Tag: Ethernet Storage

Are Ethernet-attached SSDs Brilliant?

Ted Vojnovich Leave a comment

Several solid state disk (SSD) and networking vendors have demonstrated ways to connect SSDs directly to an Ethernet network. They propose that deploying Ethernet SSDs will be more scalable, easier to manage, higher performance, and/or lower cost than traditional storage networking solutions that use a storage controller (or hyperconverged node) between the SSDs and the network.

Who would want to attach SSDs directly to the network? Are these vendors brilliant or simply trying to solve a problem that doesn’t exist? What are the different solutions that could benefit from Ethernet SSDs? Which protocols would one use to access them? How will orchestration be used to enable applications to find assigned Ethernet SSDs? How will Ethernet SSDs affect server subsystems such as Ethernet RAID/mirroring and affect solution management such as Ethernet SAN orchestration?  And how do Ethernet SSDs relate to computational storage?  

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Intro to Incast, Head of Line Blocking, and Congestion Management

Tim Lustig Leave a comment

For a long time, the architecture and best practices of storage networks have been relatively well-understood. Recently, however, advanced capabilities have been added to storage that could have broader impacts on networks than we think.

The three main storage network transports – Fibre Channel, Ethernet, and InfiniBand – all have mechanisms to handle increased traffic, but they are not all affected or implemented the same way. For instance, utilizing a protocol such as NVMe over Fabrics will offer very different methodologies for handling congestion avoidance, burst handling, and queue management when looking at one networking in comparison to another.

Unfortunately, many network administrators may not understand how different storage solutions place burdens upon their networks. As more storage traffic traverses the network, customers face the risk of congestion leading to higher-than-expected latencies and lower-than expected throughput.

That’s why the SNIA Networking Storage Forum (NSF) is hosting a live webcast on June 18, 2019, Introduction to Incast, Head of Line Blocking, and Congestion Management where our NSF experts will cover:

  • Typical storage traffic patterns
  • What is Incast, what is head of line blocking, what is congestion, what is a slow drain, and when do these become problems on a network?
  • How Ethernet, Fibre Channel, InfiniBand handle these effects
  • The proper role of buffers in handling storage network traffic
  • Potential new ways to handle increasing storage traffic loads on the network

Register today to save your spot for June 18th. As always, our experts will be available to answer your questions. We hope to see you there.

Would You Like Some Rosé with Your iSCSI?

J Metz Leave a comment

Would you like some rosé with your iSCSI? I’m guessing that no one has ever asked you that before. But we at the SNIA Ethernet Storage Forum like to get pretty colorful in our “Everything You Wanted To Know about Storage But Were Too Proud To Ask” webcast series as we group common storage terms together by color rather than by number.

In our next live webcast, Part Ros̩ РThe iSCSI Pod, we will focus entirely on iSCSI, one of the most used technologies in data centers today. With the increasing speeds for Ethernet, the technology is more and more appealing because of its relative low cost to implement. However, like any other storage technology, there is more here than meets the eye.

We’ve convened a great group of experts from Cisco, Mellanox and NetApp who will start by covering the basic elements to make your life easier if you are considering using iSCSI in your architecture, diving into:

  • iSCSI definition
  • iSCSI offload
  • Host-based iSCSI
  • TCP offload

Like nearly everything else in storage, there is more here than just a protocol. I hope you’ll register today to join us on March 2nd and learn how to make the most of your iSCSI solution. And while we won’t be able to provide the rosé wine, our panel of experts will be on-hand to answer your questions.

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

We’ve Been Thinking…What Does Hyperconverged Mean to Storage?

John Kim Leave a comment

Here at the SNIA Ethernet Storage Forum (ESF), we’ve been discussing how hyperconverged adoption will impact storage. Converged Infrastructure (CI), Hyperconverged Infrastructure (HCI), along with Cluster or Cloud In a Box (CIB) are popular trend topics that have gained both industry and customer adoption. As part of data infrastructures, CI, HCI, and CIB enable simplified deployment of resources (servers, storage, I/O networking, hypervisor, application software) across different environments.

But what do these approaches mean for the storage environment? What are the key concerns and considerations related specifically to storage? How will the storage be connected to (or included in) the platform? Who will protect and backup the data? And most importantly, how do you know that you’re asking the right questions in order to get to the right answers?

Find out on March 15th in a live SNIA-ESF webcast, “What Does Hyperconverged Mean to Storage.” We’ve invited expert Greg Schulz, founder and analyst of Server StorageIO, to answer the questions we’ve been debating. Join us, as Greg will move beyond the hype (pun intended) to discuss:

  • What are the storage considerations for CI, CIB and HCI
  • Why fast applications and fast servers need fast I/O
  • Networking and server-storage I/O considerations
  • How to avoid aggravation-causing aggregation (bottlenecks)
  • Aggregated vs. disaggregated vs. hybrid converged
  • Planning, comparing, benchmarking and decision-making
  • Data protection, management and east-west I/O traffic
  • Application and server north-south I/O traffic

Register today and please bring your questions. We’ll be on-hand to answer them during this event. We hope to see you there!

