Category: NVMe

Life of a Storage Packet (Walk): Q&A

J Metz

We got a lot of great questions at our recent Ethernet Storage Forum webcast “The Life of a Storage Packet (Walk).” As promised, we’ve compiled all the questions with fairly detailed answers. We hope this blog helps to clear up any confusion or uncertainties. If you think of additional questions, please comment below and we’ll get back to you as soon as we can. Thanks to everyone who watched the live webcast. If you missed it, it’s now available on-demand.

Q. Does size of a block depend on OS?

A. Yes, some OS’ only support 512 byte blocks. Some OS’ provide a method to support both 512 byte blocks and 4096 (aka 4K) byte blocks; for example, via a qualifier in their format command. Some devices are built using 4K block sizes, but then emulate 512 byte blocks to the host (aka 512E devices). Many modern versions of OS’ automatically detect the block size of each device they discover and do the right thing based on what they discover. You have to check the documentation for your OS to know its capabilities.

Q. CIFS is not a file system.

A. Not in the same way that ext, ntfs, FAT, HFS, UFS, etc. are, no. However, in terms of certain functionalities that we discussed on the presentation – that is, the ability to manipulate files with operations such as read, write, create, delete, and rename – are all file system functionalities. The difference being, of course, that the files are not on the local computer and are actually on a remote computer.

For what it’s worth, the term ‘CIFS’ has been deprecated from usage, and SMB is the preferred term for precisely the reasons that it should not be confused with local OS systems like the ones mentioned above.

Q. Is it safe to say that a “block” at the file system level is equal to an IO?

A. Not specifically. The difference in the use of those terms, is that a “block” is a place that data is stored – it has an address and it contains data (512 byte of data or 4K bytes of data). An IO is an operation that requests access to a block. An IO may perform a read operation on a block, or it may perform a write operation on a block.

Remember, the term IOPS is I/O operations per second – so it is not about blocks or bytes or bits – it is operations. If the operations are performed on a 512 byte block, they produce a different number of MB/sec than if they operate on a 4K byte block.

So, let’s take an example. A 1MB/second bandwidth on a 4K block size device is the same speed as 1MB/second bandwidth on a 512 byte block size device (observe, however, that the 4K block size device will have only 1/8 the IOPS that the 512 byte device has – because it will take only 1/8 the operations to transfer the same number of Mega-bytes). However, 1M IOPS on a 4K block size device is much better than 1M IOPS on a 512 byte block size device (because the 4K block size device is moving 8X the amount of data than the 512 byte block size device in each operation.

There is an excellent explanation and walk-through of IOPS in our Storage Performance Benchmarking webinar.

Q. Don’t all those Inodes also live on the disk and so don’t the IOs to read those blocks also have to go to the SCSI controller?

A. Well-spotted!

The communication back and forth between the file system and the Inodes traverses the controller for all access to blocks on disk. This is one of the reasons why needing to access disk is considered “expensive.” When you add in a network to the mix, these kinds of situations need particular careful consideration.

Q. Shall we allocate blocks and inodes or is it an automatic process?

A. It’s an automated process, and there is no user intervention at all.

Q. Are Inodes created during OS installation?

A. Inodes are a particular block type, among some other block types (e.g., data blocks, boot blocks, superblocks, group descriptor blocks). These block types are combined into functional groups. These groups are OS-dependent. The block layout therefore, including Inodes, is created during OS installation. If the filesystem needs more Inodes after the OS installation, the OS dynamically adds them to the Inode pool.

Q. Which physical hardware does volume manager reside?

A. Volume managers are not hardware. They are software layers that create pseudo devices that are presented to layers of the OS above them (typically to the file system layer). The volume manager software fits into the OS to accept requests from the file system and pass those requests down to the device driver. In some systems they might be called “filter drivers”.

Q. In flash media, is there also an iSCSI controller that converts PCIe into iSCSI to interact with the flash?

A. I want to make sure that the answer to the question is clear.

On the host, we need to convert PCIe commands to SCSI, so we send them to an iSCSI controller/adapter to be sent across the Ethernet wire. . That adapter can be either software or hardware.

Flash drives are basic media, just like spinning disk drives. Flash will have its own controller, which can be SCSI. If you wish to access the drive over Ethernet using the iSCSI protocol, you will have an adapter on the flash drive (which can be either software or hardware) that will do the SCSI translation between the Flash media and the SCSI commands. This is often called the FTL – or the flash translation layer. Again, the SCSI commands are translated into an Ethernet-friendly packet to be sent along the wire.

