Cleaning Up the Cable Mess

Between 6 and 12 I/O cables on one server is quite a bit. You end up with a complicated NIC configuration as illustrated in this VMware/Intel white paper.


The white paper does not consider the storage I/O, but if you use two FC cards, you are adding another 14W (7W x 2) and two cables. So if we take such a heavy consolidated server as an example you end up with about:

  • 10 I/O Cables (without KVM and Server management)
  • 2 quad-port NICs x 5W + 2 FC cards x 7W = 24W

24W is not enormous, but in reality this is the best case. Dual socket servers typically need between 200 and 350W, quad socket servers about 250 to 500W in total. So the I/O power consumption is about 5—15% of the total power consumption.

The 10 I/O cables are a bigger problem. The more ports and cables, the higher the chances are that something gets badly configured and the harder it gets to troubleshoot. It does not take much imagination to see that this kind of cabling might waste a lot of sysadmin time and thus money.

The biggest problem is of course cost. Fibre Channel cabling is not cheap, but it is nothing compared to the huge cost of FC HBAs, FC switches, and SFPs. Cabling up to eight Ethernet cables per server is not cheap either: while the cable cost is negligible, someone has to do the pulling and plugging and that task isn't free.

I/O Consolidation Consolidating to the Rescue


View All Comments

  • 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: ?
  • 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. Reply
  • blowfish - Thursday, November 25, 2010 - link

    oh, thanks for the clarification! Reply
  • 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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