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

Questions on the 2017 Ethernet Roadmap for Networked Storage

Fred Zhang Leave a comment

Last month, experts from Dell EMC, Intel, Mellanox and Microsoft convened to take a look ahead at what’s in store for Ethernet Networked Storage this  year. It was a fascinating discussion of anticipated updates. If you missed the webcast, “2017 Ethernet Roadmap for Networked Storage,” it’s now available on-demand. We had a lot of great questions during the live event and we ran out of time to address them all, so here are answers from our speakers.

Q. What’s the future of twisted pair cable? What is the new speed being developed with twisted pair cable?

A. By twisted pair I assume you mean USTP CAT5,6,7 etc.  The problem going forward with high speed signaling is the USTP stands for Un-Shielded and the signal radiates off the wire very quickly.   At 25G and 50G this is a real problem and forces the line card end to have a big, power consuming and costly chip to dig the signal out of the noise. Anything can be done, but at what cost.  25G BASE-T is being developed but the reach is somewhere around 30 meters.  Cost, size, power consumption are all going up and reach going down – all opposite to the trends in modern high speed data centers.  BASE-T will always have a place for those applications that don’t need the faster rates.

Q. What do you think of RCx standards and cables?

A.  So far, Amphenol, JAE and Volex are the suppliers who are members of the MSA. Very few companies have announced or discussed RCx.   In addition to a smaller connector, not having an EEPROM eliminates steps in the cable assembly manufacture, hence helping with lowering the cost when compared to traditional DAC cabling. The biggest advantage of RCx is that it can help eliminate bulky breakout cables within a rack since a single RCx4 receptacle can accept a number of combinations of single lane, 2 lane or 4 lane cable with the same connector on the host. RCx ports can be connected to existing QSFP/SFP infrastructure with appropriate cabling.  It remains to be seen, however, if it becomes a standard and popular product or remain as a custom solution.

Q. How long does AOC normally reach, 3m or 30m?  

A. AOCs pick it up after DAC drops off about 3m.  Most popular reaches are 3,5,and 10m and volume drops rapidly after 15,20,30,50, and100. We are seeing Ethernet connected HDD’s at 2.5GbE x 2 ports, and Ceph touting this solution.   This seems to play well into the 25/50/100GbE standards with the massive parallelism possible.

Q. How do we scale PCIe lanes to support NVMe drives to scale, and to replace the capacity we see with storage arrays populated completely with HDDs?

A.  With the advent of PCIe Gen 4, the per-lane rate of PCIe is going from 8 GT/s to 16GT/s. Scaling of PCIe is already happening.

Q. How many NVMe drives does it take to saturate 100GbE?

A.  3 or 4 depending on individual drives.

Q. How about the reliability of Ethernet? A lot of people think Fibre Channel has better reliability than Ethernet.

A.  It’s true that Fibre Channel is a lossless protocol. Ethernet frames are sometimes dropped by the switch, however, network storage using TCP has built in error-correction facility. TCP was designed at a time when networks were less robust than today. Ethernet networks these days are far more reliable.

Q. Do the 2.5GbE and 5GbE refer to the client side Ethernet port or the server Ethernet port?

A.  It can exist on both the client side and the server side Ethernet port.

Q. Are there any 25GbE or 50GbE NICs available on the market?

A.  Yes, there are many that are on the market from a number of vendors, including Dell, Mellanox, Intel, and a number of others.

Q.  Commonly used Ethernet speeds are either 10GbE or 40GbE. Do the new 25GbE and 50GbE require new switches?

A. Yes, you need new switches to support 25GbE and 50GbE. This is, in part, because the SerDes rate per lane at 25 and 50GbE is 25Gb/s, which is not supported by the 10 and 40GbE switches with a maximum SerDes rate of 10Gb/s.

Q.  With a certain number of SerDes coming off the switch ASIC, which would you prefer to use 100G or 40G if assuming both are at the same cost?

A.  Certainly 100G. You get 2.5X the bandwidth for the same cost under the assumptions made in the question.

Q.  Are there any 100G/200G/400G switches and modulation available now?

A.  There are many 100G Ethernet switches available on the market today include Dell’s Z9100 and S6100, Mellanox’s SN2700, and a number of others. The 200G and 400G IEEE standards are not complete as of yet. I’m sure all switch vendors will come out with switches supporting those rates in the future.

Q. What does lambda mean?

A.  Lambda is the symbol for wavelength.

Q. Is the 50GbE standard ratified now?

A. IEEE 802.3 just recently started development of a 50GbE standard based upon a single-lane 50 Gb/s physical layer interface. That standard is probably about 2 years away from ratification. The 25G Ethernet Consortium has a ratified specification for 50GbE based upon a dual-lane 25 Gb/s physical layer interface.

Q. Are there any parallel options for using 2 or 4 lanes like in 128GFCp?

A.  Many Ethernet specifications are based upon parallel options. 10GBASE-T is based upon 4 twisted-pairs of copper cabling. 100GBASE-SR4 is based upon 4 lanes (8 fibers) of multimode fiber. Even the industry MSA for 100G over CWDM4 is based upon four wavelengths on a duplex single-mode fiber. In some instances, the parallel option is based upon the additional medium (extra wires or fibers) but with fiber optics, parallel can be created by using different wavelengths that don’t interfere with each other.