There are other types of communication forms for working with Flash, too. The most recent is NVMe. You can see the SNIA webinar on The Performance Impact of NVMe and NVMe over Fabrics for more information.

Q. Why is there a SCSI language in between Storage and the hosts?

A. Before storage standards (like SCSI and ATA), you would purchase storage from Vendor X, and you would also buy a storage controller for that vendor X storage.  If you wanted Vendor Y storage, you could not use the vendor X controller, you had to purchase a new controller from vendor Y. Every vendor had their own language, and you had to purchase matching components.  Once you got locked into one vendor, you were stuck – at the hardware level.

Today with standards (such as SCSI), you can buy a SCSI device from any vendor and connect it to any storage controller that you buy from any vendor – and it just works.  That is the point of the SCSI standard. When the storage standardization efforts began, there were many competing ideas. It just so happened that SCSI won out, and so now it’s everywhere.

Q. If the storage side is flash, do we still need a SCSI controller between host and flash storage or is the controller different for a flash storage?

Yes… and no.

Yes, to use the SCSI part of the OS, the flash device must continue to speak the SCSI protocol and so a SCSI controller is needed. This method of communicating with flash devices enables all the existing S/W on the host OS to just continue working without even knowing it is flash.

No, flash memory chips can be connected to the system using non-SCSI methods. Most of those methods are generally special purpose applications and so the existing OS S/W simply cannot use that device (only the special purpose S/W designed for that device can use it). However, NVMe is a new protocol that is enabling more general use of the flash memory technology with the hopes to provide new capabilities that are beyond what SCSI provides. This is also discussed in our webcast on The Performance Impact of NVMe and NVMe over Fabrics.

Q. What’s the difference between partition, logical disk, volume, LUN, etc?

Excellent question!

Let’s work this one backwards – LUN – that is a SCSI term (actually an acronym) that refers to the Logical Unit Number – it is part of the address used to access a logical unit. Small SCSI devices (such as a single spindle disk drive) have only a single logical unit. Large storage arrays may contain 100s of logical units. Each of those logical units appears to the host OS as if it were a single spindle disk. So, the logical unit is the SCSI object that contains the blocks where the data is stored (where that object may be an individual piece of hardware, or a logical entity within a larger SCSI device). To access a SCSI logical unit, the OS must specify the address of the SCSI device, and then the LUN (logical unit number) for the logical unit within that SCSI device.

The other terms (partition, logical disk, and volume) are OS terms that have to do with virtualization and how the storage blocks are managed. When a SCSI (or other storage device) is formatted by the OS, it may be broken into multiple partitions. Each partition is then treated by the OS as if it were a unique device (a virtual device, or a logical disk). Each of those partitions may then be used independently.

For example, on a Unix system, the “a” partition may contain a file system that has all the files that are necessary to boot. The “b” partition may be setup without a file system and used as the swap or paging storage for use by the virtual memory subsystem. The “d” partition may then contain a file system that contains all the user’s data files. Each partition is unique storage space and may even use a different file system to organize the data located there.

Notice, that I skipped the “c” partition. That partition is often setup to access all the blocks of the physical device. So, on a 500GB disk, maybe “a” contains 10GB, “b” contains 90GB, and “d” contains 400GB; while “c” contains all 500GB. Partitions “a” and “d” may be backed up or restored independently, and partition “c” may be used to perform an image copy of the entire device.

Now, to the other terms:

Logical disk and volume are terms often related to volume managers.

  • Logical disk may be a term used to refer to a partition, but usually that is not the case. Logical disks are typically the devices created by the volume management layer when they combine individual devices into a single larger device (a.k.a. a logical disk).
  • Volume managers also may be used to divide up a large device into small chunks (just like partitions), and those smaller chunks are referred to as logical disks.
  • Volume is a more vague term that typically is used as another term for a logical disk. In some circles, the term Volume is used to refer to a RAID set in a SCSI storage controller (but this is a much less often used definition for Volume).

Q. Will the “complete” presentation somewhere we can go review?

Yes. Click here to access the on-demand webcast as well as a PDF of the webcast slides

Next Webcast: The 2015 Ethernet Roadmap for Networked Storage

David Fair

The ESF is excited to announce our next live Webcast, “The 2015 Ethernet Roadmap for Networked Storage.”

