure they're out there, but
I don't see them as
frequently) then you may do just fine with that. In fact, I run
Exchange on a single processor because it's a test lab in a VS
environment. VS 2005 only supports 1 processor for VM. Not a choice
at this point no matter how much hardware is presented.
In an earlier email I asked why even 2-proc let alone 4-proc and
suggested
that perhaps there may be a tendency to over-spec as a CYA measure. Let
me
give you some specifics. Of the Exchange servers we manage, two are
single
CPU servers running P4/2.4 GHz and 512MB of RAM. Each of these two
servers
has about 100 users on it, moderate traffic and mailbox sizes (limited to
100MB or less in most cases).
The servers perform just fine. I routinely monitor the following
performance specs: CPU load, memory percent use, response speed,
complaints
of slowness.
Results:
CPU load: hardly a blip (generally under 5% or 10% load at any time even
at
peak time of day.
Memory: well below 512MB usage. generally around 200MB or less.
Bandwisdth/Network traffic: low usage. Well below 5% ustilization.
Response speed: zero speed complaints in last 12 months (compared to
other
mailservers we have particularly a Windows based iMail server).
Hence my comment about over-spec'd servers. From the emails on this
topic,
the consensus seems to be that a minimum 2-proc server is necessary for
an
Exchange installation, and I just don't see that based on the data I
have.
Best regards
Mike
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