Understanding SANs with regard to exchange sizing

I am looking at sizing a SAN for exchange.
I have never used SANs before and am trying to understand the basics after going through some documentation.
We may in future be sizing a SAN for an exchange site

What I dont understand is it seems you size the storage according to exchange performance - i.e. number of disks, etc yet i see no references to exchage storage requirements in terms of size?

It also seems the required storage in the example case studies far exceeds what may be necessary.

Are the disks arranged purely for fast access times and in essence only  a portion of the disks physical storage is used? how is this storage calculated?
Would appreciate anyone who could ellaborate on this as much as possible.
I want to get these basics straight in my own head befroe i start speaking to HP, EMC or any other SAN vendors.
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Hanno P.S.IT Consultant and Infrastructure ArchitectCommented:
Basically, there is not much difference for designing storage to be used for file service data or exchange.

Your major concern would be the size required for your exchange data.

Usually, if you do not use size limits for user mailboxes, you will easily find users allocation more than one GB per year!

If you plan to implement a SAN for exchange data storage, you must have some more reasons to use SAN. Otherwise, there is no need to spend the extra money for service, support, maintenance and administration -- as well as adding and complexity.
If you only need "big disks" to be used, you can more easily to this direct attached (SCSI or FC storage subsystem attached to exhange server(s))
http://h71019.www7.hp.com/ActiveAnswers/cache/80663-0-0-225-121.html takes into account size of mailboxes as well as performance,  you have to create a user account first but that's free. If you don't use the expert mode it tends to default to the MSA1500 which isn't very fast.

I suppose that the reason most don't take into account size is that it is IOPS that matters and  size is something you can easily calculate for yourself by counting your users and guessing how big their mailboxes will be.

JustUNIX is right that unless there's a reason for using shared storage SAN is a waste of time, you can direct attach several hundred SAS disks to a 2U server so why would you need a SAN?
Forced accept.

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