For over three decades, Ethernet has advanced on a simple “powers-of-ten” speed increases, and this model has served the industry well.  Ethernet is changing in big ways and the Ethernet Alliance has captured the latest changes in the 2015 Ethernet Roadmap.

On June 30th at 10:00 a.m. PT an expert panel comprised of Scott Kipp, President of the Ethernet Alliance, David Chalupsky, Chair IEEE P802.3bq/bz TFs and the Ethernet Alliance BASE-T Subcommittee and myself will present the Ethernet Alliance’s 2015 Ethernet Roadmap for the networking technology that underlies most of future network storage.

SNIA has focused on protocols and usage models and more or less just takes Ethernet for granted.  The biggest technology disruption in the storage space is the emergence into the mainstream of Non-Volatile Memory (NVM), FLASH in particular.  NVM increasingly moves system bottlenecks from the storage subsystem to the network.  Developments in NVM — most recently 3D FLASH — assure that the cost per GB will continue aggressive declines and demand for bandwidth will go up.  NVM will become more prevalent, making the roadmap for Ethernet increasingly more important to the storage networking community.

This will be a live and interactive session. I encourage you to register now and bring your questions for our experts. I hope to see you on June 30th.

The Performance Impact of NVMe and NVMe over Fabrics – Q&A

J Metz

More than 400 people have already seen our recent live ESF Webcast, “The Performance Impact of NVMe and NVMe over Fabrics.” If you missed it, it’s now available on-demand. It was a great session with a lot of questions from attendees. We did not have time to address them all – so here is a complete Q&A courtesy of our experts from Cisco, EMC and Intel. If you think of additional questions, please feel free to comment on this blog.

Q. Are you saying that just by changing the interface to NVMe for any SSD, one would greatly bump up the IOPS?

A. NVMe SSDs have higher IOPs than SAS or SATA SSDs due to several factors, including the low latency of PCIe and the efficiency of the NVMe protocol.

Q. How much of the speed of NVMe you have shown is due to simpler NVMe protocol vs. using Flash? i.e how would the SAS performance change when you are attaching SSD to SAS

A. The performance differences shown comparing NVMe to SAS to SATA were all using solid-state drives (all NAND Flash). Thus, the difference shown was due to the interface.

Q. Can you comment on the test conditions these results were obtained under and what are the main reasons NVMe outperforms the others?

A. The most important reason NVMe outperforms other interfaces is that it was architected for NVM – rather than inheriting the legacy of HDDs. NVMe is built on the foundation of a very efficient multi-queue model and a simple hardware automatable command set that results in very low latency and high performance. Details for IOPs and bandwidth comparisons are shown in the footnotes of the corresponding foils. For the efficiency tests, the detailed setup information was inadvertantly removed from the backup. This will be corrected.

Q. What is the IOPS difference between NVMe and SAS at the same queue depth?

A. At a queue depth of 32, for the particular devices shown with 4K random reads is NVMe =~ 267K IOPs and SAS =~ 149K IOPs. SAS does not improve when the queue depth is increased to 128. NVMe performance increases to ~ 472K IOPs at a queue depth of 128.

Q. Why not use PCIe directly instead of the NVMe layer on PCIe?

A. PCI Express is used directly. NVM Express is the standard software interface for high performance PCI Express storage devices. PCI Express does not define register, DMA, command set, or feature set for PCIe storage devices. NVM Express replaces proprietary software interfaces and drivers used previously by PCIe SSDs in the market.

Q. Is the Working Group considering adding things like enclosure identification in the transport abstraction so the host/client can identify where the NVMe drives reside?

A. The NVM Express organization is developing a Management Interface specification set for release in Q1’2015 that will enable standardized enclosure management. The intent is that these features could be used regardless of fabric type (PCIe, RDMA, etc).

Q. Are there APIs in the software interface for device query information and device RAID configuration?

A. NVMe includes an Identify Controller and Identify Namespace command that provides information about the NVMe subsystem, controllers, and namespaces. It is possible to create a RAID controller that uses the NVMe interface if desired. Higher level software APIs are typically defined by the OSV.

Q. 1. Are NVMe drivers today multi-threaded? 2. If I were to buy a NVMe device today can you suggest some list of vendors whose solutions are used today in data centers (i.e production and not proof of concept or proto)?

