The Final Piece of the Puzzle: SR-IOV

The final step is to add a few buffers and Rx/Tx descriptors to each queue of your multi-queued device, and a single NIC can pretend to be a collection of tens of “small” NICs. That is what PCI SIG did, and they call each small NIC a virtual function. According to the PCI SIG SR-IOV specification you can have up to 256 (!) virtual functions per NIC. (Note: the SR-IOV specification is not limited to NICs; other I/O devices can be SR-IOV capable too.)

Courtesy of the excellent Youtube movie: "Intel SR-IOV"

Make sure there is a chipset with IOMMU/VT-d inside the system. The end result: each of those virtual functions can DMA packets in and out without any help of the hypervisor. That means that it is not necessary anymore for the CPU to copy the packages from the memory space of the NIC to the memory space of the VM. The VT-d/IOMMU capable chipset ensures that the DMA transfers of the virtual functions happen and do not interfere with each other. The beauty is that the VMs are connecting to these virtual functions by a standard paravirtualized driver (such as VMXnet in VMware), and as a result you should be able to migrate VMs without any trouble.

There you have it: all puzzles pieces are there. Multiple queues, virtual to physical address translation for DMA transfers, and a multi-headed NIC offer you higher throughput, lower latency, and lower CPU overhead than emulated hardware. At the same time, they offer the two advantages that made virtualized emulated hardware so popular: the ability to share one hardware device across several VMs and the ability to decouple the virtual machine from the underlying hardware.

SR-IOV Support

Of course, this is all theory until all software and hardware layers work together to support this. You need a VT-d or IOMMU chipset, the motherboard’s BIOS has to adapted to recognize all those virtual functions, and each virtual function must get memory mapped IO space like other PCI devices. A hypervisor that supports SR-IOV is also necessary. Last but not least, the NIC vendor has to provide you with an SR-IOV capable driver for the operating system and hypervisor of your choice.

With some help of mighty Intel, the opensource hypervisors (Xen, KVM) and the commercial product derivatives (Redhat, Citrix) were first to market with SR-IOV. At the end of 2009, both Xen and KVM had support for SR-IOV, more specifically for Intel 10G Ethernet 82599 controller. The Intel 82599 can offer up to 64 VFs. Citrix announced support for SR-IOV in Xenserver 5.6, so the only ones missing in action are VMware’s ESX and Microsoft’s Hyper-V.

Solving the Virtualization I/O Puzzle Meet the NICs


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  • blosphere - Wednesday, November 24, 2010 - link

    Oh my cable arms on the first page pic :(

    And about the consolidation, you don't want to do it that way. The proper way is to have two 1-port 10g cards or if you're counting every dollar, one 2-port card. Then you set the production traffic to active/standby config (different vlans of course) and when configuring the vmotion/vkernel port you go and override the port failover order to reverse the port priority from the production traffic (own vlans of course).

    This way you utilise both ports on the cards and you have mediocre HA (not that vmware should be called a HA system in the first place) since the production would failover to the vmotion/vkernel port and vice versa.

    All this stuff is in the vmware/cisco whitepaper. Deployed already a few years ago to our datacentres worldwide, around 100 esxi hosts and 3000+ vm guests, works like charm when things start going wrong. Of course vmware itself does cause some problems in a port loss situation but that's a different story.
  • mino - Wednesday, November 24, 2010 - link

    Agreed, Agreed and again Agreed :). Reply
  • Dadofamunky - Thursday, November 25, 2010 - link

    Two thumbs up for this. Reply
  • DukeN - Wednesday, November 24, 2010 - link

    And what type of switch would actually have the switching capacity to push this type of traffic through in a dedicated manner? That is a cost to be considered.

    That being said, I think well priced FC might still be better from a CPU usage standpoint.
  • mino - Wednesday, November 24, 2010 - link

    FC is better at everything! Problem being, it is a "bit" more expensive.

