Transitioning to 64bit, is it worth it, and how
* a) What are the pros and cons in switching to 64 bit mode?
* - Is it faster?
I don't know - never ran my Opteron in 32-bit mode. I was wishing 64-bit floating point operations would be faster but they're not because AMD didn't really improve the math unit. In principle it should be faster if only because you're running in Native mode rather than Compatibility mode.
* - Are the packages less stable?
No. It is possible that some 'unclean' code which is being transitioned to 64-bit will cause problems but I am not familiar with any. OpenOffice was unclean at one stage but the fix-up was completed at least a year ago. (Having said that, I had fixed up 64-bit issues in 2 "open source" libraries.)
* - Does it need significantly more memory?
No, memory use doesn't change that much except for addresses now being 64-bit (well, something like 48 really) rather than 32-bit. 32-bit addresses limited you to 4GB of virtual memory. With 64-bit addressing you probably cannot afford to buy enough memory to make use of all 48 address bits.
* - Can I use 32 bit apps from the repository if I have problems with an
identically named one in the 64 bit repository (with pinning maybe)
Yes, but it's tricky. The kernel needs to support the 32-bit system call format (I think the Debian kernels enable this by default) and you need a 32-bit dynamic loader and 32-bit libraries. Your 32-bit program can only make system calls and link to 32-bit libraries.
* b) Is there a simple transition path?
Yes, install from scratch.
* - Is there any howto?
Numerous howtos on the Debian AMD64 website.Have a look at the links near the bottom of this:
http://www.debian.org/ports/amd64/
* - My current unstable repository seems to have a 64 bit kernel - do I just install that and then change where my sources.list points>
No. Install from scratch. Install 32-bit libraries as needed.
Some software (notably numerous proprietary stuff - people have already mentioned Acrobat Reader) are available as 32-bit only. Other proprietary binaries that come to mind are Flash Player and Skype. I've nagged Skype for a few months but they really don't care. I don't use any of these 32-bit only software because I refuse to waste disk space installing 32-bit libs. If you use Wine or CrossoverOffice or Cedega you will have pretty much a complete 32-bit installation in parallel with your 64-bit. If you really use a lot of 32-bit stuff, stick to 32-bit until things improve (that may be a few more years). The exception to that of course is if you're one of those people who really wants to squeeze something out of every CPU cycle.
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