A. The NVM Express drivers are designed for multi-threading – each I/O queue may be owned/controlled by one thread without synchronization with other driver threads. A list of devices that have passed NVMe interoperability and conformance testing are on the NVMe Integrator’s List.

Q. When do you think the market will consolidate for NVMe/PCIE based SSDs and End of SATA era ?

A. By 2018, IDC predicts that Enterprise SSDs by Interface will be PCIe=38%, SAS=28%, and SATA=34%. By 2018, Samsung predicts over 70% of client SSDs will be PCIe. Based on forecasts like this, we expect strong growth for NVMe as the standard PCIe SSD interface in both Enterprise and Client segments.

Q. why can’t it be like a graphic card which does memory transactions?

A. SSDs of today have much longer latency than memory – where a read from a typical NAND page takes > 50 microseconds. However, as next generation NVM comes to market over the next few years, there may be blurring of the lines between storage and memory, where next generation NVM may be used as very fast storage (like NVMe) or as memory as in NVDIMM type of usage models.

Q. It seems that most NVMe drive vendors supply proprietary drivers for their drives. What’s the value of NVMe over proprietary interfaces given this? Will we eventually converge on the open source driver?

A. As the NVMe ecosystem matures, we would expect most implementations would use inbox drivers that are present in many OSes, like Windows, Linux, and Solaris. However, in some Enterprise applications, a vendor may have a value added feature that could be delivered via their own software driver. OEMs and customers will decide whether to use inbox drivers or a vendor specific driver based on whether the value provided by the vendor is significant.

Q. To create an interconnect to a scale-out storage system with many NVMe drives does that mean you would you need an aggregated fabric link (with multiple RDMA links) to provide enough bandwidth for multiple NVMe drives?

A. Depends on the speed of the fabric links and the number of NVMe drives. Ideally, the target system would be configured such that the front-end fabric and back-end NVMe drives were bandwidth balanced. Scaling out multi-drive subsystems on a fabric may require the use of fat-tree switch topologies which may be constructed using some form of link aggregation. The performance of the PCIe NVMe drives is expected to put high bandwidth demands on the front-end network interconnect. Each NVMe SFF8639 2.5” drive has a PCIe Gen3/x4 interconnect with the capability to product 3+ GB/s (24gbps) of sustained storage bandwidth. There are multiple production server systems with 4-8 NVMe sff8639 drive bays, which puts these platforms in the 200Gbps capability when used as NVMe over fabrics storage servers. The combination of PCIe NVMe drives and NVMe over fabric targets is going to have a significant impact on datacenter storage performance.

Q. In other forums we heard about NVMe extensions to deliver vendor specific value add features. Do we have any updates?

A. Each vendor is allowed to add their own vendor specific features and value. It would be best to discuss any vendor specific features with the appropriate vendor.

Q. Given that PCIe is not a scalable fabric at least from a storage perspective, do you see the need for SAS SSDs to increase or diminish over time? Or is your view that NVMe SSDs will populate the tier between DRAM and say, rotating media like SAS HDDs?

A. NVMe SSDs are the highest performance SSDs available today. If there is a box of NVMe SSDs, the most appropriate connection to that JBOD may may be Ethernet or another fabric that then fans out inside the JBOD to PCIe/NVMe SSDs.

Q. From a storage industry perspective, what deficiencies does NVMe have to displace SAS? Will that transition ever happen?

A. NVMe SSDs are seeing initial broad deployment primarily in server use cases that prize the high performance. Storage applications require a robust high availiability interface. NVMe has defined support for dual port, reservations, and other high availability features. NVMe will be used in storage applications as these high availability features mature in products.

Q. Will NVMe over fabrics allow to dma read/write the NVMe device directly (without going through system memory)?

A. The locality of the NVMe over Fabric buffers on the target-side are target implementation specific. That said, one could construct a target that used a 
pool of PCIe NVMe subsystem controller resident memory as the source and/or sink buffers of a fabric’s NIC’s NVMe data exchanges. This type of configuration will have the limitation of having to pre-determine fabric data to NVMe device locality else the data could end up in the wrong drive’s controller memory.

Q. Intel True scale fabric technology was based on Fulcrum ASIC. Could you please provide an input how Intel Omni Scale differs from Intel True Sclae fabric?

