PCI Saturation - What happens when you get it ?

[jose]

Hello everyone,

My friend is upgrading his email server (sendmail & Linux). He's looking at the Mylex AccelleRaid 170 (32bit scsi) and the 3Ware 8 port sata card (32/64bit) to do raid 5.

Now both cards will saturate the pci buse w/ a 3.0ghz/1gig/P4c800e mobo using raports or Atlas 10k4's .

How does it affect the OS ? Currently he's using a p400 w/ 40gig hd w/ 256mb of memory serving 1000+ users.
He's also using spam assasin & iptables & squid.

Comments appreciated,

Regards,
Jose

 

[us3rnotfound]

It'll go all crazy and such.

 

[drag]

There is a limited amount of bandwidth aviable on a PCI bus. This must be shared by all the devices on it. The vid card (unless it's agp) pci raid cards, network, sound etc etc.


The pci bus is generally 33mhz, with a 133MB/s (1064Mb/s for networking terms). There is a standard for 66mhz (with 2x the bandwidth) pci bus, but I am not sure about how common it is.

When a PCI bus is saturated it is maxed out, this limits the performance potential of your computer.

One of the nice thing about 64bit motherboards is that you get 2 times the bandwidth, 33mhz = 266MB/s vs 133MB/s for 32bit. Also from what I seen 66mhz is more common and PCI-express will have 1066MB/s with a 133mhz bus, which should be plenty for a long time.

Now a normal IDE drive like a 7200 rpm 120gig will use up to about 44MB/s of thouroghput.

Now I don't know how much overhead is built into the "networking" part of the bus, or how much a network/vid/sound/etc card uses, but I am pretty sure with a nice high speed SCSI array it would be pretty easy to max out a x86 machine.


(of course if your running a internet server or a 100Mb/s network then you probably won't max it out since the network would be the limitation. You probably won't be using more then 8-10MB/s of the bus for the networking aspect.


Mb=Megabits MB=megabytes

 

[Gunbuster]

A Xeon server board will give you "Three independent PCI buses on six slots: one 64-bit/133MHz PCI-X, two 64-bit/100MHz PCI-X, and three PCI 32-bit/33MHz"