Have a really easy / basic question. Have a poweredge 2950, 8 slots, 4 in use (1gb strips) so have 4 remaining. I would like to double it to 8, looked and see I can go with 2 2gb strips right now for a good price.

The box is running CentOS 4.6, with the following in dmesg;
Linux version 2.6.9-67.0.1.ELsmp (mockbuild@builder6.centos.org) (gcc version 3.4.6 20060404 (Red Hat 3.4.6-9)) #1 SMP Wed Dec 19 16:01:12 EST 2007

I started reading on PAE, etc. and wonder before I make this purchase if there is anything I need to do on the box for the OS to see it, like install a new kernel, etc.

Thanks

Since you have a full 4GB already, you can easily check whether all that PAE stuff is working now.

1) Look at the BIOS memory map from dmesg (If it is too long since you booted Linux that map might not be there. I don't know how else to get it, short of rebooting.)

For comparison, look at the map in this post
http://www.linuxquestions.org/questi...10#post3455110
Notice the line
That shows the usable memory outside the first 4GB of address space.

If your current 4GB of ram is all usable, some of it (probably less than in the quoted example) will be outside the first 4GB of address space.

2) If it is OK at the BIOS map level, use the free command to see that Linux has about 4GB (not just about 3.5).

If that isn't working, additional ram won't work either. If that is working, most issues are OK for additional ram, but it is hard to be sure without trying.

Well a;
cat /var/log/dmesg |grep BIOS shows the following;

BIOS-provided physical RAM map:
BIOS-e820: 0000000000000000 - 00000000000a0000 (usable)
BIOS-e820: 0000000000100000 - 00000000cffa8000 (usable)
BIOS-e820: 00000000cffa8000 - 00000000cffb7c00 (ACPI data)
BIOS-e820: 00000000cffb7c00 - 00000000d0000000 (reserved)
BIOS-e820: 00000000e0000000 - 00000000f0000000 (reserved)
BIOS-e820: 00000000fe000000 - 0000000100000000 (reserved)
BIOS-e820: 0000000100000000 - 0000000130000000 (usable)
PCI: PCI BIOS revision 2.10 entry at 0xfb1fe, last bus=16

So if I read this correctly, I should be okay already then.

So the BIOS is OK. You have 3327.28MB in the first 4GB of address space plus 768MB outside the first 4GB of address space.

I don't happen to know which Centos kernels are PAE.

What was the output from free? Is the total memory slightly less (there is some overhead) than the 3327MB in the first 4GB or slightly less than 3327+768 = 4095MB?

total used free shared buffers cached
Mem: 4149104 2824188 1324916 0 30604 2428136
-/+ buffers/cache: 365448 3783656
Swap: 1052248 23296 1028952

Your BIOS and Linux Kernel are both OK for the upgrade to 8GB. No changes required to either.

Dell servers tend to be quite picky about exactly the right kind of RAM. Are you sure the good price you found is on the ram it can use?

The quote from the spec sheet is "Fully Buffered DIMMs (FBD) in
matched pairs, 533MHz or 667MHz".

I expect the speed needs to match for all the DIMMs. "Matched pairs" means in each pair, the size must also match (you already indicated you will get the size matching per pair). I'm not certain about matching speed. Most systems operate all DIMMs at the speed of the slowest DIMM, rather than rejecting the memory if there is a speed difference. I don't know for sure that Dell is pickier about that.