Why x64?

Exchange 2007 is only supported on 64-bit hardware and operating systems. This will cost a lot of people a lot of time and money to replace servers. Why did Microsoft do this? I’ll try to explain the thinking here.

Disk i/o operations per second (iops) is the single biggest constraint in Exchange 2003. A heavy user in E2K3 uses about 1 iops to the disk subsystem. A typical disk drive running at 10,000 rpm can deliver about 100 iops (your mileage may vary depending upon how much caching is in place in the disk subsystem. For example, a SAN will typically cache a lot and give you an effective iops more like 150 for the same disk).

Unfortunately, iops are SLOW, slow like molasses compared to quick in-memory access. An iop needs to wait for disks to spin, and disk heads to move, and processor interrupts, and finally transfers of data over impossibly slow busses. That means that users accessing large mailboxes are slowed way down as well.

The time-honored, brute force solution to slow iops and not enough of them is to keep recently used data mirrored in RAM (called caching). RAM is very speedy and the number of mops (memory operation per second) that can be done is vastly greater. Consider 1066 MHz RAM: that can do 1,000,000 mops. A lot faster than the 100 iops a disk can do (the comparison is a bit bogus because transfer size is not considered, but you get the idea).

Exchange 2003 used as much cache as it possibly could within the confines of a 4GB memory size imposed by 32-bit hardware and operating systems. They even put in a special /3GB switch to tell Exchange to use up to 3GB of cache on a machine configured with 4GB of RAM to tell Exchange that there is nothing else running on the machine and it's ok for it to hog it all like that. So, 32-bit machines are limited to addressing 4GB of memory. 64-bit machines, on the other hand, can address, well, LOTS of RAM (about 17 million Terabytes, which Bill assures us "ought to be enough for anybody", but who's counting).

So by going to 64-bit hardware (which in turns needs an operating system specially compiled and optimized to use the 64-bit hardware) this allows Exchange to make the database cache as large as you can afford.

The difference between a server with 4GB or RAM and one with 32GB of RAM is about $13K. The difference between one with 4GB RAM and one with 16GB is $2K. I think I'll stick to 16GB servers for now, thanks all the same. Microsoft reckons you need 2GB of RAM plus anywhere from 2MB up to 5MB extra per mailbox depending upon how heavy the user is (no fat jokes, please).

So a server with 16GB of RAM can host about 3000 heavy users. 3000 heavy users used to require 3000 iops, which can only be supplied by a whopping 30 disk drives. Definitely expensive SAN territory. And at about $1400 for a 10krpm 300GB Fiber Channel SAN disk drive, that's an extra $42K for disk drives.

Microsoft also reckons that the iops load drops to about 30% of what is was in E2K7. That means that the same storage can be had from fewer, less expensive 500GB 7200 rpm disks, which cost about the same as the smaller faster 300GB drives. We can make do with 18 of these disks instead (or fewer if you venture into RAID5 territory).

So we spend $2K extra for the RAM, but we save $17K on disk drives. Makes sense to me. And it will keep making better and better sense as RAM prices continue to drop, and as the size of disks continue to get larger (for the same $) but spin no faster (hence no extra iops).

Not only is it cheaper, but much larger mailboxes, no longer constrained by the speed of iops, can be used in Exchange 2007, and the same-sized mailboxes are faster to access on loaded servers. All good news.

Couldn’t Microsoft have also put out a 32-bit version of Exchange 2007 to support up to 400 heavy users? Actually, the eval is 32-bits, so it’s clearly possible. Why not support it in production? The cynic might say “sell more hardware and OS licenses?”. As a developer, every extra platform supported costs extra money, and if the world, and especially Longhorn is going 64, well…

So while this is great news if you're going out to buy a brand new Exchange 2007 back-end server, if you have an existing Exchange server you would like to upgrade, chances are it's all wrong for you. Oh well, you needed a beefy 32-bit server for that old accounting package anyways.