Real benefits of RHEL over Fedora?

[Marko Vojinovic]

On Monday 06 June 2011 14:22:23 Alex wrote:
> None of the servers I will be building will have Xorg installed, so
> this should make upgrading significantly easier, should I choose the
> fedora path. This is a pretty critical server in the role that it will
> be implemented, so stability is a concern, but it's really only
> performing basic email and web functions which are pretty well tested
> in fedora.

You can think of the term "stability" as "the server app I use will not change 
its version during the daily process of regular updating". In this sense, 
Fedora is *not* stable, since the *features* of the software get updated on a 
daily basis. This may occasionally break your existing mission-critical app, 
and require an afternoon of downtime and work to reconfigure/customize/deal 
with. This doesn't happen often, of course, but there is no guarantee that it 
cannot happen.

OTOH, RHEL/CentOS is stable in precisely this sense --- you set up your server 
once, and you can be quite sure that the regular updates will not break any 
existing functionality for the lifetime of the release (aka 7 years).

> - Are there improvements made to RHEL beyond what is available in the
> most stable version of fedora? Does RHEL effectively use the same
> kernel as some version of fedora? In other words, once the kernel has
> been time-tested on fedora, doesn't it become the basis for the RHEL
> kernel? Are there kernel, filesystem, or memory tuning improvements
> that don't ever appear in fedora?

The kernel version used by RHEL is the kernel version used by Fedora at the 
time of the creation of RHEL release. After that it doesn't change in RHEL, 
and it does in Fedora. That's the story with "stability" --- if you build some 
custom drivers/modules against the RHEL kernel, they will continue to work for 
a looong time. If you do the same in Fedora, they will need to be rebuilt on 
every kernel bump, several of which may happen during a single Fedora release.

The RHEL kernel gets bugfixes and security fixes backported when necessary, so 
that it stays the same version. In Fedora a newer version of the kernel is 
preferable than backporting to the old version. This can break existing 
software modules (typical examples are nVidia graphics drivers and VMware 
kernel drivers).

AFAIK, there is no additional tuning and improvements that are being done in 
the RHEL world as opposed to Fedora. Such kind of tuning is more likely to 
happen in Fedora first.
 
> - I think if it was just a matter of stripping out the trademark
> stuff, CentOS would be released much more closely to the RHEL release
> than it is, so I suspect there is more to it than that.

Oh yes, it's about reverse-engineering the build environment. Compiling a 
distro from source code is nowhere near as trivial as it might seem.
 
> - It appears RH does not add new features through the seven year
> lifespan of RHEL, only security and bug fixes, so I would most likely
> have to upgrade more frequently anyway. In other words, the hardware I
> install RHEL on initially won't be the hardware I'm using seven years
> later.

Why? If all you are going to run is a web&mail server, why not keep the old 
hardware for all 7 years, if it does its job well? I mean, it's not like you 
are going to need a fancy new graphics card for it, or drivers for your brand 
new USB sound card, or such. The whole point of a long lifespan is that it 
keeps working 24/7 for several years, with minimum downtime, on a given 
hardware. Provided, of course, that the hardware doesn't fail in that period.
 
> - I believe the fedora lifespan for security updates is at least a
> couple of years, correct?

It should be 13 months, give or take a few days. So just over a year.
 
> - Much of the focus for RHEL seems to be on virtual machines, not
> installing on the bare metal. Is that correct?

That's a matter of taste, really. RHEL does implement and support virtual 
environments, since many businesses require or benefit from them. But it 
doesn't mean that bare metal installs are not "in the focus" or whatever. RHEL 
will work equally well in both physical and virtual environment. The point of 
the virtualization hype is that you can use RHEL to *deploy* virtual guests, 
without using third party software (such as VMware or VirtualBox). If you need 
virtualization, it's there for you to use it. If you don't need it, RHEL will 
work just as well on bare metal.

> What is a "socket" as
> referred to in the RHEL subscription information? Does this just mean
> an available processor on the server for that virtual machine?

I have no idea on this one. ;-)

As a final note, I would suggest using RHEL/CentOS for a web&mail server over 
Fedora anytime. A server really doesn't need the latest&greatest software 
features, just security and bugfixes. If you choose to go with Fedora, be 
prepared for occasional afternoon of downtime now and then --- either because 
you'll be installing a new Fedora version every year, or because some new 
fancy system update might break your custom scripts and configuration --- not 
too often, but I've seen it happen more often than on RHEL.

HTH, :-)
Marko