A.  In the context of NVMe over Fabrics, Intel Omni-Path fabric is a possible future fabric candidate for an NVMe over Fabrics definition. Specifics on the fabric itself are outside of the scope of NVMe over Fabrics definition. For information on Omni-Path file, please refer to http://www.intel.com/content/www/us/en/omni-scale/intel-omni-scale-fabric-demo.html?wapkw=omni-scale.

Q. Can the host side NVMe client be in user mode since it is using RDMA?

A. It is possible since RDMA QP communications allow for both user and kernel mode access to the RDMA verbs. However, there are implications to consider. The NVMe host software currently resides in multiple operating systems as a kernel level block-storage driver. The goal is for NVMe over Fabrics to share common NVMe code between multiple fabric types in order to provide a consistent and sustainable core NVMe software. When NVMe over Fabrics is moved to user level, it essentially becomes a separate single-fabric software solution that must be maintained independently of the multi-fabric kernel NVMe software. There are some performance advantages of having a user-level interface, such as not having to go through the O/S system calls and the ability to poll the completions. These have to be weighed against the loss of kernel resident functionality, such as upper level kernel storage software, and the cost of sustaining the software.

Q. Which role will or could play the InfiniBand Protocol in the NVMe concept?

A. InfiniBand™ is one of the supported RDMA fabrics for NVMe over RDMA. NVMe over RDMA will support the family of RDMA fabrics through use of a common set of RDMA verbs. This will allow users to select the RDMA fabric type based on their fabric requirements and not be limited to any one RDMA fabric type for NVMe over RDMA.

Q. Where is this experimental code for NVMeOF for Driver and FIO available?

A. FIO is a common Linux storage benchmarking tool and is available from multiple Internet sites. The driver used in the demo was developed specifically as a proof of concept and demonstration for the Intel IDF 2014. They were based on a pre-standardized implementation of the NVMe over RDMA wire protocol. The standard NVMe over RDMA wire protocol is currently being under definition in NVM Express, Inc.. Once the standard is complete, both Host and reference Target drivers for Linux will be developed.

Q. Was polling on the completion queue used on the target side in the prototype?

A. The target side POC implementation used a polling technique for both the NVMe over RDMA CQ and NVMe CQ. This was to minimize the latency by eliminating the interrupt latency on the target for both CQs. Depending on the O/S and both the RDMA and NVMe devices interrupt moderation settings, interrupt latency is typically around 2 microseconds. If polling is not the desired model, Intel processors enable another form of event signaling called Monitor/Mwait where latency is typically in the 500ns latency range.

Q. In the prototype over iWARP, did the remote device dma write/read the NVMe device directly, or did it go through remote system memory?

A. In the PoC, all NVMe commands and command data went through the remote system memory. Only the NVMe commands were accessed by the CPU, the data was not touched.

Q. Are there any dependencies between NVMEoF using RDMA and iWARP? Can standard software RDMA in Linux distributions be used without need for iWARP support?

A. As mentioned, the NVMe over RDMA will be RDMA type agnostic and work with all RDMA providers that support a common set of RDMA verbs.

Q. PCIe doesn’t support multi-host access to devices. Does NVMe over fabric require movement away from PCIe?

A. The NVMe 1.1 specification specifically added features for multi-host support – allowing NVMe subsystems to have multiple NVMe controllers and multiple fabric ports. This model is supported in PCI Express by multi-function/ multi-port PCIe drives (typically referred to as dual-port). Dependng on the fabric type, NVMe over Fabrics will extend to configurations with many hosts sharing a single NVMe subsystem consisting of multiple NVMe controllers.

Q. In light of the fact that NVMe over Fabrics reintroduces more of the SCSI architecture, can you compare and contrast NVMe over Fabrics with ‘SCSI Express’ (SAM/SPC/SBC/SOP/PQI)?

A. NVMe over Fabrics is not a SCSI model, it’s extending the NVMe model onto other fabric types. The goal is to maintain the simplicity of the NVMe model, such as the small amount of NVMe command types, multi-queue interface model, and efficient NVM oriented host and controller implementations. We chose the RDMA fabric as the first fabric because it too was architected with a small number of operations, multi-queue interface model, and efficient low-latency operations.

Q. Is there an open source for NVMe over Fabrics, which was used for the IDF demo? If not, can that be made available to others to see how it was done?

A. Most of the techniques used in the PoC drivers will be implemented in future open-source Host and referent Target drivers. The PoC was both a learning and demonstration vehicle. Due to the PoC drivers using a pre-standards based NVMe over RDMA protocol, we feel it’s best not to propagate the implementation.

