I/O Consolidation

In our previous article, we showed that thanks to multi-queue technology, 10G Ethernet can deliver about 9 Gbit/s per second. We compared it with the typical link aggregated quad-port gigabit “NIC”. Quad-port link aggregation (IEEE 802.3 ad) is considered the “sweet spot”, both from a performance and an economic point of view. The results made 10G Ethernet really attractive: 10Gbit Ethernet actually consumed less CPU cycles than the quad-port link aggregated solution while delivering more than twice the bandwidth (9.5 Gbit/s vs 3.8 Gbit/s). It also features lower latency than quad-port NICs.

And 10G is hardly expensive: the least expensive dual 10G NICs cost about 50% ($600-700) more than the quad-port gigabit NICS ($400-450). Even the more expensive 10G cards (>$1000) offer a competitive bandwidth (2x 10G) per dollar ratio and offer a much better performance per watt ratio too. Typical power usage of dual 10G card is between 6W to 14W. The best quad gigabit NICs go as low as 4.3 W, although 8.4W is also possible.

10G Ethernet is much more than “a bigger pipe” for your (virtualized) network traffic. Let our knowledgeable IT professionals commenting on our last 10G Ethernet article enlighten you:

“In the market for a new SAN for a server in preparation for a consolidation/virtualization move, one of my RFP requirements was for 10GbE capabilities now. Some peers of mine have questioned this requirement stating there is enough bandwidth with etherchanneled 4Gb NICs and FC would be the better option if that is not enough. The biggest benefit for 10Gb is not bandwidth, it's port consolidation, thus reducing total cost.

To understand this just look at the picture below.

A virtualized server might need I/O ports for:

  • Console and management traffic (Ethernet)
  • VM migration (Ethernet)
  • VM Application network I/O (Ethernet)
  • Block Storage I/O (Fibre Channel)
  • File Storage I/O (Ethernet)

For speed and availability reasons, you quickly end up with two ports for each traffic flow, so you might end up with up to 10 ports coming out of one server. You might even need more: an IP based KVM to access the physical host and another port for server management interface (ILO, DRAC…).

Cleaning Up the Cable Mess
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  • mino - Thursday, November 25, 2010 - link

    Well the main issue with 10G, especially copper, is that the medium frequencies are nowhere near FC.
    Then there is the protocol overhead for iSCSI, FCoE is musch better in that respect though.

    That IOps measure was a reliably achievable peak - meaning generally with <4k IO operations.
    10k on Gbit can be done in the lab easily, but it was not sustainable/reliable-enough to consider it for production use.

    Those disk arrays have caches, today they even have SSD's etc. etc. then there is the dark fiber DR link one has to wait for ...

    But yes, in a typical virtualized web-serving or SMB scenario 10G makes all the sense in the world.

    All I ask is that you generally not dismiss FC without discussing its strengths.
    It is a real pain explaining to a CIO why 10G will not work out when a generally reputable site AT says it is just "better" than FC.
  • JohanAnandtech - Thursday, November 25, 2010 - link

    I do not dismiss FC. You are right that we should add FC to the mix. I'll do my best to get this is in another article. That way we can see if the total latency (= actual response times) are really worse on iSCSI than on FC.

    Then again, it is clear that if you (could) benefit from 8 gbit/s FC now, consolidating everything into a 10 Gbit pipe is a bad idea. I really doubt 10 GbE iSCSI is worse than 4 Gb FC, but as always, the only way to check this, is to measure.
  • gdahlm - Thursday, November 25, 2010 - link

    The main issue is that Ethernet is designed to drop packets. This means that to be safe all iSCSI writes need to be synchronous and this means you will be hitting the disks hard or you are going to risk data loss if congestion starts dropping packets or you fill your ring buffer etc..

    Even with ZFS and a SSD ZIL you will be slower then ram based WriteBack Cache.

    As an example here are some filebench oltp results from a linux based host to a zfs array over 4GB FC.

    Now this is a pretty cheap array but will show the difference between a SSD backed ZIL and using the memory in writeback mode.

