Using Round Robin multipathing, what is the maximum number of paths that can be used to send data to the VMFS

An administrator is configuring an ESXi 5.x host with two dual-port Fibre Channel (FC) HBAs. The FC storage array has four active storage processor ports. Zoning is configured on both fabrics, with ESXi HBA ports and the array ports in the same zone. There are two LUNs with VMFS datastores configured.

Using Round Robin multipathing, what is the maximum number of paths that can be used to send data to the VMFS datastores at any given time?

A.
2

B.
3

C.
1

D.
4

11 Comments on “Using Round Robin multipathing, what is the maximum number of paths that can be used to send data to the VMFS

  1. JL says:

    This is a tricky question.

    The Maximum Paths to a FC LUN is 32. This is nu limitation in this case.

    If this SAN is properly designed using a SAN switch each HBA port can connect to 2 active storage processer ports at the same time.
    In this case there are 8 paths for the host to reach each LUN.

    RoundRobin doesn’t provide SIMULTANIOUS connections over multiple paths, but it is switching after a set number of IOPS.

    RoundRobin will switch through all available ports, so for one LUN only one path will be Active during the set number of IOPS at any given time. After the set number of IOPS has passed, it will switch to another path and process the same number of IOPS again.
    Effectively there is only ONE path per lun active at any given time when using RoundRobin.

    There are 2 LUN’s so the answer is 2.

    1. ittelettnei says:

      Good answer.

      The number of iops going down one path before switching to the next is 1000 btw.

      But consider “at any given time” in the following respect when we are fully utilizing the queued depth of 32 for the Datastore:
      1. At some point in time we will have 16 in flight on two iopaths to a datastore.
      2. If we modify the setting from 1000 to 3? Then it will switch 10 times before draining the queue, in effect for this example we will have IOs in flight down all paths. In total 8 paths in used at the same time.

  2. angryant says:

    IMO
    – Total number of paths = 16 ( http://www.vmware.com/pdf/esx_san_cfg_technote.pdf )
    – Total number of active paths at any time = 4
    – The answer is 2, because this is how RoundRobin works when using 4 paths
    – In this case number of LUN’s is irrelevant (you don’t look for path/LUN separation e.g. http://blogs.vmware.com/vsphere/tag/target )
    – yes, another tricky question (a single array with 4 active ports – ??? – give me a break)

    1. JL says:

      Why is the number of LUN’s irrelevant?

      When a VM on LUN A writes data to it’s vdisk, this data will be processed and results in iops to LUN A.
      There are multiple patchs to this LUN and RoundRobin is configured by default to process 1000IOPS accross path 1, then switch to the next active path and process another 1000IOPS and than it switches to the next active path.
      Although there are, for example, 2 active paths, only one is used in a single moment of time.

      When a VM on LUN B writes data to it’s vdisk, this data will be processed and this results in iops to LUN B. Just following the exact same routing as writen here above.

      There is only one problem any HBA port can only handle IO from or to one LUN at a time.

      In this case port one of HBA one can process the data for LUN A and port two of HBA two can process the data for LUN B. Meaning two data streams.
      Since RoundRobin is configured on a LUN rather than on a device, and the principle behind RoundRobin is that data processing is switched after a given number of IOPS between the available ACTIVE paths, the number of LUNs, to me, seem very relevant!

  3. Bob says:

    It’s 2, but I dont know why. There are only 4 total paths in this scenaric: 2 hbas, 1 to each fabric, two connections to each SP from each fabric. So each HBA sees 2 paths. 2 + 2 is 4. If you use round robin and watch the paths using “manage paths”, you’ll see that only two are active at any given time. I just wish I knew why that was the case.


Leave a Reply

Your email address will not be published. Required fields are marked *