SANs that mix the best of both worlds
- 06 August, 2003 09:50
When the news broke that HP and Compaq would merge, one of the questions was what would happen to their respective storage virtualisation plans. HP had taken the in-band route, acquiring Storage Appliance and its SAN controller technology, while Compaq's VersaStor project took the out-of-band approach.
In the event the question has proved redundant, says Mike Feinberg, the vice president and chief technology officer of HP Network Storage Solutions. Instead HP has combined the two in a hybrid approach which Feinberg says is a natural evolution, adding the intelligent software that is missing from most of today's SANs.
Virtualisation hides the physical storage, instead presenting it as a pool of logical blocks. In-band uses a Windows or Linux-based storage controller which sits across the data path, accepting requests from servers and translating them as needed, for example routing a request for mirrored storage to two sets of blocks.
Out-of-band relies on block maps loaded into the servers or switches to handle the translation, which its proponents say avoids the potential bottleneck of the in-band controller.
"Everyone wants to say 'Where do you put virtualisation?' It's a really silly question - the answer is 'Yes'!," says Feinberg. "Multiple answers exist and can all be correct. The war between in-band and out-of-band virtualisation made good press, but technically they're complementary and they work together."
He points out that in-band virtualisation engines have been shipping and working since 1999, yielding more than enough performance benefit for their users. And in any case, there is no such thing as an out-of-band solution, because even if the host has a block map it will still have to route queries through the storage controller.
"VersaStor still needed a management device to handle exceptions and push out the mapping tables," he adds. "You always have to map [block addresses] somewhere, and you need to do it at line speed."
HP's route is therefore to use in-band virtualisation, via the Rhapsody intelligent switch technology that it buys in from Brocade, alongside its own CASA out-of-band storage controller. The block maps are loaded into blades on the switch and they redirect traffic as required, for example a read request goes direct to disk and a replication request to CASA.
It seems as if it has taken ages for virtualisation to get this far, but Feinberg insists it has been little different to any other new technology: "Virtualisation went through the typical industry hype cycle. Things have a natural adoption curve - people think some technologies will jump past that, but they don't."
He adds that the next big thing will be storage resource management (SRM), and more importantly the concept of data lifecycle management. Virtualisation can help here by adding new capabilities, for example creating different pools of storage (replicated, plain disk, RAID-5, etc) and allocating applications to them based on their needs.
"Many administrators spend all their time keeping things going and never get to the higher levels of how to do more, how to give storage new capabilities," Feinberg says. "Lifecycle data management changes the way people think about storage. The question is, is it more cost-effective to manage efficiently or inefficiently?
"SRM helps you understand what you have and how it's utilised. If you don't measure something, you can't say if you're doing well or badly. What does it mean if you have too many copies? If you have eight replicas, there's also the processes and policies needed to manage those, to back them up."
SRM and data lifecycle management will not bring revenue, but they can - and should - add value to the business, he says.
"Utilisation hovers around 35 percent, we know we can get it to 70 percent or 80 percent. So do some modelling - what does moving from 35 percent to 70 percent utilisation mean to me? It's not just the utility cost, could consolidation also let me negotiate bigger discounts?
"Think about your assets too - they have a useful life, perhaps dictated by a lease or company policy. It would be silly for your provisioning system to use a device in its last month. But how many provisioning tools are tied to your asset management system?
"It's not more work, it's 'We'll give you tools.' We're not going to increase your work if we're sensible, but we do want to tie replication to backup, and link backup to archives, for example, to make sure stuff gets erased at the right time."