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Introduction
Recently, an independent testing laboratory found NT on a 4 way Xeon
processor machine to be faster than Linux on the same hardware. While
this tidbit is interesting for many reasons, not the least of which is
the questionability of the testing procedures and the name calling and
mud slinging that has ensued, most people have missed the really important
point, which is: Quad Xeon processor boxes are a very weak value
proposition for Web servers.
Let's do a quick case study of the price / performance ratio of a quad
Xeon processor box versus a load balanced server farm suitable for either
SMB file serving or Web serving. The extra details pertaining to
network configuration will not be covered in this document, only the
servers
themselves.
First, the prices for hardware I will be quoting will be from May 6,
1999. I will use Dell as the standard supplier, although any supplier
will likely give similar prices for the type of machines we are looking
for.
Objective
Build a solid, reliable server for enterprise class use. Uptime should
be as high as possible, preferably 100%. Cost should be kept low,
but performance should not suffer simply to save money.
Specifications
The server(s) should be able to provide 100Mbit/sec or better throughput
under load as either a file server or as a static content HTML server.
It should house 25 Gigabytes of online storage in a RAID
or redundant configuration.
Discussion
While the quad Xeon machine can meet and exceed most of these goals, it
does so at tremendous expense. The cost of a machine of this caliber
runs from between $25,000 and $50,000, depending on what options you
choose.
That's a big chunk of change no matter how you look at it.
Let's compare that to a farm of Linux (or FreeBSD/NetBSD/OpenBSD) boxes
running under load balanced switches. Since a web serving farm scales
at a nearly linear rate, there is no sense in buying the fastest machines
made. What we want to shoot for is the most bandwidth per buck in
each unit. For a Linux farm, dual CPU machines represent a fairly
good trade off of price and performance.
We basically want to maximize performance and minimize cost. Since
licensing isn't an issue with Linux or FreeBSD, and the energy required
for each machine is fairly low, we probably have to worry more about shelf
space than anything else.
From Dell, a Dual Pentium III 450 with 256 Megs of RAM, dual 100BastTX
NICs,
low end video card, and four 9 Gig Ultra Wide hard drives sells for $4636.
Four of these machines will cost us $18,544 plus shipping. We'll
call it $20,000 for four machines. Note that we will have a total
of eight 450 MHz CPUs, with a total of 1 Gig of RAM between them. Note
that if you don't already have a load balancing switch to use, you may
need to buy another machine for $3,000 for a machine to balance to the
loads. While this machine will not need the large RAID array of the
of the farm servers, it will need plenty of memory (hey, if you're
building a balancer, you should put squid on it too) and four or more
NICs, and probably a FDDI card as well. That will bring our bill
to just under the cost of the lower end Quad Xeon machines.
Comparison Chart
Quad Xeon
Farm
Cost:
$25,000 (to $50,000)
$24,000
Operating System
Windows NT
Linux
# of CPUs:
4 x 500MHz (1 Meg cache)
8 x 450MHz (512k cache)
RAM
1 gig
1 gig
Aggregate network bandwidth
400Mb/s
800Mb/s
Drive storage (RAID 5)
27 gigs TOTAL hardware RAID
27 gigs software RAID / machine
Even at the lowest cost of $25,000, the Xeon machine is still running
half as many CPUs, and has fairly poor fault tolerance.
Price / Performance evaluation
While the quad Xeon machine may be as fast as the Linux farm, the
cost of the operating system ($799 more) and lack of redundancy make it
a weak value proposition. Also, its poor options for upgrades make
it a dead end system.
Failure Mode Analysis
We now look at the common types of failures for servers, and
speculate
about how these failures would affect each type of web server environment.
Failure type
Effect on quad Xeon
Effect on Farm
Hardware failure, soft
System slows down as a whole.
Depending on the failure you may have to take down your server and
replace
the failed component (i.e. NIC failure, single hard drive failure, single
power supply failure.)
Performance loss could be anywhere 25% if a NIC fails to 70% or more
if a hard drive fails.
System slows down as a whole
One of the four servers may be significantly slowed down, and may need
to be taken down for a few hours to be fixed. However, the rest of
the farm stays up.
A single machine failing may result in 25% performance loss
maximum. Other minor failures (one NIC failing, one hard drive failing)
could
result in 8 to 12% performance degradation.
Hardware failure, hard
Systems stops.
You must repair the system to bring it back online. At best you
may be able to restore partial operability after removing the failed
component
and restarting without it.
Performance loss is 100% until the server is fixed.
System slows by 25%
You must repair the one bad server and bring it back online. If
a component is not available, but the server can still operate in a
degraded
manner, you may be able to reinsert it into the farm until you can get
the component. Note that high end workstations could be tasked to
take over this job until the parts arrive.
System stays up in a slightly degraded manner.
Software failure, soft
System stops temporarily. Users must wait while Server or a service
restarts.
25% performance degradation. System back at 100% when
the failed server is restarted.
Software failure, hard
System stops. System Administrator must be called on to bring
system back up. Depending on damage, this may take several hours.
25% performance degradation. System administrator must be
called on to bring system back up. Depending on damage, this may
take several hours.
Failure types:
Hardware, soft: Single item fails, but does not shut down server.
System can be reconfigured on the fly to overcome these problems.
Example: Failed NIC or hard drive in a RAID
Hardware, hard: Single or multiple items fail. Results
in server shutdown of the unit affected. Example: CPU locks
up memory bus, server catches on fire, power spike kills both power
supplies
in the Xeon. etc...
Software, soft: Server or a service on it crashes. Must reboot
server or restart service.
Software, hard: Server or service software becomes heavily corrupted.
Requires OS and / or service to be completely reinstalled and tested
before being placed back online.
Upgrading
So, imagine you've got one of these two setups running, and they are
working fine. Average CPU loads are sitting at below 50%, and
the customers are happy. Then, you get a large contract or you start
to advertise. Suddenly, your CPUs are averaging 80% during the
day, peaking at 100%, and you are getting error_logs full of messages about
time-outs.
It's time to upgrade. How do you upgrade the quad Xeon machine?
You don't, really. You replace it. At $25,000 it's hard to
convince to boss to replace a server that's less then one year old, when
the new server will only buy you another 6 months at best.
With the farm, you just buy new machines as you need them. And,
as faster machines come in, you can add them to a cluster one at a time
as you need them. Since they are standard dual processor workstations,
they can arrive much faster as well, days instead of weeks. As long
as you don't outrun your network connection speed, you can keep increasing
your farm size as needed.
Well, that's my take on it, what's yours?
Scott Marlowe is from Jacksonville, Florida and
currently resides in Englewood, Colorado. Scott is a curricululm
developer, Web author, and system engineer at a medium sized Internet
oriented company. He is the father of two, an amateur saxophonist and a
Linux enthusiast.
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