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

 

 

Ethernet Networked Storage – FAQ

Fred Zhang Leave a comment

At our SNIA Ethernet Storage Forum (ESF) webcast “Re-Introduction to Ethernet Networked Storage,” we provided a solid foundation on Ethernet networked storage, the move to higher speeds, challenges, use cases and benefits. Here are answers to the questions we received during the live event.

Q.  Within the iWARP protocol there is a layer called MPA (Marker PDU Aligned Framing for TCP) inserted for storage applications. What is the point of this protocol?

A. MPA is an adaptation layer between the iWARP Direct Data Placement Protocol and TCP/IP. It provides framing and CRC protection for Protocol Data Units.   MPA enables packing of multiple small RDMA messages into a single Ethernet frame.   It also enables an iWARP NIC to place frames received out-of-order (instead of dropping them), which can be beneficial on best-effort networks. More detail can be found in IETF RFC 5044 and IETF RFC 5041.

Q. What is the API for RDMA network IPC?

The general API for RDMA is called verbs. The OpenFabrics Verbs Working Group oversees the development of verbs definition and functionality in the OpenFabrics Software (OFS) code. You can find the training content from OpenFabrics Alliance here. General information about RDMA for Ethernet (RoCE) is available at the InfiniBand Trade Association website. Information about Internet Wide Area RDMA Protocol (iWARP) can be found at IETF: RFC 5040, RFC 5041, RFC 5042, RFC 5043, RFC 5044.

Q. RDMA requires TCP/IP (iWARP), InfiniBand, or RoCE to operate on with respect to NVMe over Fabrics. Therefore, what are the advantages of disadvantages of iWARP vs. RoCE?

A. Both RoCE and iWARP support RDMA over Ethernet. iWARP uses TCP/IP while RoCE uses UDP/IP. Debating which one is better is beyond the scope of this webcast, but you can learn more by watching the SNIA ESF webcast, “How Ethernet RDMA Protocols iWARP and RoCE Support NVMe over Fabrics.”

Q. 100Gb Ethernet Optical Data Center solution?

A.  100Gb Ethernet optical interconnect products were first available around 2011 or 2012 in a 10x10Gb/s design (100GBASE-CR10 for copper, 100GBASE-SR10 for optical) which required thick cables and a CXP and a CFP MSA housing. These were generally used only for switch-to-switch links. Starting in late 2015, the more compact 4x25Gb/s design (using the QSFP28 form factor) became available in copper (DAC), optical cabling (AOC), and transceivers (100GBASE-SR4, 100GBASE-LR4, 100GBASE-PSM4, etc.). The optical transceivers allow 100GbE connectivity up to 100m, or 2km and 10km distances, depending on the type of transceiver and fiber used.

Q. Where is FCoE being used today?

A. FCoE is primarily used in blade server deployments where there could be contention for PCI slots and only one built-in NIC. These NICs typically support FCoE at 10Gb/s speeds, passing both FC and Ethernet traffic via connect to a Top-of-Rack FCoE switch which parses traffic to the respective fabrics (FC and Ethernet). However, it has not gained much acceptance outside of the blade server use case.

Q. Why did iSCSI start out mostly in lower-cost SAN markets?

A. When it first debuted, iSCSI packets were processed by software initiators which consumed CPU cycles and showed higher latency than Fibre Channel. Achieving high performance with iSCSI required expensive NICs with iSCSI hardware acceleration, and iSCSI networks were typically limited to 100Mb/s or 1Gb/s while Fibre Channel was running at 4Gb/s. Fibre Channel is also a lossless protocol, while TCP/IP is lossey, which caused concerns for storage administrators. Now however, iSCSI can run on 25, 40, 50 or 100Gb/s Ethernet with various types of TCP/IP acceleration or RDMA offloads available on the NICs.

Q. What are some of the differences between iSCSI and FCoE?

A. iSCSI runs SCSI protocol commands over TCP/IP (except iSER which is iSCSI over RDMA) while FCoE runs Fibre Channel protocol over Ethernet. iSCSI can run over layer 2 and 3 networks while FCoE is Layer 2 only. FCoE requires a lossless network, typically implemented using DCB (Data Center Bridging) Ethernet and specialized switches.

Q. You pointed out that at least twice that people incorrectly predicted the end of Fibre Channel, but it didn’t happen. What makes you say Fibre Channel is actually going to decline this time?

A. Several things are different this time. First, Ethernet is now much faster than Fibre Channel instead of the other way around. Second, Ethernet networks now support lossless and RDMA options that were not previously available. Third, several new solutions–like big data, hyper-converged infrastructure, object storage, most scale-out storage, and most clustered file systems–do not support Fibre Channel. Fourth, none of the hyper-scale cloud implementations use Fibre Channel and most private and public cloud architects do not want a separate Fibre Channel network–they want one converged network, which is usually Ethernet.

