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    An FAQ to Make Your Storage System Hum

    May 23rd, 2017

    In our most recent “Everything You Wanted To Know About Storage But Were Too Proud To Ask” webcast series – Part Sepia – Getting from Here to There, we discussed terms and concepts that have a profound impact on storage design and performance. If you missed the live event, I encourage you to check it our on-demand. We had many great questions on encapsulation, tunneling, IOPS, latency, jitter and quality of service (QoS). As promised, our experts have gotten together to answer them all.

    Q. Is there a way to measure jitter?

    A. Jitter can be measured directly as a statistical function of the latency, typically as the Variance or Standard Deviation of the latency. For example a storage device might show an average latency of 5ms with a standard deviation of 1.5ms. This means roughly 95% of the transactions have a latency between 2ms and 8ms (average latency plus/minus two standard deviations), however many storage customers measure jitter indirectly by showing the 99.9%, 99.99%, or 99.999% latency. For example if my storage system has 99.99% latency of 8ms, it means 99.99% of transactions have latency <=8ms and 1/10,000 of transactions have latency >8ms. Percentile latency is an indirect measure of jitter but often easier to calculate or understand than the actual jitter.

    Q. Can jitter be easily characterized for storage, media, and networks.  How and what tools are available for doing this?

    A. Jitter is usually easy to measure on a network using standard network monitoring and reporting tools. It may or may not be easy to measure on storage systems or storage media, depending on the tools available (either built-in to the storage OS or using an external management or monitoring tool).  If you can record the latency of each transaction or packet, then it’s easy to calculate and show the jitter using standard statistical measures such as Variance or Standard Deviation of the latency. What most customers do is just measure the 99.9%, 99.99%, or 99.999% latency. This is an indirect measure of jitter but is often much easier to report and understand than the actual jitter.

    Q.  Generally IOPS numbers are published for a particular block size like 8k write/read size, but in reality, IO request per second could be of mixed sizes, what is your perspective on this?

    A. Most IOPS benchmarks test only one I/O size at a time. Most individual real workloads (for example databases) also use only one I/O size.  It is true that a storage controller or HDD/SSD might need to support multiple workloads simultaneously, each with a different I/O size.  While it is possible to run benchmarks with a mix of different I/O sizes, it’s rarely done because then there are too many workload combinations to test and publish. Some storage devices do not perform well if they must handle both small random and large sequential workloads simultaneously, so a smart storage controller might assign different workload types to different disk groups.

    Q. One often misconfigured parameter is queue depth. Can you talk about how this relates to IOPS, latency and jitter?

    A. Queue depth indicates how many tasks or I/Os can be lined up for a particular controller, interface, or CPU. Having a higher queue depth ensures the CPU (or controller or interface) always has a new task to do as soon as it finishes its current task(s). This can result in higher IOPS because the CPU is less likely to have idle time between transactions. But it could also increase latency because the CPU is more likely to be multi-tasking and context switching between different tasks or workloads.

    Q. Can you please repeat all your examples of tunneling? GRE, MPLS, what others? How can it be IPv4 via IPv6?

    A. VXLAN, LISP, GRE, MPLS, IPSEC.  Any time you encapsulate and send one protocol over another and decapsulate at the other end to send the original frame that process is tunneling. In the case we showed of IPv6 over IPv4, you are taking an original IPv6 frame with its IPv6 header of source address to destination address all IPv6 and sending it over and IPv4 enabled network we are encapsulating the IPv6 frame with an IPv4 header and “tunneling” IPv6 over the IPv4 network.

    Q. I think it’d be possible to configure QoS to a point that exceeds the system capacity. Are there any safeguards on avoiding this scenario?

    A. Some types of QoS allow over-provisioning and others do not. For example a QoS that imposes only maximum limits (and no minimum guarantees) on workloads might not prevent many workloads from exceeding system capacity. If the QoS allows over-provisioning, then you should use system monitoring and alerts to warn you when system capacity has been exceeded, or when any workloads are not getting their minimum guaranteed performance.

    Q. Is there any research being done on using storage analytics along with artificial intelligence (AI) to assist with QoS?  

    A. There are a number of storage analytics products, both third party and storage vendor specific that help with QoS. Whether any of these tools may be described as using AI is debatable, since we’re in the early days of using AI to do much in the storage arena. There are many QoS research projects, and no doubt they will eventually make their way into commercially available products if they prove useful.

    Q. Are there any methods (measurements) to calculate IOPS/MBps in tier capable storage? Would it be wrong metric if we estimate based on medium level, example tier 2 (between 1 and 3)?

    A. This question needs refinement, since tiering is sometimes a cache model rather than a data movement model. And knowing the answer may not actually help! Vendors do have tools (normally internal, since they are quite complex) that can help with the planning of tiered storage.

    By now, we hope you’re not “too proud” to ask some of these storage networking questions. We’ve produced four other webcasts in this “Everything You Wanted To Know About Storage,” series to date. They are all available on-demand. And you can register here for our next one on July 6th where we’ll bring in experts to discuss:

    • Storage APIs and POSIX
    • Block, File, and Object storage
    • Byte Addressable and Logical Block Addressing
    • Log Structures and Journaling Systems

    The Ethernet Storage Forum team and I hope to see you there!

     

     


    Too Proud to Ask Webcast Series Continues – Getting from Here to There Pod

    May 4th, 2017
    As part of the SNIA Ethernet Storage Forum’s successful “Everything You Wanted To Know About Storage But Were Too Proud To Ask” series, we’ve discussed numerous topics about storage devices, protocols, and networks. As we examine some of these topics further, we begin to tease out some subtle nuances; subtle, yet important nevertheless. On May 9th we’ll take on the terms and concepts that affect Storage Architectures as a whole in “Everything You Wanted To Know About Storage But Were Too Proud To Ask – Part Sepia – Getting from Here to There.”  Continue Reading...

    Latency Budgets for Solid State Storage Access

    March 7th, 2017

    New solid state storage technologies are forcing the industry to refine distinctions between networks and other types of system interconnects.  The question on everyone’s mind is: when is it beneficial to use networks to access solid state storage, particularly persistent memory?

    It’s not quite as simple as a “yes/no” answer. The answer to this question involves application, interconnect, memory technology and scalability factors that can be analyzed in the context of a latency budget.

    On April 19th, Doug Voigt, Chair SNIA NVM Programming Model Technical Work Group, returns for a live SNIA Ethernet Storage Forum webcast, “Architectural Principles for Networked Solid State Storage Access – Part 2where we will explore latency budgets for various types of solid state storage access. These can be used to determine which combinations of interconnects, technologies and scales are compatible with Load/Store instruction access and which are better suited to IO completion techniques such as polling or blocking.

    In this webcast you’ll learn:

    • Why latency is important in accessing solid state storage
    • How to determine the appropriate use of networking in the context of a latency budget
    • Do’s and don’ts for Load/Store access

    This is a technical seminar built upon part 1 of this series. If you missed it, you can view it on demand at your convenience. It will give you a solid foundation on this topic, outlining key architectural principles that allow us to think about the application of networked solid state technologies more systematically.

    I hope you will register today for the April 19th event. Doug and I will be on hand to answer questions on the spot.