With the announcement that you can order an XIV with 3 TB SAS disks, IBM now have some amazing capacity options and some equally clever growth options with XIV Gen3.
As you hopefully know, the XIV consists of modules that each contain 12 disks. An XIV can have 6, 9, 10, 11, 12, 13, 14 or 15 modules (all modules must have the same size disk). You can start at any of those points and then grow without interruption or outage up to 15 modules (that's 243.3 TB!). There is practically no planning required to do a capacity upgrade and the data relocation to re-balance between the nodes is done automatically by the machine (without any end-user intervention).
The useable capacity sizing with 3 TB drives stretches from 84.1 TB with 6 modules to 243.3 TB with 15 modules (these are decimal TB).
However the Capacity on Demand (CoD) options are far more interesting. With CoD you effectively buy a certain amount of capacity up front but also get up to 3 more modules shipped with the machine. You can start using this extra capacity when your business requirements demand it, at which point you will be asked by IBM to purchase it. The advantage here is that you physically get a bigger machine up front with all the performance benefits that bestows, plus you don't have to contact IBM to start using that extra capacity. Lets look at the possible configurations.
So lets take a scenario. You need 100 TB today, but you know this will grow to 130 TB over the next 12 months. So you could purchase an XIV with 9 physical modules (using 3 TB drives), with 7 CoD activations. This means IBM ship a machine that physically has 132 TB and that physically has 108 drives in 9 modules. Your data will be spread over all these drives and all of these modules will be active and working. However you have effectively only paid for 103 TB of that space up front. If you order extra CoD activations, you could also order extra physical modules. As long as you stick to the chart above and have at least one un-activated module, you stay in the CoD program.
When your data requirements exceed 103 TB you just start using the extra space, no license keys or special tasks required. Nice!
So having told you how great it is... are there any disadvantages?
1) You need to actually buy the storage... eventually. Depending on the CoD contract there will be a point when IBM expect you to purchase this extra capacity. The whole point of CoD is that it is like pre-ordering capacity without actually paying for it up front. If your really not certain you need extra capacity, your probably better off not ordering CoD capacity in the first place. Instead order capacity upgrades as you require them.
2) There is nothing to stop you using the storage. Now this is a curious disadvantage because it means that if you have paid for 103 TB and you start using 105 TB, the machine will not tell you off, or yell at you. So is this a good thing or a bad thing? Well I really like the flexibility so I think it is a good thing. Plus there is a nice command called cod_list which displays consumed capacity to help keep you on the path. You can also display it in the GUI. So it just means you need to keep an eye on volume and pool creation to ensure you don't start configuring extra capacity until your prepared to pay for it.
You can also use CoD with 2 TB drives on XIV Gen3 so this is another option. With 2 TB drives, the useable capacities look like this:
Questions? Fire away....