Q. What is the overhead of the protocol? Did you try putting NVMe in front just DRAM? I’d assume you’ll get much better results, and understand the limitations of the protocol much better. In front of DRAM it won’t be NVM, but it will give good data regarding protocol latency.

A. The overhead of the protocol on the host-side matched the PCIe NVMe driver. On the target, the POC protocol efficiency was around 600ns of compute latency for a complete 4K I/O. For low-latency media, such as DRAM or next generation NVM, the reduced latency of a solution similar to the PoC will enable the effective use of the media’s low latency characteristics.

Q. Do you have FC performance comparison with NVMe?

A. We did implement an 100GBE/FCoE target with NVMe back-end storage for an Intel IDF 2012 demonstration. FCoE is a combination of two models, FCP and SCSI. Our experience with this target implementation showed that we were adding a significant amount of computational latency on both the host (initiator) and target FCoE/FC/SCSI storage stacks that reduced the performance and efficiency advantages gained in the back-end PCIe NVMe SSDs. A significant component of this computational latency was due to the multiple storage models and associated translations that occurred between the host application and back-end NVMe drives This experience led us down the path of enabling an end-to-end NVMe model through expanding the NVMe model onto a range of fabric types.

A Beginner’s Guide to NVMe

J Metz

When I first started in storage technology (it doesn’t seem like that long ago, really!) the topic seemed like it was becoming rather stagnant. The only thing that seemed to be happening was that disks were getting bigger (more space) and the connections were getting faster (more speed).

More speed, more space; more space, more speed. Who doesn’t like that? After all, you can never have too much bandwidth, or too much disk space! Even so, it does get rather routine. It gets boring. It gets well, “what have you done for me lately?”

Then came Flash.

People said that Flash memory was a game changer, and though I believed it, I just didn’t understand how, really. I mean, sure, it’s really, really fast storage drives, but isn’t that just the same story as before? This is coming, of course, from someone who was far more familiar with storage networks than storage devices.

Fortunately, I kept that question to myself (well, at least until I was asked to write this blog), thus saving myself from potential embarrassment.

There is no shortage of Flash vendors out there who (rightfully) would have jumped at the chance to set my misinformed self on the straight and narrow; they would have been correct, too. Flash isn’t just “cool,” it allows the coordination, access, and retrieval of data in ways that simply weren’t possible with more traditional media.

There are different ways to use Flash, of course, and different architectures abound in the marketplace – from fully “All Flash Arrays” (AFA), “Hybrid Arrays” (which are a combination of Flash and spinning disk), to more traditional systems that have simply replaced the spinning drives with Flash drives (without much change to the architecture).

Even through these architectures, though, Flash is still constrained by the very basic tenets of SCSI connectivity. While this type of connectivity works very well (and has a long and illustrious history), the characteristics of Flash memory allow for some capabilities that SCSI wasn’t built to do.

Enter NVMe.

What’s NVMe?

Glad you asked. NVMe stands for “Non-Volatile Memory Express.” If that doesn’t clear things up, let’s unpack this a bit.

Flash and Solid State Devices (SSDs) are a type of non-volatile memory (NVM). At the risk of oversimplifying, NVM is a type of memory that keeps its content when the power goes out. It also implies a way by which the data stored on the device is accessed. NVMe is a way that you can access that memory.

If it’s not quite clear yet, don’t worry. Let’s break it down with pretty pictures.

Starting Before the Beginning – SCSI

For this, you better brace yourself. This is going to get weird, but I promise that it will make sense when it’s all over.

Let’s imagine for the moment that you are responsible for programming a robot in a factory. The factory has a series of conveyor belts, each with things that you have to put and get with the robot.

Graphic 1

The robot is on a track, and can only move from sideways on the track. Whenever it needs to get a little box on the conveyor belt, it has to move from side to side until it gets to the correct belt, and then wait for the correct orange block (below) to arrive as the belt rotates.

Now, to make things just a little bit more complicated, each conveyor belt is a little slower than the previous one. For example, belt 1 is the fastest, belt 2 is a little slower, belt 3 even slower, and so on.

Graphic 2

In a nutshell, this is analogous to how spinning disk drives work. The robot arm – called a read/write head – moves across a spinning disk from sector to sector (analogous, imperfect as it may be, to our trusty conveyor belts) to pick up blocks of data.

