Storage energy use in the data center: Ask the Expert podcast

As more storage is added to the data center, how does one deal with increased energy demands to power these storage arrays and to keep them cool?

Energy is an important concern for every data center. But the focus is often on server energy and while servers can take the lion’s share of a power budget, data center owners can’t forget about the energy demands of storage. An organization can own multiple storage arrays with thousands of disks, so storage can have a surprising impact on total energy use in the data center.

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Storage energy use in the data center

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Hello, I’m Stephen Bigelow, Senior Technology Editor, and to tell us more about data center storage and its potential energy implications I’m speaking with Robert McFarlane, principal with Shen Milsom and Wilke, a consulting and international technology design firm headquartered in New York City.

Bob heads the data center design specialty in the firm, so Bob thanks for joining us today.

Robert McFarlane: Thanks for having me!

Bigelow: Bob, we talk a lot about power and cooling demands for servers, but how big a factor is storage in data center facilities planning?

McFarlane: Well it can be a relatively small percentage or a very significant one. It all depends on the type of the organization and the type of storage they use. If it’s heavy on tape utilization — and a surprising number of operations still are — then the space requirements can be significant but the power and heat loads are minimal and not even constant. But if we’re talking about disk-based storage — which we usually are when we’re talking about significant power and heat loads — then the opposite is going to be true.

In most cases the space requirements are really a very small part of the data center but the power and heat loads can be significant. Many enterprises think that the total load is quite high. Users also tend to think they have an enormous amount of storage and, compared with several years ago, they actually do. But when you look at today’s storage densities, you can pack an enormous amount into a cabinet.

Not long ago a few gigabytes of storage was considered a big number but today we’ve got that in our ultrathin laptops, and we carry it around in our pockets! I have a small but rapidly growing client that keeps warning me about their huge storage requirements — afraid we wouldn’t plan enough capacity for their new data centers — so we ran the most aggressive growth extrapolation we could justify and what we came up with was that they wouldn’t grow beyond one cabinet in five years. But then we have another client that stores legal data for major trials. Nearly 25% of their floor space will be storage arrays, and that will account for some 27% of the projected power and cooling load even though half that space is high-density 12 to 15 KW per capita. So it can be very misleading.

Bigelow: And Bob, certainly you spoke to the fact that we do see a lot of growth in data center storage partly because of server virtualization, partly because of increased data retention and protection needs and partly because more business applications are simply creating more data. So how do you see storage needs moving into the future and are there any emerging strategies to deal with future storage burdens?

McFarlane: That’s kind of a big complicated question, but the answer is that storage requirements do keep growing for all the reasons you named, plus one other that’s almost never talked about. We just never clean house anymore.

Back in the olden days — I can talk about those — when paper file cabinets used to get full and there was no room to put another one in somebody’s office, then we went through and we got rid of the old and unnecessary papers. In our paperless society — and I use the word advisedly because we probably print and copy more today than we ever did — we just buy more storage. And because we can’t really trust electronic storage — like when we put paper inside fire-retardant metal file cabinets — we back it up. And most people will tell you “back it up twice,” because if one fails then Murphy’s Law says the second one will fail too.

In addition to that, much of our data — much of our material — exists in the electronic files of several other people—all of which is also backed up twice. And none of us has time to go through it and delete what’s no longer useful. Or it’s been forwarded to all, along with all previous replies again and again and again along with all the attachments. It was actually easier to do file purges with paper because you could quickly flip through the pages and visually scan the content and drop the unwanted stuff into the trash. But electronic documents, as we know, are invisible. They’re rarely named clearly, we have to point, click, open, read, scroll, read, close, delete, confirm — you know the routine.

It takes too long to go through the thousands of documents we now send and receive every day so what do we do? We just save, save, save.

Bigelow: Bob, tactics like thin provisioning and data deduplication, those can certainly help to contain storage growth but how do you see organizations dealing with today’s power and cooling demands in storage?

McFarlane: Well, data deduplication is starting to address the problem, but it’s made only minimal inroads so far and that just reduces the amount of storage—or storage growth. The power and heat load densities for a given amount of storage are still the same. There are predictions that solid-state storage usage will increase in 2012 as costs come more into line with disk storage, but we’re seeing very little interest level and acceptance from our clients. That would certainly be a help because SSD uses less space and less power, but right now it’s still expensive in enterprise-grade systems and not yet considered by many to be as reliable as spinning disks.

There may be one good piece of news on the horizon, however, depending on your situation. Spinning disks seem to have pretty much reached their speed limit. By that I mean running them any faster uses too much energy so the storage developers that I’ve talked with seem to think that we’ve reached a speed plateau. We may find ways to pack more into the disk space; in other words more on a given disk surface—but if we don’t spin them any faster, then increasing the storage density won’t make much of a difference in power and heat load.

So we may be close to a place of stability at today’s disk densities. Yeah, we’re going have to use more floor space which spreads the average power and heat load out but it doesn’t look like we’re going to be dealing with particularly significantly higher densities per cabinet in terms of power and heat load. And, I guess that could be considered good news if you’ve got the space.

Bigelow: So Bob, does storage fit well with current data center approaches like hot aisle/cold aisle type containment? How do you see large storage users deploying storage cooling technologies?

McFarlane: Well, yes, it does fit well with that kind of approach because what we call “high-density storage” is really only medium-density power and cooling by today’s definitions. There’s a well-known storage system out there—major storage system—where 2 petabytes of storage consisting of 14 cabinets is listed by the manufacturer at just under 80 KW. Now that’s less than 12 tons of cooling for the whole 2 petabyte array. Come right down to it, that’s not a huge amount. The loads are fairly uniform — they average less than 6 KW a cabinet and no particular cabinet is really a high power cabinet. That requires less than 1,000 CFM of air per cabinet based on a 20 degree temperature rise — 20 degree F temperature rise.

That’s just not huge, so in legacy terms, which we try not to use anymore but to which most people can still relate, that’s around 80 to 95 watts per square foot when the cabinets are spread out on the floor like they’re usually installed. So what it comes down to is that in spite of the fact that cooling power and heat loads have grown considerably in the last several years, with a decently designed under-floor air and containment system — and I’m not advocating under-floor, I’m just saying that’s what most people are still used to — with a decently designed under-floor air and containment system, it shouldn’t really be difficult to cool and, if it is, then quite frankly you should start questioning your cooling designer because it’s not that big a challenge today.

Bigelow: And Bob, of course that brings us down to our last question today. Are there any other best practices for mitigating power and cooling needs in data center storage that you could share with us?

McFarlane: Yeah, it’s really like everything else in cooling today. The air has to be controlled. You happened to mention containment which does not have to mean full containment. It can be partial containment — plastic strips do a pretty good job. You don’t have to go the whole route of building a room within a room. But you have to control the return air — that’s the key to all cooling problems.

That’s what’s usually responsible for messing up otherwise good air delivery designs. And since storage is really only medium density heat load, as we said, then it can get challenging if we mix it in with true high-density cabinets that are running 12 to 15 KW or more a cabinet. So it really does help to have a designated storage area in a data center that can be designed for the cooling it requires and be designed for the kind of uniformity and constant air control that the storage really enables us to do.

Bigelow: Terrific. Well Bob those are some great insights today, but that is all the time that we have. On behalf of Robert McFarlane, I’m Stephen Bigelow and you can learn more about data center energy and other facilities topics at

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