Re: ram/raid1

[Erik Mouw <erik@harddisk-recovery.com>]

On Fri, Apr 07, 2006 at 06:59:29AM +0200, Francesco Pietra wrote:
> n setting up a workstation with 
> 
> --two amd6a 265 opterons dual core
> --Tyan K8WE S2895SA3NRF main board
> --two 360GB raid1 hd (raid 1 software by debian)
> --ram 2GB (Kingston KVR400D4R3A/2G - DDR 400 Ecc Registered), is any reason to 
> prefer two slots of memories 1GB each instead of a single 2GB slot?

Most certainly.

> The technician here maintains that two slots are needed to have needed two 
> channels for raid1; it is unclear to me.

The memory slots have nothing to do with the RAID. The reason you want
2x 1GB is that dual (or more) Opteron designs are not SMP (Symmetric
Multi Processor), but NUMA (Non-Uniform Memory Architecture). With SMP
the two CPUs share the same bus to memory, but with NUMA each CPU has
some local memory connected to a local memory bus. The other CPU can
still get to that memory, but it's a bit slower. If you would only put
in 1x 2GB, you will severely slow down the other CPU cause it has to go
through the other CPU to do memory accesses.

To see what I mean, get the board datasheet at
ftp://ftp.tyan.com/datasheets/d_s2895_101.pdf and look at the block
diagram on the second page. The Linux virtual memory subsystem is NUMA
aware, especially in the latest kernels (i.e.: 2.6.15 and better): it
will take care managing the memory in such a way to minimize the
amount of traffic between the CPUs.

> Incidentally, the 2GB Kingston is charged in Italy six hundred euros, that is 
> more than twice the price in US. This is to recognize that we can circumvent 
> the market leader software houses (and be more efficient) but we cannot avoid 
> the system in our country which favors handlers against citizen (and against 
> scientific research activities). The results of such policy are under the 
> eyes.

You can order memory everywhere in the EU, that's what the "no
obstacles for trading goods" rules are for. If you can find memory
cheaper in (for example) Germany, buy it over there and have it shipped
to Italy. Be sure to let your Italian vendor know that he missed a
sale.


Erik

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