I’ve been catching up on a few RSS feeds today (I’m currently standing at around 8000 unread articles) and was browsing through this from Barry Burke (TSA) -1.051: skate to where the puck is going. Whilst the content of the article itself isn’t that illuminating (a discussion on how wonderful EMC are to be selling bigger SSDs in their arrays), what caught my eye was the mention of the term “slot cost”.
For years I’ve been banging on about slot cost. Imagine a storage array that supports 1000 drives. The cost of the chassis, front-end ports, power/cooling etc all contribute to the overall running of an array. Selecting the components that stay constant, irrespective of the number or size of disks in the array, the effective “overhead” of maintaining those disks can be calculated. That overhead is there whether there’s 1 or 1000 disks in the array and is constant whether there are 73GB or 1TB drives in use. The “slot cost” is the cost of servicing that slot (or drive) in an array.
I like to use slot cost (or the relative efficiency of a slot) as a measure of how efficiently configured my storage array is. Arrays with a higher relative slot cost stand out as candidates for replacement or upgrade. Here are some example metrics:
- GB/slot (bigger is better)
- KVA/slot (smaller is better)
- £/slot (smaller is better)
These calculations can be made against both available slots and slots in use.
Another interesting idea is to measure the slot cost against different storage vendors and technologies – compare DMX/USP/DS8000 for example or modular versus monolithic. This gives a good comparison of the overall cost of maintaining that storage and can highlight where data could be more appropriately stored on a cheaper platform.
4 Responses to Enterprise Computing: Slot Cost
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Great metric Chris. As 2.5″ drives enter storage, it will be interesting to watch how the definition of “good” slot cost evolves. Thoughts?
Top loading arrays (e.g. Sun Thumper and 7210) make for interestingly low slot cost (though only applicable to SATA drives in most cases). With the comment on 2.5″ drives, I wonder if there will be a top loading 2.5″ equivalent on the way? It could make for some extreme density!
Pete, that’s a good point and slightly preempts a post I was about to put up! Clearly at the moment, everything is based around the 3.5″ format, except for a few arrays out there using 2.5″ (most notably for me the Sun USS 7110 Array). As 2.5″ drives become more widely adopted, then we’ll have to consider power/cooling/space/weight as part of the equation and I actually do calculate those metrics too when considering KPIs for arrays. It’s amazing looking at the differences between monolithic/enterprise and modular for this! Bring on 2.5″ drives, I say.
The 7000 is interesting because there’s no reliance on fast drives, so the array can be easily converted to 2.5″ if the GB/cm3 density works.