This is a series of posts that cover the features of Hitachi’s new enterprise storage platform, the VSP (Virtual Storage Platform), also sold by HP as the P9500 array.
The VSP is the first enterprise array from Hitachi that offers support for 2.5″ drives. We’ve seen these Small Form Factor (SFF) models for some time now and other manufacturers of modular storage arrays have chosen to use them. Why is this move being made now by Hitachi and what can we expect from the market in general?
I’ve made many comments on 2.5″ drives over the years (here and here for example) and from time to time I’ve tracked the capacities of 2.5″ drives compared to their 3.5″ counterparts. The tipping point has pretty much been reached, with 2.5″ 10K 600GB/450GB/300GB/146GB models available from both Seagate and Western Digital and 15K drives available up to 146GB from Seagate. Whilst 15K 2.5″ drives have yet to catch up to their 3.5″ counterparts, 10K drives are clearly on a par (the largest 10K 3.5″ drives are still only 600GB) and consequently offer much higher volume density with lower power/cooling requirements. As an example, a typical 600GB 10K 2.5″ drive will run at 8.32W compared to 10W for a 3.5″ equivalent; the 2.5″ drive will idle at 4.6W compared to 6.49W for its 3.5″ equivalent. Although these numbers are small, scale them up across 1000 drives and you’re looking at a saving of nearly 2000W, or 30% irrespective of the number of drives. Now cost is a factor; if the price per GB of 2.5″ and 3.5″ drives isn’t comparable, then their use may not stack up, however a quick street price check shows 600GB 2.5″ Savvio drives at £318 and 600GB 3.5″ Cheetah drives at £333, making the 2.5″ drive cheaper.
10K speed drives may not be suitable in all occasions and it’s true that the 15K 2.5″ drives still lag behind in capacity. However the VSP also provides block-level tiering, enabling a much finer level of granularity in terms of active data placement. This means bulk deployment of 15K drives isn’t as important; the high performance requirements can be met with 15K or SSD drives. I’ll discuss more on block-level tiering in another post.
Hitachi have made a decision with the VSP not to support fibre channel drives and all back-end drive connectivity is now SAS (Serial Attached SCSI). This has facilitated the move to 2.5″ drives but also provides greater stability over traditional FCAL (Fibre Channel Arbitrated Loop) configurations. SAS is a point-to-point protocol rather than loop-based which means failing drives cause less impact. I’ve seen examples on FCAL arrays where the loss of a drive triggered a failure in the entire loop and 40 drives dropping out of their RAID set. Although no data was lost, the configuration was severely compromised and all of the failing RAID groups required a rebuild when the problem was corrected. With up to 2048 drives per array, back-end drive reliability is essential.
SAS has been available for some time on the Hitachi AMS models and other vendors have already moved their technology over. In terms of bus speeds, SAS now supports 6Gb/s, with Fibre Channel languishing behind at 4Gb/s. Remember that even though the drives can’t provide sustained transfers over around 130-140MB/s, the speed of the backplane is important as it enabled high burst speeds and affects the overall throughput writing to large numbers of drives.
Hitachi have moved the back-end architecture on to the next level. The use of SAS and 2.5″ drives is a step over the competition (VMAX still uses FCAL and 3.5″ drives) and provides higher performance and reliability and reduced cost through space & power/cooling savings. Although 2.5″ drives are slower speed, this is mitigated by the use of block-level tiering. It will be interesting to see how long it takes other vendors to follow Hitachi’s lead.