(As an aside, this is the reason why there are differences in performance between long contiguous blocks to read and write – called sequential data – and randomly placing blocks down willy-nilly in various sectors/conveyor belts – called random read/writes.)

Now remember, our trusty robot needs to be programmed. That is, you – in the control room – need to send instructions to the robot so that it can get/put its data. The command set that is used to do this, in our analogy, is SCSI.

Characteristics of SCSI

SCSI is a tried-and-true command structure. It is, after all, the protocol for controlling most devices in the world. In fact, its ubiquity is so prevalent, most people nowadays think that it’s simply a given. It works on so many levels and layers as an upper layer protocol, with so many different devices, that it’s easily the default “go-to” application. It’s used with Fibre Channel, FCoE, iSCSI (obviously), InfiniBand – even the hard drives in your desktop and laptops.

It was also built for devices that relied heavily on the limitations of these conveyor belts. Rotational media has long been shown as superior than linear (i.e., tape) in terms of speed and performance, but the engineering required to make up for the changes in speeds from the inside of the drive (where the rotational speed is much slower) to the outside – similar to the difference between our conveyor belts “4” and “1” – results in some pretty fancy footwork on the storage side.

Because the robot arm must move back and forth, it’s okay that it can only handle one series of commands at a time. Even if you could tell it to get a block from conveyor belt 1 and 3 at the same time, it couldn’t do it. It would have to queue the commands and get to each in turn.

So, the fact that SCSI only sent commands one-at-a-time was okay, because the robot could only do that anyway.

Then came Flash, and suddenly the things seemed a bit… constricted.

The Flash Robot

Let’s continue with the analogy, shall we? We still have our mandate – control a robot to get and put blocks of information onto storage media. This time, it’s Flash.

First, let’s do away with the conveyor belts altogether. Instead, let’s lay out all the blocks (in a nice OCD-friendly way) on the media, so that the robot arm can see it all at once:

Graphic 3

From its omniscient view, the robot can “see” all the blocks at once. Nothing moves, of course, as this is solid-state. However, as it stands, this is how we currently use Flash with a robot arm that responds to SCSI commands. We still have to address our needs one-at-a-time.

Now, it’s important to note that because we’re not talking about spinning media, the robot arm can go really fast (of course, there’s no real moving read/write head in a Flash drive, but remember we’re looking at this from a SCSI standpoint). The long and the short of it is that even though we can see all of the data from our vantage point, we still only ask for information one at a time, and we still only have one queue for those requests.

Enter NVMe

This is where things get very interesting.

NVMe is the standardized interface for PCIe SSDs (it’s important to note, however, that NVMe is not PCIe!). It was architected from the ground-up, specifically for the characteristics of Flash and NVM technologies.

Most of the technical specifics are available at the NVM Express website, but here are a couple of the main highlights.

First, whereas SCSI has one queue for commands, NVMe is designed to have up to 64 thousand queues. Moreover, each of those queues in turn can have up to 64 thousand commands. Simultaneously. Concurrently. That is, at the same time.

That’s a lot of freakin’ commands goin’ on at once!

Let’s take a look at our programmable robot. To complete the analogy, instead of one arm, our intrepid robot has 64 thousand arms, each with the ability to handle 64 thousand commands.

Graphic 4

 

Second, NVMe streamlines those commands to only the basics that Flash technologies need: 13 to be exact.

Remember when I said that Flash has certain characteristics that allow for radically changing the way data centers store and retrieve data? This is why.

Flash is already fast. NVMe can make this even faster than we do so today. How fast? Very fast.

Graphic 5

Source: The Performance Impact of NVMe and NVMe over Fabrics 

This is just an example, of course, because enterprise data centers have more workloads than just those that run 4K random reads. Even so, it’s a pretty nifty example:

  • For 100% random reads, NVMe has 3x better IOPS than 12Gbps SAS
  • For 70% random reads, NVMe has 2x better IOPS than 12Gbps SAS
  • For 100% random writes, NVMe has 1.5x better IOPs than 12Gbps SAS

What about sequential data?

Graphic 6

Source: The Performance Impact of NVMe and NVMe over Fabrics 

Again, this is just one scenario, but the results are still quite impressive. For one, NVMe delivers greater than 2.5Gbps read performance and ~2Gbps write performance:

  • 100% reads: NVMe has 2x performance of 12Gbps SAS
  • 100% writes: NVMe has 2.5x performance of 12Gbps SAS

Of course, there is more to data center life than IOPS! Those efficiencies of command structure that I mentioned above also reduce CPU cycles by half, as well as reduce latency by more than 200 microseconds than 12 Gbps SAS.