    Host: noop elevator with Direct IO to SSD ZIL
    6472: 77.657: IO Summary: 486127 ops, 8099.587 ops/s, (4040/4018 r/w), 31.6mb/s, 683us cpu/op, 48.4ms latency

    Host: noop elevator with Direct IO with WB cache on the target.
    18042: 73.066: IO Summary: 767336 ops, 12778.487 ops/s, (6373/6340 r/w), 50.0mb/s, 481us cpu/op, 29.6ms latency

    Basic FC switches are cheap compared to 10gig switches at this point in time too.
  • JohanAnandtech - Friday, November 26, 2010 - link

    "The main issue is that Ethernet is designed to drop packets. This means that to be safe all iSCSI writes need to be synchronous"

    IMHO, this only means that congestion is a worse thing for iSCSI (so you need a bit more headroom). Why would writes not be async? Once you hit the cache of your iSCSI target, the packets are in the cache, Ethernet is not involved anymore. So a smart controller can perform writes async. As matt showed with his ZFS article.

    "Even with ZFS and a SSD ZIL you will be slower then ram based WriteBack Cache."

    Why would write back cache not be possible with iSCSI?

    "Basic FC switches are cheap compared to 10gig switches at this point in time too. "

    We just bought a 10GbE switch from Dell: about $8000 for 24 ports. Lots of 24 port FC Switches are quite a bit more expensive. It is only a matter of time before 10GbE switches are much cheaper.

    Also, with VLAN, the 10GbE can be used for both network as SAN traffic.
  • gdahlm - Friday, November 26, 2010 - link

    I may be missing the part where Matt talked about async on ZFS. I only see where he was discussing using SSDs for an external ZIL. However writing to the ZIL is not an async write, it is fully committed to disk even if that commit is happening on an external log device.

    There are several vendors who do support async iSCSI writes using battery backed cache etc.. But to move up into the 10G performance level puts them at a price point where the costs of switches is fairly trivial.

    iSCSI is obviously is a route-able protocol and thus often it is not just traversing one top of rack switch. Due to this lots of targets and initiators tend to be configured a conservative manor. COMSTAR is one such product, all COMSTAR iSCSI writes are synchronous, thus the reason you gain any advantage from an external ZIL. It appears that the developers assumed that FC storage is “reliable” and thus by default (at least in sol 11 express) zvols that are exported through COMSTAR are configured as writeback by default. You need to actually use stmfadm to specifically enable the honoring of sync writes and thus the use of the ZIL on pool or ssd.

    I do agree that 10Gig Ethernet will be cheaper soon. I do not agree that it is cheaper at the moment.

    Dell does have a 10GbE switch for about 8K but that is without the SFP modules. Qlogic has a 20 port 8GB switch that can be purchased for about 9K with the SFP modules.

    If you have a need for policy on your network side or other advanced features the cost per port for 10GbE goes up dramatically.

    I do fully expect that this cost difference will change dramatically over the next 12 months.

    Ideally had SUN not been sold when they were we would probably have a usable SAS target today, but it appears that work on the driver has stopped.

    This would have enabled the use of LSI's SAS switches which are dirt cheap to provide full 4 lane 6Gbs connectivity to hosts through truly inexpensive SAS HBAs.
  • Photubias - Friday, November 26, 2010 - link

    quote "That same vendor has observed the best 1GbE solutions choke at <5k IOps..."

    This guy achieves 20k IOPS through 1GbE: http://www.anandtech.com/show/3963/zfs-building-te... ?
  • blowfish - Wednesday, November 24, 2010 - link

    Trying to read this article is making my brain hurt! ;(

    It surprises me to see what looks like pci connectors on the NIC;s though. Are servers slower to adopt new interfaces?
  • Alroys - Wednesday, November 24, 2010 - link

    They are not PCI, they actually are PCIe x8.
  • blowfish - Thursday, November 25, 2010 - link

    oh, thanks for the clarification!
  • blandead - Wednesday, November 24, 2010 - link

    On that note, does anyone know how to combine ports to aggregate with an extreme switch rather than a fail-over solution like as mentioned above.. combining 1GbE ports. Just a general idea of commands will point me in right direction : )
    Much appreciated if anyone replies!

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