ORIGINALLY POSTED 7th January 2010

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 Happy New Year to all. As you may know, or have seen the UK is currently under the grip of winter. A few inches of snow and the country comes to a stand-still. Maybe I’m getting old an cynical (some may say no maybe about it!) but I do remember walking to school in drifts of over 2 foot back in Glasgow… I don’t remember a day when the school was closed due to snow… maybe the odd teacher from far a field couldn’t make it in, but most did.

Now you may say that Glasgow / Scotland are more used to this kind of weather and so just get on with it. That maybe true, and maybe its also true that some 25+ years ago, most of us actually just went to the ‘local’ school and walked – i.e. didn’t need to be driven miles to make it to school and hence add to the rush hour traffic etc etc.

I was also told the other day that not only is there a shortage of grit, but that they have been putting less sugar in it this year. I didn’t know (nor do I know if its true) but said person suggested that because of that, its not sticking to the road, and so when just icy, the grit powders up and blows away.

All in all, I think that in general the country just isn’t spending money on the infrastructure as it used to. Where are all the yellow grit bins I remember as a kid, which were always full. Round here, we have 2 that I know of, and have never been filled in the 7+ years I’ve lived  here.

OK, rant over, and it does make a difference having the Land Rover these days, compared with the RX-8 a few years back. Small, lightweight rear wheel drive + slight bit of snow = forget it. I guess my point is that unless you have the right equipment, its difficult to make things happen. The councils certainly don’t have the snow clearing hardware, and grit they need, and as such the few inches we have around here (yes we seem to have got off lightly) still causes chaos.

The same can is true in most environments, storage included. We often find that users who have been using SVC for a while are the most evangelical sales people for us. They wonder how they coped before. The mobility of data and flexibility of heterogeneous (i.e. non-vendor specific) storage it provides cuts monthly migrations into days.

One common question we get asked was repeated in a recent post on Linked In, under the SVC experts group. Not only is this actually the wrong question, but it can only be FUD thats spread by our competitors. The question was more of a statement, and was a user asking if it was true.

I have heard recently that their are some scalability issues with IBM’s SVC. The rumors indicate that a single two node cluster shouldn’t be pushed past 12TB

So why is this the wrong question. Well, SVC scalability is 8PB in a single cluster, so its not a capacity scalability comment. Nor is capacity addressability a limiting factor. Scalability for a node pair is purely driven by performance. Each node has a certain level of IOPs or MB/s that it can sustain before the internal CPU cores get saturated. Capacity is therefore the wrong thing to measure, its IOPs that we use to decide when a node pair is saturated.

Based on internal measurements I’ve made, a CF8 based node pair maxes out at about 400,000 IOPs (read miss 4KB) and 150,000 IOPs (write miss 4KB) – in OLTP terms that’s about 250,000 IOPs (70/30 mixed miss 4KB) – Similarly from a bandwidth perspective we are talking about 6GB/s read and 3GB/s write.

These are all node pair numbers, and are of course driving the nodes to 100% saturation, which we wouldn’t recommend. But if the 12TB number were to be taken into account, and it was made up from 1TB SATA drives, then that would be 12x 100 IOPs – or 1,200 IOPs. So really if you were using 1TB drives, you’d need 2,500 of them before you saturated the nodes – and thats 2.5PB of data, not 12TB…

That’s an extreme, and of course you don’t use SATA drives for performance orientated workloads… or at least not yet. As we previewed back in November, this year we will be adding a Smart Tiering solution to SVC. Think of this as moving just the extents of your workload that need the performance onto high performance storage, while leaving the other areas of your applications on slower storage. This dynamic tiering, can really help to reduce costs. In theory one or two SSD combined with a collection of SATA drives, will easily outperform today’s 15K RPM drives – under certain workloads.

Most storage vendors have previewed that such functions are coming this year, and will very soon be a ‘must have’ tick box feature. But remember that SVC has been splitting up storage into usable ‘extents’ and providing the extent level online migration for the last 6 and a half years – so the underlying primitives have been tried and testing in over 5,000 production environments.