I know it sounds like I’m picking on poor 12 Gbps SAS, but at the moment it is the closest thing to the NVMe-type of architecture. The reason for this is because of NVMe’s relationship with PCIe.

Relationship with PCIe

If there’s one place where there is likely to be some confusion, it’s here. I have to confess, when I first started going deep into NVMe, I got somewhat confused, too. I understood what PCIe was, but I was having a much harder time figuring out where NVMe and PCIe intersected, because most of the time the conversations tend to blend the two technologies together in the discussion.

That’s when I got it: they don’t intersect.

When it comes to “hot data,” we in the industry have been seeing a progressive migration towards the CPU. Traditional hosts contain an I/O controller that sits in-between the CPU and the storage device. By using PCIe, however it is possible to eliminate that I/O controller from the data path, making the flows go very, very quick.

Because of the direct connection to the CPU, PCIe has some pretty nifty advantages, including (among others):

  • Lower latency
  • Scalable performance (1 GB/s, per lane, and PCI 3.0 x8 cards have 8 lanes – that’s what the “x8” means)
  • Increased I/O (up to 40 PCIe lanes per CPU socket)
  • Low power

This performance of PCIe, as shown above, is significant. Placing a SSD on that PCIe interface was, and is, inevitable. However, there needed to be a standard way to communicate with the SSDs through the PCIe interface, or else there would be a free-for-all for implementations. The interoperability matrices would be a nightmare!

Enter NVMe.

NVM Express is that standardized way of communicating with a NVM storage device, and is backed by an ever-expanding consortium of hardware and software vendors to make your life easier and more productive with Flash technologies.

Think of PCIe as the physical interface and NVMe as the protocol for managing NVM devices using that interface.

Not Just PCIe

Just like with SCSI, an interest has emerged in moving the storage outside of the host chassis itself. It’s possible to do this with PCIe, but it requires extending the PCIe hardware interface outside as well, and as a result there has been some interest in using NVMe with other interface and storage networking technologies.

Work is progressing on not just point-to-point communication (PCIe, RDMA-friendly technologies such as RoCE and iWARP), but also Fabrics as well (InfiniBand-, Ethernet- and Fibre Channel-based, including FCoE).

By expanding these capabilities, it will be possible to attach hundreds, maybe thousands, of SSDs in a network – far more than can be accommodated via PCIe-based systems. There are some interesting aspects for NVMe using Fabrics that are best left for another blog post, but it was worth mentioning here as an interesting tidbit.

Bottom Line

NVM Express has been in development since 2007, believe it or not, and has just released version 1.2 as of this writing. Working prototypes of NVMe using PCIe- and Ethernet-based connectivity have been demonstrated for several years. It’s probably the most-developed standards work that few people have ever heard about!

Want to learn more? I encourage you to listen/watch the SNIA ESF Webcast “The Performance Impact of NVMe and NVMe over Fabrics” which goes into much more technical detail, sponsored by the NVMe Promoter’s Board and starring some of the brainiacs behind the technical work. Oh, and I’m there too (as the moderator, of course).

Keep an eye open for more information about NVMe in the coming months. The progress made is remarkably rapid, and more companies are joining each year. It will be extremely interesting to see just how creative the data center architectures can get in the coming years, as Flash technology comes to realize its full potential.

 

 

It’s “All About M.2 SSDs” In a New SSSI Webcast June 10

Marty Foltyn

Interested in M.2, the new SSD card form factor?

The SNIA Solid State Storage Initiative is partnering with SATA-IO and NVM Express to give you the latest information on M.2, the new SSD card form factor.  Join us “live” on Tuesday, June 10, at 10:00 am Pacific time/1:00 pm Eastern time.

Hear from a panel of experts, including Tom Coughlin of Coughlin Associates, Jim Handy of Objective Analysis, Jon Tanguy of Micron, Jaren May of TE Connectivity, David Akerson of Intel, and Eden Kim of Calypso Systems.  You will leave this webinar with an understanding of the M.2 market, M.2 cards and connection schemes, NVM Express, and M.2 performance. You’ll also be able to ask questions of the experts.

You can access this webcast via the internet.  Click here, or visit http://snia.org/news_events/multimedia#webcasts