Q. Which storage protocols support RDMA over Ethernet?

A. The Ethernet RDMA options for storage protocols are iSER (iSCSI Extensions for RDMA), SMB Direct, NVMe over Fabrics, and NFS over RDMA. There are also storage solutions that use proprietary protocols supporting RDMA over Ethernet.

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

 

 

 

 

 

 

 

 

 

 

 

Storage Basics Q&A and No One’s Pride was Hurt

J Metz Leave a comment

In the first of our “Everything You Wanted To Know About Storage But Were Too Proud To Ask – Part Chartreuse,” we covered the storage basics to break down the entire storage picture and identify the places where most of the confusion falls. It was a very well attended event and I’m happy to report, everyone’s pride stayed intact! We got some great questions from the audience, so as promised, here are our answers to all of them:

Q. What is parity? What is XOR?

A. In RAID, there are generally two kinds of data that are stored: the actual data and the parity data. The actual data is obvious; parity data is information about the actual data that you can use to reconstruct it if something goes wrong.

It’s important to note that this is not simply a copy of A and B, but rather a logical operation that is applied to the data. Commonly for RAID (other than simple mirroring) the method used is called an exclusive or, or XOR for short. The XOR function outputs true only when inputs differ (one is true, the other is false).

There’s a neat feature about XOR, and the reason it’s used by RAID. Calculate the value A XOR B (let’s call it AxB). Here’s an example on a pair of bytes.

A                                                                   10011100

B                                                                   01101100

A XOR B is AxB              11110000

Store all three values on separate disks. Now, if we lose A or B, we can use the fact that AxB XOR B is equal to A, and AxB XOR A is equal to B. For example, for A;

B                                                                   01101100

AxB                                                           11110000

A XOR AxB is A              10011100

We’ve regenerated the A we lost. (If we lose the parity bits, they can just be reconstructed from A and B.)

Q. What is common notation for RAID? I have seen RAID 4+1, and RAID (4,1). In the past, I thought this meant a total of 5 disks, but in your explanation it is only 4 disks.

A. RAID is notated by levels, which is determined by the way in which data is laid out on disk drives (there are always at least two). When attempting to achieve fault tolerance, there is always a trade-off between performance and capacity. Such is life.

There are 4 common RAID levels in use today (there are others, but these are the most common): RAID 0, RAID 1, RAID 5, and RAID 6. As a quick reminder from the webinar (you can see pictures of these in action there):

  • RAID 0: Data is striped across the disks without any parity. Very fast, but very unsafe (if you lose one, you lose all)
  • RAID 1: Data is mirrored between disks without any parity. Slowest, but you have an exact copy of the data so there is no need to recalculate anything to reconstruct the data.
  • RAID 5: Data is striped across multiple disks, and the parity is striped across multiple disks. Often seen as the best compromise: Fast writes and good safety net. Can withstand one disk loss without losing data.
  • RAID 6: Data is striped across multiple disks, and two parity bits are stored on all the disks. Same advantages of RAID 5, except now you can lose 2 drives before data loss.

Now, if you have enough disks, it is possible to combine RAID levels. You can, for instance, have four drives that combine mirroring and striping. In this case, you can have two sets of drives that are mirrored to each other, and the data is striped to each of those sets. That would be RAID 1+0, or often called RAID 10. Likewise, you can have two sets of RAID 5 drives, and you could stripe or mirror to each of those sets, and it would be RAID 50 or RAID 51, respectively.

Erasure Coding has a different notation, however. It does not use levels like RAID; instead, EC identifies the number of data bits and the number of parity bits.

So, with EC, you take a file or object and split it into ‘k’ blocks of equal size. Then, you take those k blocks and generate n blocks of the same size, such that any k out of n blocks suffice to reconstruct the original file. This results in a (n,k) notation for EC.

Since RAID is a subset of EC, RAID6 is the equivalent of EC or RAID(n,2) or n data disks and 2 parity disks. RAID(4,1) is RAID5 with 4 data and 1 parity, and so on.

Q. Which RAIDs are classified/referred to as EC? I have often heard people refer to RAID 5/6 as EC. Is this only limited to 5/6?

A. All RAID levels are types of EC. The math is slightly different; traditional RAID uses XOR, and EC uses Galois Fields or polynomial arithmetic.

Q. What’s the advantage of RAID5 over RAID1?

A. As noted above, there is a tradeoff between the amount of capacity that you need in order to stay fault tolerant, and the performance you wish to have in any system.

RAID 1 is a mirrored system, where you have a single block of data being written twice – one to each disk. This is done in parallel, so it doesn’t take any extra time to do the write, but there’s no speed-up either. One advantage, however, is that if a disk fails there is no need to perform any logical calculations to reconstruct data – you already have a copy of the intact data.

RAID 5 is more distributed. That is, blocks of data are written to multiple disks simultaneously, along with a parity block. That is, you are breaking up the writing obligations across multiple disks, as well as sending parity data across multiple disks. This significantly speeds up the write process, but more importantly it also distributes the recovery capabilities as well so that any disk can fail without losing data.