    So for an SMB or storage IO light apps? 10G all the way.

    For an enterprise database stuff? Think about it very thouroughly before commiting to 10G. And even then,you better forget about iSCSI.

    Consolidating everything-ethernet info 2*10G ? Great. Just do it!
    But do not forget to get security boys on-board before making a proposal to your CIO :D
    No, even Nexus 1000V would not help you ex-post ...
  • Inspector2211 - Wednesday, November 24, 2010 - link

    Myricom was one of the 10G pioneers and now has a 2nd generation lineup of 10G NICs, with any phsyical connection option you can imagine (thick copper, thin copper, long range fiber, short range fiber).

    I picked up a pair of new first-gen Myricom NICs on eBay for $200 each and will conduct my own performance measurements soon (Linux box to Linux box).
  • iamkyle - Wednesday, November 24, 2010 - link

    Last I checked, Myricom has no 10G over CAT5e/6 UTP product available. Reply
  • mianmian - Wednesday, November 24, 2010 - link

    I guess the lightpeak products May first hit the 10G Ethernet market. it will greatly reduce the cost&energy for those servers. Reply
  • mino - Wednesday, November 24, 2010 - link

    There is not mentioned in the article what kind of setup you are simulating.
    Surely the network(HTTP ?) latency is not in tens of milliseconds, is it ?

    Port consolidation? Yes, a great thing, but do not compare oranges to apples!
    There is a huge difference in consolidating those 10+ Ethernet interfaces (easy) and joining in a previously FC SAN (VERY hard to do properly).

    You are pretending that Ethernet (be it 1Gb or 10Gb) is in the performance class of even 4G FC SAN's is a BIG fail.

    10Gb Ethernet SAN (dedicated!) is a great el-cheapo data streaming solution.
    Rather try not hitting that with a write-through database.

    If your 4G SAN utilization is in the <10% range and you have no storage-heavy apps, FCoE or even iSCSI is a very cost-effective proposition.
    Yet even then it is prudent to go for a 2*10G + 2*10G arrangement of SAN + everything else.

    I have yet to see a shaper who does not kill latency ...

    Provided no test description was given, one has to assume you got ~4x the latency when shaping as well.

    The article on itself was enlightening so keep up the good work!

    Please, try not thinking purely SMB terms. There are MANY apps which would suffer tremendously going from FC latency to Ethernet latency.

    FYI, One unnamed storage virtualization vendor has FC I/O operation pass-through-virtualization-box capability of well under 150us.
    That same vendor has observed the best 1GbE solutions choke at <5k IOps, 10GbE at ~10k IOps while a basic 2G FC does ~20k IOps, 4G ~40k IOps and 8G up to ~70k IOps.
  • JohanAnandtech - Thursday, November 25, 2010 - link

    I agree with you that consolidating storage en network traffic should not be done on heavy transaction databases that already require 50% of your 10 GbE pipe.

    However, this claim is a bit weird:

    "That same vendor has observed the best 1GbE solutions choke at <5k IOps, 10GbE at ~10k IOps while a basic 2G FC does ~20k IOps, 4G ~40k IOps and 8G up to ~70k IOps."

    Let us assume that the average block size is 16 KB. That is 5000x16 KB or 80 MB/s for the 1 G solution. I can perfectly live with that claim, it seems very close to what we measure. However, claiming that 10G ethernet can only do twice as much seems to indicate that the 10G solution was badly configured.

    I agree that the latency of FC is quite a bit lower. But let us put this perspective: those FC HBA have been communicating with disk arrays that have several (in some cases >10) ms of latency in case of write-through database. So 150us or 600us latency in the HBA + cabling is not going to make the difference IMHO.

    To illustrate my point: the latency of our mixed test (Iometer/IxChariot) is as follows: 2.1 ms for the disktest (Iometer 64 KB sequential), 330 us for the networktest (high performance script of IxChariot). I think that is very acceptable to any application.

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