Not much point in putting in ( and paying for ) 16 gigs of RAM if the 32 bit OS can't talk with all of it :)

32 bit is going to be able to see and talk with, a little short of 4 gigs..

IIWY I'd get 64 bit system set up from the start.."clean" install is always better than "upgrade"..

[edited by: Leosghost at 3:40 am (utc) on Dec 7, 2011]

Does 64 bit really matter than much?

It does if you want to actually use all that RAM

Or what about if I started off with 32 bit, then did an upgrade of the OS, is that likely to work?

Waste of time.

Either do it or don't.

I've been running RHE 64-bit for a few years now, worked fine first time out of the box.

Never looked back.

@wheel..what flavour are you on now in 32 ..and what flavour are you looking at in 64 ?

64 bit compiled applications use more memory than their 32 bit versions, mainly because code and data structures are aligned in blocks of 8 bytes rather than 4 bytes. Expect an average increase of your application footprint around 20%. So if you need 1GB for a running application on a 32 bit box, expect the same application with the same load to use around 1.25GB on a 64 bit platform.

Furthermore a number of applications are more buggy on 64 bit than on 32 bit, although they seem to work. This is often caused by long/int conversions because these type definitions on C/C++ changed slightly between the two platforms. One such a problem child I have found is the ntpd time server which produces false statistics on 64 bit in a number of distributions.

Some applications are not able to share data between 32 and 64 bit. AWStats data files collected on 32 bit Linux can't be moved to a 64 bit Linux machine for example.

But having said that, 64 bits gives you a lot of extra memory space if you have installed the RAM and it provides a growth path to the future. I am currently running three 32 bit Linux servers and two 64 bit servers and planning to upgrade them all to 64 bit in the near future.

Another way of looking at it is endure the pain now, or at a later time. Move to 64bit was done some time back. There's a reason why it is called the bleeding edge of technology.

When I moved to 64 bit a couple of years ago I ran a few tests to see which was faster.

32-bit passed all tests as the fastests but obviously there is the 4Gb limit - which is going to be useless if you are running a meaty DB.

You want the O/S and apps to use the RAM for caching, buffers and stuff. Thus for any rig with > 4Gb you are better off with 64-bit.

One alternative is 32-bit in PAE mode. This allows 32-bit to break the 4Gb barrier. But there is a slight performance hit with this.

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As the others have said you are better off doing a fresh install with 64-bit version.

One gotcha with Centos 64-bit is that you can get into a mess with it installing some 32-bit apps and some 64-bit. In yum.conf put

exclude = *.i?86

right at the start, before you do any further yum install / yum updates

Yes, with 16gb of memory, the 32-bit linux os is not utilizing any but 3gb of it.

@apachewebserverguy Only if you have done nothing to enable it to do so. Google PAE.

As far as I'm concerned, there's no issue with going 64 bit. Linux has been 64 bit friendly almost since the get-go. I have had absolutely no issues running 64 bit for the last 6+ years.

Welcome here on WebmasterWorld zaktoo and thanks for your insights. With 6+ years of experience, you are certainly one of the early users of 64 bit Linux and have a wealth of experience.