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    Q&A on All Things iSCSI

    Posted by AlexMcDonald

    In the recent SNIA Ethernet Storage Forum iSCSI pod webcast, from our “Everything You Wanted To Know About Storage Part Were Too Proud to Ask” series, we discussed all things iSCSI. If you missed the live event, it’s now available on-demand. As promised, we’ve compiled all the webcast questions with answers from our panel of experts. If you have additional questions, please feel free to ask them in the comment field of this blog. I also encourage you to check out the other on-demand webcasts in this “Too Proud To Ask” series here and stay informed on upcoming events in this series by following us on Twitter @SNIAESF.

    Q. What does SPDK stand for?

    A. SPDK stands for Storage Performance Development Kit. It is comprised of tools and libraries for developers to write high performance and scalable storage applications in user-mode. For details, see www.spdk.io.

    Q. Can you elaborate on SPDK use? A quick search seems to indicate it is a “half-baked” solution, and available only on Linux systems.

    A. SPDK isn’t a solution, per se – it’s a development kit, intended to provide common building blocks (NVMe drivers, NVMe over Fabrics targets & host/initiator, etc.) for solutions developers who care about latency, license (BSD) and efficiency.

    Q. Is iSCSI ever going to be able to work with object storage?

    A. iSCSI is a block storage protocol while object storage is normally accessed using a RESTful API such as Amazon’s S3 API or the Swift API. For this reason, iSCSI is unlikely to be used for direct access to object storage. However, an object storage system controller could use iSCSI—or other block protocols–to access remote storage enclosures or for data replication. There also could be storage systems that support both iSCSI/block and object storage access simultaneously.

    Q. Does a high-density virtualized workload represent something better served with a full or partial offload solution?

    A. The type of workload that is better served with full or partial offload will really depend more on what that workload is doing. If you are processing a lot of very large data segments, LSO or LRO might be very helpful. If you have a lot of smaller data sets, you might be able to benefit from checksum or chimney offload. Unfortunately, the best way to see is to test things out (but not on production, obviously).

    Q. How does one determine if TOE NIC cards are worth the cost?

    A. This is a really tough question to answer without context. The best way to look at it is do some digging into what your CPU and memory utilization and IO patters look like on your servers and try to map that to TCP connections. If you have a lot of iSCSI IO and a large amount of TCP connections on a server, that might be a candidate for TOE. That’s just a technical response, but then comes the really tricky part – the QUANTITY measurement of how many dollars it is worth… that’s way more challenging. For example, if I have a regular 10G NIC that costs $200 and a TOE card that costs 3x that and only saves 5% CPU, then it may not have enough value. On the other hand, if that 5% CPU can be used by your application to transact enough business to pay for the extra $400, then it’s worth it. Sorry to say that I have seen no scientific way to enumerate that value outside of specific hands-on testing of the solution with and without TOE NICs.

    Q. What is the difference between a stateless and stateful TCP offload? Are RSS and TSS (receive-side and transmission-side scaling) offloads a type of TCP offload or are they operating at a lower level like Layer 2?

    A. Stateless offloading is basically any offload function that can be done without the NIC needing to maintain a connection state table. Checksum offloads are an example. Stateful offloading is any offloading that requires the NIC to maintain a full state connection table. Receive Side Scaling has to do with distributing inbound connections in order to alternate connections coming into the server to different CPUs on a multi-CPU server. There are also some other performance-enhancements that can be done such as RPS, RFS, XPS and some others. These are more about how to get data from the network to the CPU, but are not really specifically TCP functions, as they have to do with uniform processing, not necessarily to do with the TCP stack.

    Q. Is using the host CPU to run iSCSI really a downside?

    A. There may be applications where this is a problem, but you’re generally right; it’s not too much of an issue today. But there are iSCSI-based storage solutions coming up where a consistent 100s of nanoseconds to low microseconds of latency from the device is possible – and that’s very fast indeed. So an iSCSI stack in these circumstances needs to ensure that its consumption of CPU doesn’t increase the latency (even very efficient stacks can add 100s of micro- to milliseconds of latency), or cause contention for the CPU (busy CPUs mean you may queue for compute resources).

    Q. Is the term “onload” for iSCSI new – never heard this before?

    A. It was intended as a quick shorthand word to stand in contrast to iSCSI offload. It will probably not catch on!

    Update: If you missed the live event, it’s now available on-demand. You can also download the webcast slides.




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