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Hitachi Virtual Storage Platform: Disk Drive Architecture

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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?

Drive Size

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.

Drive Connectivity

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.

Summary

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.

About Chris M Evans

Chris M Evans has worked in the technology industry since 1987, starting as a systems programmer on the IBM mainframe platform, while retaining an interest in storage. After working abroad, he co-founded an Internet-based music distribution company during the .com era, returning to consultancy in the new millennium. In 2009 Chris co-founded Langton Blue Ltd (www.langtonblue.com), a boutique consultancy firm focused on delivering business benefit through efficient technology deployments. Chris writes a popular blog at http://blog.architecting.it, attends many conferences and invitation-only events and can be found providing regular industry contributions through Twitter (@chrismevans) and other social media outlets.
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  • http://nigelpoulton.com Nigel Poulton

    Top notch post Chris.

    Couple of things –

    1. Its just my opinion, but the 15K drive is on its way out. I dont expect drive manufacturers to invest heavily in 15K any more. 10K is the future for FC and SAS drives (IMHO). 15K requires a lot more effort to design and manufacture and the effort is not worht it for the small increase in perf. Especially with SSD providing a huge performance incerase.

    2. Enterprise arrays like USP V etc have implemented switched FC-AL backends for a while now. The protocol was of course FC-AL but they hung thr drives off of switches so that you dont get those issues where a faulting (LIPing) drive takes the rest of the loop out. The move to SAS is still great though!

    Nigel

  • http://www.brookend.com Chris Evans

    Nigel, interesting thought regarding 15K versus 10K. If the manufacturers are thinking of dropping 15K, why do you think we haven’t seen more hybrid drives with 10K spindles? It seems to me that this combo could perform as well or better than 15K or even perform well in arrays where SSDs are only shoehorned in. Perhaps it’s a cost thing.

    Thanks for reminding me about switched FC-AL. You are right of course, the physical loop hasn’t existed for some time. But as we all know, full FC-AL loops don’t scale well so, SAS is a much better direction.

    Cheers
    Chris

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  • shjacks55

    Pardon my math: for the same bit density, data is read at the same rate for a 15K 2.5″ drive as a 10K 3.5″ drive. And if Higher quality SATA drives can also attach to SAS controller over the same connector using the same cabling, and ATAPI supports the SCSI layer that SAS uses the what’s the diff? The better quality drives have multiple LUNs in each hard drive SCSI ID (SATA hard disk; ATAPI). The multiple LUNs allow each head to behave as a separate subsystem. The reliability of large multi-terrabyte drives (based on mathematics) . suggests that RAID 6 is the minimum to protect from data loss.

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