Q. So RAID improves WRITES? I guess because it breaks the data into smaller pieces that can be written in parallel. If this is true, then why will READ not benefit from RAID? Isn’t it that those pieces can be read and re-combined into a larger piece from parallel sources would be faster?

A. RAID and the “striping” of IO can improve writes by reducing serialization by allowing us to write anywhere. But a specific block can only be read from the disk it was written to, and if we’re already reading or writing to that disk and it’s busy – we must wait.

Q. Why is EC better for object stores than RAID?

A. Because there’s more redundancy, EC can be made to operate across unreliable and less responsive links, and at potentially geographic scales.

Q: Can you explain about the “RAID Penalty?” I’ve heard it called “Write Penalty” or “Read before Write penalty.”

A. When updating data that’s already been written to disk, there’s a requirement to recalculate the parity data used by RAID. For example, if we update a single byte in a block, we need to read all the blocks, recalculate the parity, and write back the updated data block and the parity block (twice in the case of dual parity RAID6).

There are some techniques that can be used to improve the performance impact. For example, some systems don’t update blocks in place, but use pointer-based systems and only write new blocks. This technique is used by flash-based SSDs as the write size is often 256KB or larger. This can be done in the drive itself, or by the RAID or storage system software. It is very important to avoid when using Erasure Coding as there are so many data blocks and parity blocks to recalculate and rewrite that it would become prohibitive to do an update.

Q.  What is the significance of RAIN? We have not heard much about it.

A.A Redundant Array of Independent Nodes works under the same principles of RAID – that is, each node is treated as a failure domain that must be avoided as a Single Point of Failure (SPOF).Where as RAID maintains an understanding of data placement on individual drives within a node, RAIN maintains an understanding of data placement on nodes (that contain drives) within a storage environment.

Q.  Is host same as node?

A.  At its core, a “node” is an endpoint. So, a host can be a node, but so can a storage device at the other end of the wire.

Q.  Does it really matter what Erasure Coding (EC) technologies are named or is EC just EC?

A.  A. Erasure Coding notation refers to the level of resilience involved. This notation underscores not only the write patterns for storage of data, but also the mechanisms necessary for recovery. What ‘matters’ really will depend upon the level of involvement for those particular tasks.

Q. Is the Volume Manager concept related to Logical Unit Numbering (LUNs)?

A.  It can be. A volume manager is an abstraction layer that allows a host operating system to create a Volume out of one or more media locations. These locations can be either logical or physical. A LUN is an aggregation of media on the target/storage side. You can use a Volume Manager to create a single, logical volume out of multiple LUNs, for instance.

A. For additional information on this, you may want to watch our SNIA-ESF webcast, “Life of a Storage Packet (Walk).”

Q. What’s the relationship between disk controller and volume manager?

A. Following on the last question, a disk controller does exactly what it sounds like – it controls disks. A RAID controller, likewise, controls disks and the read/write mechanisms. Some RAID controllers have additional software abstraction capabilities that can act as a volume manager as well.

We hope these answers clear things up a bit more. As you know, our “Everything You Wanted To Know About Storage, But Were Too Proud To Ask” is a series, since this Chartreuse event, we’ve done “Part Mauve – The Architecture Pod” where we explained channel vs. bus, control plane vs. data plane and fabric vs. network. Check it out on-demand and follow us on Twitter @SNIAESF for announcements on upcoming webcasts.

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

 

The Everything You Want To Know About Storage Is On Again With Part Mauve – The Architecture Pod

J Metz Leave a comment

The first installment of our “colorful” Webcast series, “Everything You Wanted To Know about Storage But Were Too Proud To Ask – Part Chartreuse,” covered the fundamental elements of storage systems. If you missed it, you can check it out on-demand. On November 1st, we’ll be back at it, focusing on the network aspect of storage systems with “Everything You Wanted To Know About Storage But Were Too Proud To Ask – Part Mauve.”

As with any technical field, it’s too easy to dive into the jargon of the pieces and expect people to know exactly what you mean. Unfortunately, some of the terms may have alternative meanings in other areas of technology. In this Webcast, we look at some of those terms specifically and discuss them as they relate to storage networking systems.

In particular, you’ll find out what we mean when we talk about:

  • Channel versus Busses
  • Control Plane versus Data Plane
  • Fabric versus Network

Register now for Part Mauve of “Everything You Wanted To Know About Storage But Were Too Proud to Ask.

For people who are familiar with data center technology, whether it be compute, programming, or even storage itself, some of these concepts may seem intuitive and obvious… until you start talking to people who are really into this stuff. This series of Webcasts will help be your Secret Decoder Ring to unlock the mysteries of what is going on when you hear these conversations. We hope to see you there!

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

 

 

Principles of Networked Solid State Storage – Q&A

J Metz Leave a comment

At this month’s SNIA Ethernet Storage Forum Webcast, “Architectural Principles for Networked Solid State Storage Access,” Doug Voigt, Chair of the SNIA NVM Programming Technical Working Group, and a member of the SNIA Technical Council, outlined key architectural principles surrounding the application of  networked  solid state technologies. We had a flurry of questions near the end of the Webcast that we did not have enough time to answer. Here are Doug’s answers to all the questions we received during the event:

Q. Are there wait cycles in accessing persistent memory?

A. It depends entirely on which persistent memory (PM) technology is being accessed and how the memory interconnect is used.   Some technologies have write times that are quite different from read times.   When using tightly timed interconnects such as DDR with those technologies it may be difficult to avoid wait cycles.

