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Full Discussion: Path missing???
Operating Systems AIX Path missing??? Post 302883449 by bakunin on Tuesday 14th of January 2014 08:54:43 AM
Old 01-14-2014
Quote:
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.

bakunin
 

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LEARN ABOUT HPUX

vxdmp

vxdmp(7)						 Miscellaneous Information Manual						  vxdmp(7)

NAME

       vxdmp - Veritas Volume Manager multipathing disk devices

DESCRIPTION

       Multipathed  disk  devices  in Veritas Volume Manager (VxVM) represent virtual devices with one or more physical access paths to a physical
       disk.  The Multipathing feature provides disk access reliability by dynamically switching to another physical path if there is a path fail-
       ure.

       Every disk that is visible to VxVM is represented by a multipath device node in the /dev/vx/dmp and /dev/vx/rdmp directories for block type
       and character type interfaces.  A simple disk is represented by a multipath device having one physical path, whereas a disk that is part of
       a multiported disk array is represented by a multipath device with many device access paths depending upon the type of the disk array.

       VxVM  uses  standard  disk  device  naming conventions to name multipath devices.  For example, if there is a disk device with two physical
       access paths such as c1t0d0 and c2t0d0, the multipathing device node for this particular disk is created under /dev/vx/dmp and /dev/vx/rdmp
       as c1t0d0 or c2t0d0.

       To  provide  multipathing,  VxVM  maintains an internal database of all the multipathed devices detected by the system and their respective
       paths.  The database is created at system boot time.  When disks are added or removed from the system, VxVM provides support to reconfigure
       the multipath device database to reflect new states of the hardware, while the system is online.

       To reconfigure the DMP database, run ioscan followed by insf, and then invoke vxdctl enable (see vxdctl(1M)).

       The  various  access  paths to a multipathed device can be seen by using a vxdisk command (see vxdisk(1M)).  These commands open the multi-
       pathing configuration device /dev/vx/dmpconfig to obtain the required information about the various paths to a multipathed device and their
       states.

       VxVM also provides the facility to enable or disable a disk controller. This feature can be used to stop all I/Os through a particular con-
       troller to perform maintenance operations on it.  Use the vxdmpadm command to do this and also perform other operations	on  the  multipath
       device database.

       VxVM  can  provide notification about events that occur in the DMP database due to a change in hardware.  See the vxnotify(1M) man page for
       more information.

       VxVM is capable of multipathing both Active/Active and Active/Passive disk arrays.

FILES

       /dev/vx/dmp		     Multipathing block devices.

       /dev/vx/rdmp		     Multipathing character devices.

SEE ALSO

       ioctl(2), vxdctl(1M), vxdisk(1M), vxdmpadm(1M), vxnotify(1M)

VxVM 5.0.31.1							    24 Mar 2008 							  vxdmp(7)
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