Disk arrays are the bane of green storage. A myriad of spinning platters eats up electricity and creates heat, which then require cooling, using yet more power.
The average utilisation in terms of written data on such disks is between 10% and 25%, and at the same time 80% of that data will be inactive. It's incredibly wasteful when you consider those figures. So, what are vendors trying to do it?
Few make any claim to be doing anything about the fundamental fact that spinning disks means using power. In reality there's little you can do – if a disk has to spin, it has to spin, although IBM and EMC have slugged it out over the relative power consumption of their DS8000 and DMX3 arrays, with both sides claiming victory.
Most vendors, however, place the emphasis on techniques which improve utilisation – such as information lifecycle management, virtualisation and thin provisioning. IBM, for example, makes much of its ability to gain energy efficiency at the software layer with ILM and disk and tape virtualisation. Meanwhile 3Par claims its customers have replaced 11,000 terabytes of traditional storage capacity with 4,367 TB of thin-provisioned storage capacity in 2007.
Other vendors – EMC, HP and Netapp, for example – emphasise ways of optimising the datacentre to maximise power and cooling efficiency. They offer white papers as well as consulting services to that end, such as EMC's Energy Efficiency Services and HP's Datacentre Thermal Assessment Service.
So, if you can't help but have disks spinning the best you can do is to use them more effectively. Tiering of data within arrays is offered by vendors including Pillar and Compellent.
Each of these uses a quite different approach. Pillar – in its Axiom appliances – assigns a priority to classes of data which determines where on the disk it is held. The most mission critical, with priority of access and I/O speed is stored on the outside – the fastest moving part – of the platter.
Compellent, meanwhile, allows customers to specify a mix of Fibre Channel and SATA drives of different speeds so they can write to that which is most appropriate to the value and throughput needs of specific data. In this way users can, for example, use 15,000rpm drives for tier 1 storage and 10,000rpm drives for tier 2, thus saving energy.
Another approach is to use smaller drives. Infortrend does this with its disk arrays which use 2.5" disks rather than the standard 3.5". Use of such attacks the space problem – they fit the same capacity into 1U as would occupy 2U with 3.5" disks while also using 40% less power for the same amount of drives and a mean time between failure that is about 15% greater. There is a cost though, and that is that with greater density of drives cooling requirements are greater.
HDS also offers the ability to tier, with the option of 7,200rpm SATA drives in its Advanced Modular Storage systems. You also have the option to power down RAID groups in this product family. 3Par offers tiering in its InServ storage servers, and also claims its can offer 'nearline-for-online' capability with SATA drives used where traditionally Fibre Channel disks would have been required.
That principle – that if spinning disks are the enemy of energy efficiency, why not turn them off if they're not being used? – underpins MAID (Massive Array of Idle Disks) technology. It can lay good claim to being one of the most energy efficient disk array configurations and it appeals to businesses that need online archiving, to speed up back up or for use in VTL or continuous data protection type environments.
Copan is the vendor which has made the biggest splash over the past year, and it claims the industry's highest storage density of 44.8TB/sq ft and a power density of 90TB/kilowatt, which compares to 4TB/kilowatt for Fibre Channel and 17TB/kilowatt for SATA disk products. Idle disks are spun and their health verified periodically in what Copan calls 'disk aerobics'. Some users have, however, complained about the form factor of Copan's devices, which are not standard rack size.
The challenge with MAID is primarily that data access times are longer than they would be for permanently spinning disks though they are far more rapid than would be the case with tape-held data. Nexsan addresses this with AutoMAID, which it incorporates into its SATABeast and SATABoy arrays. Dubbing it MAID 2.0, Nexsan users can configure spindles according to the response times they require of spindles, with spinning, reduced speed and sleep modes possible. Like Copan, Nexsan's MAID also powers up idle drives for a surface scan periodically.
Gresham also uses MAID technology in its Clareti VTL devices.
Which is the greenest storage technology of them all? Why tape, of course. Unlike disks, which need to spin constantly, tape is only in motion for as long as it is being written to or read from. Estimates in the industry reckon tape consumes 5% of the power that disk systems use and generates 2% of the heat.
Juergen Arnold, chairman of SNIA Europe, says, "Two years ago at storage shows you'd often see the slogan, 'tape sucks'. Last year that had changed to 'tape is cool', which goes to show the recognition of tape as an energy saving medium that can decrease the need for power and cooling in a tiered storage architecture."
Tape's inherent advantage over spinning disks is the key message pushed by most vendors. In addition watch out for reduced form factors with half height devices available from Quantum, HP, IBM and Tandberg – as well as the ability of the unit to power itself down when no tape activity is taking place.
Where tape vendors do go head to head is on the power used for tape cartridge exchange. While unable to stake out any bragging rights on tape drive electricity consumption, Sun claims the energy cost per cartridge mount for its StorageTek SL8500 is two thirds that of IBM's TS3500.
An alternative pitched at the tape-phobic who are considering disk for long term archiving is Plasmon's UDO (ultra density optical), which is aimed at the standards compliant segment of the market. The UDO media come in solid cartridges and are available as rewriteable, WORM (write once read many) and 'compliant' WORM, which provides verification of data integrity by creating crystalline dots in the medium.
Fibre Channel switches are not the biggest generators of heat in the data centre, but the biggest players the space – Brocade and Cisco – have created a great deal of it in trying to prove which one of them is the most energy efficient.
Currently Brocade seems to be coming out on top, claiming its 48000 Director is up to three times more efficient than Cisco's MDS 9513. Lab validation by ESG and an independent electrician have been published which show Brocade coming out better in both power draw per port and power draw per gigabit per second bandwidth. The company also has good claims on better airflow with a back-to-front fan configuration as opposed to Cisco's side-to-side.
Brocade's assumption of the energy efficiency mantle is a reverse from its early days when the company's first Director level switch which was infamous for power and cooling issues. Those early troubles caused it to address design issue and its latest ASIC is five times more power efficient.
This was first published in June 2008