Q. How do Pmalloc and malloc share the virtual address space of the application?

A. This is entirely up to the OS and other libraries operating within any constraints of the processor architecture-specific memory management units.   A good mental model would be fairly large regions of contiguous address space in both the physical and virtual domains, where each region will comprise a single type of memory. Capacity will be reserved for pmalloc and malloc in the appropriate regions.

Q. Always flush after doing your memory-mapped IO.   Is that simply good hygiene?

A. Not exactly. The term “Memory Mapped IO” is used to reference control plane (as opposed to data plane) access.   It is often reasonable to set up control plane memory as uncacheable. The need for strict order of access to physical control plane registers is so pervasive that caching is generally not useful. Uncacheable writes are always flushed by the processor, as opposed to the application.

Generally with memory mapped IO devices the data plane uses direct memory access (DMA).   With memory mapped files (as opposed to memory mapped IO) Load/Store (more commonly referred to as “Ld/St”), not DMA, is used in the data plane. Disabling caching in the data plane is generally a big performance sacrifice for small byte range access.

In the Ld/St datapath, strategically placed flushing is required to retain both performance and power failure recovery. The SNIA NVM Programming Model describes this type of functionality.

Q. Once NVDIMM support become pervasive with support from NVMe drives in the server box, should network storage be more focused on SAS Flash or just SAS HDDs?

A. Not necessarily.   NVMe over Fabric, Fibre Channel and iSCSI are also types of networked storage that will likely retain significant market share relative to SAS.

Q. Are the ‘Big Data’ Data Warehouse applications starting to use the persistence memory and domain technologies in their applications?

A. It is too early to see much of this yet. PM technologies might become a priority as a staging area for analytic applications with high ingest or checkpoint rates. NVDIMMs are likely to be too expensive to store anything “big” for quite a while.

Q. Also, is the persistence memory/domains being used in the Hyper-converged and Converged hardware infrastructures?

A. Persistent memory is quintessentially (Hyper-) converged.   It wouldn’t be unreasonable to expect some traction with hyper-converged solutions that experience high storage-performance demand.

Q. What distance would you associate with 10’s of microseconds?

A. In terms of transmission delay, 10’s of uS align with a campus or small city scale, but the distance itself is often not the primary factor.   Switching delays, transmission line properties and software overhead are generally bigger factors.

Q. So latency would be the binding factor for distances…not a question, an observation.

A. Yes, in effect, either through transmission or relay.   See above.

Q. Aren’t there multi-threaded SSDs?

A. Yes, but since the primary metric in this presentation is latency we ignore multi-threading.   It can enable more work to get done, but it generally increases latency rather than reducing it.

Q. Is Pmalloc universal usage?

A. The term is starting to be recognized among developers and has been used in research. Various similar names have been used in early research prototypessuch as pmalloc in Mnemosyne and nvmalloc in SCMFS.

Q. So how would PM help in a (stock broking) requirement, where we currently prophesize an RDMA or iWARP solution?

A. With PM the answer is always lower latency.   PM can be litegrated like memory or like flash. RDMA network paths for both of these options were discussed in the presentation. In either case, PM is low-latency enough that networking and software overheads will completely determine performance, even when using RDMA. The performance boost from PM is greatest when it is accessed locally.   If remote access is a requirement then the new work being done in the RDMA community should help.

Q. If data stored in memory requires to be copied to a different host, memory (for consistency) how does PM assist, or is there an extension to PM? Coherency between multiple hosts in a cluster, if you will?

A. PM technology does not help with this; the methods of managing consistency across hosts remain unchanged by PM.   All PM offers is low latency persistence.

Coordination across hosts or nodes in a cluster must use existing clustering techniques such as locking and quorums. In addition, the relative timescales of memory access and network communication suggest the application of asynchronous remote replication techniques used in today’s storage solutions.

Regarding coherency, PM brings nothing new to the known techniques for managing coherency.   Classical cluster architecture must be applied outside of symmetric multi-processing coherency domains. Within coherency domains, all of the logic is above the PM level in a processor side memory controller or a software emulation of the same algorithms.

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

 

 

 

Q&A on Exactly How iSCSI has Evolved

David Fair 2 Comments

Our recent SNIA ESF Webcast, “The Evolution of iSCSI” drew a big and diverse group of attendees. From beginners looking for iSCSI basics, to experts with a lot of iSCSI deployment experience, there were plenty of good questions. Our presenters, Andy Banta and Fred Knight, did a great job answering as many as they could during the live event, but we didn’t have time to get to them all. So here are answers to them all. And by the way, if you missed the Webcast, it’s now available on-demand.

