Good performance question

A reader wrote to quiz me about my recent columns on 64-bit Exchange and the performance benefits it should offer. He asked:

In your last e-letter you mentioned the added performance boost putting Exchange on a 64-bit box. For those of us that connect our Exchange servers to an iSCSI SAN, would we not run into bottlenecks at the NIC (1Gb backbone, assuming we were not using a TOE card or maybe even if we do), before a 32-bit setup cut into performance?

I’ll trot out my all-purpose answer: “it depends.”

First, let’s assume that you have a Gigabit Ethernet connection to the iSCSI SAN, with an HBA that has a native x64 driver– no thunking required. That’s just a clarification, but in the end it doesn’t really matter. Why? Assuming that you have “enough” RAM (where the precise value of “enough” varies according to the user workload on your server), JET 12 is going to be able to cache a significantly larger portion of the EDB data than it can now, meaning that the amount of bandwidth between your server and the iSCSI cabinet becomes much less relevant from a perf standpoint. We already see a similar effect now; when SAN vendors are hunting for business, they often put lipstick on the bulldog by adding a very large cache to the controller. Of course, this only works until the disks hit 70% or so of capacity, then the cache detunes and performance drops like a rock. That’s a problem only because the SAN controller has no idea what the application is doing; it’s not a problem for Exchange in this case because ESE is in charge of the cache. Given “enough” RAM, the amount of bandwidth you use for a given set of user behaviors should decrease because you’ll be making fewer requests to the actual disk.

What about page size? My gut feel is that the page size change will be a wash; caching will reduce the total number of IOPS that have to go over the wire, but those pages that do go will be 8KB vice 4KB. I’m looking forward to seeing hard data to confirm or disprove this, though.

Why did I say “it depends”, then, if the performance news is so rosy? Because one of the key reasons people will be deploying Exchange 12 is to consolidate servers. Obviously if you take four or five Exchange 2003 servers and stuff their mailboxes onto an Exchange 12 server, the new server is going to require a significant amount of SAN bandwidth, and I suspect it’ll easily be possible to build configurations that would saturate a GigE HBA. So, don’t do that and you should be good to go!

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