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Thoughts on Sizing


Many times your SAN guy will only want to know how many GB your db needs from the SAN.  This is important but isn't the only concern, maybe not even the biggest concern.  Keeping the amortized cost per disk low is certainly a concern. 
First of all, if you are dealing with a virtualized SAN then these calculations really don't apply.  However, I'd still perform the calculations and provide them to your SAN guy as a "jumping off point". 
How Many Phys Disks Do I Need to Support My Workload?
The common formula is
Required # Disks = (Reads/Sec + (Writes/Sec * RAID adjuster)) / Disk IOPS

RAID adjuster factors in the additional writes required for RAID operations for a given RAID setup.  RAID 0 incurs no overhead so its adjuster value is 1.  RAID 1 and 10 incur two physical writes for the mirrors so the adjuster is 2.  RAID 5 requires 4 writes to maintain parity. 
My Editorial
It seems like when I am spec'ing out what I need to support a given database I see SAN guys and management that want to see high disk utilization numbers.  This means they want to know how many GB of space the db will need, then they allocate that for you.  They are trying to squeeze every last penny out of their SAN investment.  But this is the wrong way to look at the situation.  If I really just need the SAN to support my db's persistent storage then why not use a cheap SATA drive or even some USB disks?  Those will be far cheaper than any SAN.  Of course you wouldn't do that because SATA or USB wouldn't support your IOPs requirements. 
With a SAN you really have made a huge investment.  Now is not the time to skimp and choose RAID 5 over RAID 10, or not stripe your data over as many spindles as possible.  Always, always choose low utilization/high disk stripe over higher utilization/less disk stripe.  It's not uncommon to discover you really only need, say, 100GB of *real* data storage for your database, yet your data LUN is over 1TB of physical storage.  This is the price of performance.  It looks bad to management, but is best if you really care about performance.   

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