Q. What are the top 3 reasons to choose iSCSI over FC SAN?

A. 1. Use of commodity equipment and protocols. It means that you don’t have to set up a completely separate network. It means you don’t have to buy separate HBAs. 2. Inherent networking capability. Built on top of TCP/IP, it benefits from any networking technology to come along. These include routing, tunneling, authentication, encryption, etc. 3. Ease of automation and configuration. In it’s simplest form, an iSCSI host only needs to know the IP address of the target system. In more complex systems, hosts and storage provide APIs to allow automation through scripting or management tools.

Q. Please comment on why SCSI went from being a widely used protocol for all sorts of devices to being focused as only essentially a storage protocol?

A. SCSI was originally designed as both a protocol and a bus (original Parallel SCSI). Because there were no other busses, the SCSI bus did it all; disks, tapes, scanners, printers, Optical (CDs), media changers, etc. As other busses came onto the market (think USB), many of those devices moved to the new bus (CDs, printers, scanners, etc.) Commodity devices used commodity busses (IDE, SATA, USB), and enterprise devices used enterprise busses (FC, SAS); and so, disks, tapes, and media changers mostly stayed on SCSI.

The name SCSI can be confusing for some, as the term originally was used for both the SCSI protocol and the SCSI bus. The term for the SCSI protocol is all that remains today; the SCSI bus (the old SCSI parallel bus) is no longer in wide use. Today, the FC bus, or the SAS bus, or the SoP bus, or the SRP bus are used to carry the SCSI protocol. The SCSI Architecture Model (SAM) describes a very distinct separation between the device layer (the SCSI protocol) and the transport layer (the bus).

And, the SCSI command set has become the basis for many subsequent command sets. The JEDEC group used the SCSI command set as a model (JEDEC devices are in your cell phone), the ATAPI devices used SCSI commands, and many SCSI commands and SATA commands have a common heritage. The Mt. Fuji group (a standards group in Japan) also uses SCSI as the basis for new DVD and BlueRay devices. So, while not widely known, the SCSI command family has grown well beyond what is managed by the ANSI/INCITS T10 committee that originally defined SCSI in to a broad set of capabilities that are used across the industry, by a broad group of organizations. But, that all said, scanners and printers are still on USB, and SCSI is almost all about storage in one form or another.

Q. How does iSCSI support software-defined storage?

A. Answered during the talk. SDS provides more automation and knobs on the storage capabilities. But SDS still needs a way to transport the storage and iSCSI works perfectly fine for that. They are complementary technologies, not competing.

Q. With 40Gb and faster coming soon to a server near you, what kind of impact will that have on CPU utilization? Will smaller servers be able to push that much traffic?

A. More throughput simply requires more CPU. With good multithreaded drivers available, this can mean simply adding cores to keep the pipe as full as possible. As we mentioned near the end, using iSCSI with RDMA lightens the load on the CPU even more, so you’ll probably be seeing more of that.

Q. Is IPSec commonly supported on iSCSI targets?  

A. Yes, IPsec is required to be implemented on an iSCSI target to be a compliant device.   However, it is not commonly enabled by customers. If they MUST provide IPsec there are a lot of non-compliant initiators and targets on the market.

Q. I’m told direct connect with iSCSI is discouraged, that there should be a switch in place to handle the buffering, latency, acknowledgement etc….. Is this true or a best practice to make sure switches are part of the design?

A. If you have no need to connect to multiple targets or multiple initiators, there’s no harm in direct connections.

Q. Ethernet was not designed to support storage traffic. The TCP/IP protocol suite was not designed to support storage traffic. SCSI was not designed to be encapsulated. So TCP/IP FTW? I think not. The reason iSCSI is exists is [perceived] cost savings. I get fed up with people constantly looking for ways to squeeze another penny out of something. To me it illustrates that they’re not very creative. Fibre Channel is a stupid name, but it is a purpose built protocol that works as designed to.

A. Ethernet is a general purpose network. It is capable of handling lots of different traffic (including storage). By putting iSCSI onto an existing Ethernet infrastructure, it can (as you point out) create a substantial cost savings over installing a FC network (although that infrastructure savings comes with other costs – such as the impact of a shared wire). However, installing a dedicated Ethernet network provides many of the advantages of a dedicated FC network, but at an added cost over that of a shared Ethernet infrastructure. While most consider FC a purpose-built storage network, it is worth pointing out that some also consider it a general purpose network (for example FC-Avionics is built into Fighter Jets, and it’s not for storage). And while not designed to be encapsulated, (it was designed for a parallel bus), SCSI today is encapsulated on every transport that carries it (yes, that includes FCP and SAS).
There are many kinds of storage at different price points, USB storage, SATA devices, rotating media (at different RPMs), SSD devices, SAS devices, FC devices, single spindles, arrays, cloud, drop boxes, etc., all with the corresponding transport wires. iSCSI is one of those wires. Each protocol and wire offer specific advantages and disadvantages.     There can be a lot of confusion about which to use, but just as everyone does not drive the same type car (a FORD FUSION for example), everyone does not need the same type of storage (FC devices/arrays). Yes, I drive a FORD FUSION, and I like FC storage, but I use a USB stick on my laptop, and I pray my bank never puts my financial records out in the cloud. Selecting the right storage (and wire) for the job at hand can be one of a system administrators most interesting problems to solve.As for the name – that is often what happens in committees…

Q. As a best practice for Windows servers, disable hardware acceleration features in NICs (TOE etc.)? Are any NIC features valuable given modern multicore CPUs?

