Path missing???

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Operating Systems AIX Path missing???
# 1  
Old 01-13-2014
IBM Path missing???


I not familiar with MPIO pathing those stuffs!
Can any one please tell me ONE FC card can hold how many paths?

I have a stand alone server, where TWO fc card has been configured,
In one card i am getting ie., fsci0 im getting 4paths
Another card i am getting ie., fsci2 im gettin 3paths.
Path#      Adapter/Path Name          State     Mode     Select     Errors
    0           fscsi0/path0           OPEN   NORMAL      23478          0
    1           fscsi0/path1           OPEN   NORMAL      23809          0
    2           fscsi0/path2           OPEN   NORMAL      23736          0
    3           fscsi0/path3           OPEN   NORMAL      23706          0
    4           fscsi2/path4           OPEN   NORMAL      25749          0
    5           fscsi2/path5           OPEN   NORMAL      26232          0
    6           fscsi2/path6           OPEN   NORMAL      26241          0

According to my assumption One card has two port. So two paths!!!
fcs0    Available 01-00 8Gb PCI Express Dual Port FC Adapter
fcs2    Available 0C-00 8Gb PCI Express Dual Port FC Adapter

Please explain me the above configuration!!!

***Thanks in Advance***

Last edited by Franklin52; 01-13-2014 at 06:42 AM.. Reason: Please use code tags
# 2  
Old 01-13-2014
Each adapter has two ports, and each port is connected to two different adapters at storage (through Fabric). That is the reason you are seeing 4 paths for each adapter.

These connections are given for redundancy (to avoid single point of failure).

Hope this helps.
This User Gave Thanks to ibmtech For This Post:
# 3  
Old 01-13-2014
# 4  
Old 01-14-2014
fscsi0 getting 4paths !!!
but one fscsi0 dual port adapter has only2 ports = 2paths but why its getting 4paths for an Dual port adapter?

And same for fscsi2 also getting 3paths for 2ports.

Please clarify my doubt....
# 5  
Old 01-14-2014
Originally Posted by Thala
but why its getting 4paths for an Dual port adapter?
It has nothing to do with how many ports one adapter has. A "path" is just a way of looking at one disk. The short answer is, a "path" is a certain connection from the system to the disk, using a specific way.

Here is the long version: consider the following setup. You have one FC-Adapter (two ports, FCa, FCb), which is connected to two FC-switches (SWa, SWb), which both connect to a storage system. The switches are themselves connected via a crossover-link. You create a single disk on this storage system. This disk will be seen over different paths: FCa-SWa-Disk, FCa-SWa-SWb-Disk, FCb-SWb-Disk, FCb-SWb-SWa-Disk. AIX will therefore create 4 different hdisk-devices when you run cfgmgr.

Now, this is not very comfortable, especially when you create several disks. Each disk will be seen via 4 paths (if you have a fabric more complex than the one i sketched above you might have even more paths) and soon you will have trouble to remember that hdisk137 is in fact hdisk49 and also hdisk87 and hdisk3.

This is where MPIO comes into play: it takes all the hdisk-devices which represent in fact one and only one disk (its like taking a picture of a building from several angles - regardless of how many pictures you take it is always the same one building) and builds a "super-device" above them. You can use this super-device to build VGs, LVs and all other sorts of things you use normally disks for. The MPIO (Multipath-I/O) driver will take care about the relationship of the different hdisk devices and do the bookkeeping for you (and the OS as well).

And there is an additional advantage to it: suppose Switch A in the above picture would fail. Two of the four hdisks (the two paths involving SWa) would be inaccessible, because these pathes would not be usable, but the super-device still works, because it uses ALL paths and if two fail there are still two more to use. In fact this is the main reason one builds such complex fabrics with parallel FC-switches, crossover-links and the like.

I hope this helps.

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