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Holiday is over

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I’ve been off on holiday for a week (Spain as it happens), now I’m back. I’m pleased to say I didn’t think of storage once. Not true actually; the cars we’d hired didn’t have enough storage space for the luggage – why does that always happen?

So back to work and continuing on virtualisation. For those who haven’t read the previous posts, I’m presenting AMS storage through a USP. I finished the presentation of the storage; the USP can see the AMS disks. These are all about 400GB each, to ensure I can present all the data I need.

Now, I’m presenting three AMS systems through one USP. A big issue is how cache should be managed. Imagine the write process; a write operation is received by the USP first and confirmed to the host after writing to USP cache. The USP then destages the write afterwards down to the AMS – which also has cache – and then to physical disk. Aside from the obvious question of “where is my data?” if there is ever a hardware or component failure, my current concern is managing cache correctly. The AMS has only 16GB of cache, that’s a total of 48GB in all three systems. The USP has 128GB of cache, over twice the total of the AMS systems. It’s therefore possible for the USP to accept a significant amount of data – especially if it is the target of TrueCopy, ShadowImage or HUR operations. When this is destaged, the AMS systems run the risk of being overwhelmed.

This is a significant concern. I will be using Tuning Manager to keep on top of the performance stats. In the meantime, I will configure some test LUNs and see what performance is like. The USP also has a single array group (co-incidentally also about 400GB) which I will use to perform a comparison test.

This is starting to get interesting…

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.
  • Rick Freedman

    Chris:

    I’ll be very interested to hear how you progress with this. Coincidentally I am just starting to work on a whitepaper specifically on tiered storage performance. I’m just the dumb marketing guy, so I’ve been interviewing our top storage performance experts on some of the issues related to using external storage on a USP. Given my travel schedule and other projects, I probably won’t have the paper done for 60-90 days, so I’m not sure that it will be done in time to provide much insight for you.

    The use of the USP’s cache, via the cache-mode enable option, can significantly improve performance accessing data on the AMS systems. However, this can have a significant impact to everything else running on, or externally attached to, the USP. Using Hitachi Virtual Partition Manager can extremely valuable to control the amount of cache for various applications.

    If you have specific questions, I probably don’t know the answer, but have access to several performance experts in-house and I’d be pleased to help find the answers for you… and will help me make my whitepaper more complete. Please let me know if I can be of assistance.

    Regards,
    Rick Freedman, HDS, Product Marketing

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