A. Yes. Typically the only reason to disable TOE is that multiple or virtual TCP/IP stacks are going to be using the same NIC. TSO, LRO and jumbo frames will benefit any OS that can take advantage of them.

Q. What is the advantage of iSCSI when compared with NVMe?

A. NVMe and iSCSI are very different protocols. NVMe started life as a direct attach protocol to communicate to native PCIe devices (not even outside the box). iSCSI was a network protocol from day one. iSCSI has to deal with the potential for long network induced delays, and complex out of order error recovery issues. NVMe operates over an interlocked bus, and as such, does not have those issues.

But, NVMe is now being extended over fabrics. NVMe over a RoCE V1 transport will be a data center network (since there is no IP routing). NVMe over a RoCE V2 transport or an iWARP transport will have the same routing capabilities that iSCSI has.     When it comes to the raw command set, they are very similar (but there are some differences). SCSI is a more full featured command set than NVMe – it has been developed over a span of over 25 years, and has developed solutions for all the problems that have been discovered during that time span. NVMe has a more limited (or more focused) command set (for example, there are no tape commands in the NVMe command set). iSCSI is available today, as is direct attach NVMe, but NVMe over Fabrics is still in the development phases (the specification is expected to be available the first week of June, 2016). NVMe products will take some time to mature and to develop solutions for the problems they have not discovered yet. Another example of this is the ability to support shared storage – it existed on day one in iSCSI, but did not exist in the first NVMe specification. To support shared storage in NVMe over Fabrics, that capability has since been added, and it was done using a SCSI compatible method (to make it easier for host S/W that already performs this function).

There is a large community working to develop NVMe over Fabrics. As memory based storage device get cheaper, and the solution space matures, NVMe will become more attractive.

Q. How often do iSCSI installations provide encryption of data in flight? How: IPsec, IKEv2-SCSI + ESP-SCSI, etc.?

A. Rarely. More often than not, if in-flight data security is needed, it will be run on an isolated network. Well under 100% of installations are 100% compliant.  VMware never qualified IPsec with iSCSI and didn’t have any obvious switch to turn it on. Side note: We standards guys can be overly picky about words.   Since the question is “provide” the answer is – 100% of compliant installations PROVIDE encryption (IPsec V2 – see above), however, in practice, installations that require that type of security typically run on isolated networks, rather than turn on encryption.

Q. How do multiple independent applications inside the same initiator map to iSCSI sessions to the same target? E.g., iSCSI session one-to-one with application?

A. There is no relationship between applications and sessions. When an iSCSI initiator discovers a target, the initiator logs in and establishes a session. If iSCSI MCS (multi connection session) is being used, multiple TCP connections may be established and used in parallel to process operations for that session.

Applications send reads and writes to the operating system. Those IO requests make their way through the file system and caching layers into the device driver. The device driver issues the IO request to the device (over the iSCSI session) and retains information about that IO. When a completion is received from a device (the WRITE command or READ command completed), it is matched up with the request. That completion status (success or error) is passed back through the operating system (file system, etc.) to the application. So it is the responsibility of the device driver to mux/demux the requests from all the applications out over the iSCSI session and track the responses as the operations are completed.

When an operating system is using MPIO (multi-pathing), then the device driver may create multiple sessions between the initiator and the target. This is where operating system MPIO policies such as round-robin, shortest queue, LRU, etc. come into play. In this case, the MPIO driver will send an IO operation to the device using what it considers to be the most appropriate path (based on the selected policy). But again, there is no relationship between the application and the path used for IO (any application can have it’s IO send via any path).

Today, MPIO is used more commonly than MCS.

Q. Will Microsoft iSCSI implement iSER?

A. This is a question for Microsoft or iSER-capable NIC vendor that provides Microsoft drivers.

Q.Zadara has some iSER deployments using Linux and VMware clients going to the Zadara cloud storage.

A. There’s an answer, all by itself.

Q. In the case of iWARP, the TCP layer takes care of out-of-order IP packet receptions. What layer does the out-of-order management of packets in ROCE ?

A. RoCE headers contain a 24 bit “Packet Sequence Number” that is used to validate the required ordering and detect lost packets. As such, ordering still occurs, just in a different way.

Q. Correction: RoCE is over Ethernet packets and is not routable. RoCEv2 is the one over UDP/IP and *is* routable.

A. You are correct. RoCE is not routable by IP. RoCE transmits raw Ethernet frames with just Ethernet MAC headers and no IP headers, and as such, it is not routable by IP. RoCE V2 puts the information into UDP packets (with appropriate IP headers), and therefore it is routable by IP.

Q. How prevalent is iSER today in deployment? And what are some of the typical applications that leverage iSER?

A. Not terribly prevalent today, but higher speed Ethernet might drive more adoption, due to the CPU savings demonstrated.

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