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Operating Systems AIX San disk appearing double in old AIX Post 302912186 by bakunin on Wednesday 6th of August 2014 12:05:19 PM
Old 08-06-2014
To expand on what rbatte1 said: when connecting virtual disks (LUNs) from a SAN to a system one usually has a "fabric" in between. Such a fabric is basically a FC-switch, redundant adapters/FC-lines to connect to the storage system (DS8000, EMC VMax, ....) and the clients (basically your systems).

Simple "hdisk"-devices are usually representing one such path over which a disk can be seen, but there might be several parallel ways to see the disk:

Code:
+-----------------+
|  host           | 
+-----------------+
   |       |
   | 3     | 4
   |       |
+-----------------+
|  FC-Switch      |
+-----------------+
   |       |
   | 1     | 2
   |       |
+-----------------+
|  Storage        |
+-----------------+

Here a single disk from the storage can be seen over line 1 then 3, 1 then 4, 2 then 3 or 2 then 4, so you'd have 4 hdisk devices which are just different views onto the same object. (Consider 4 pictures showing the same building but from different angles.) Multipath-drivers (MPIO, SDD, EMCs Powerpath, etc.) now bind together all these views to a usable pseudo-device representing the real disk behind all these. Depending on the driver they are named differently - vpath devices, powerpath devices, but the concept is the same.

I hope this helps.

bakunin
 

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pseudo(4)							   File Formats 							 pseudo(4)

NAME
pseudo - configuration files for pseudo device drivers DESCRIPTION
Pseudo devices are devices that are implemented entirely in software. Drivers for pseudo devices must provide driver configuration files to inform the system of each pseudo device that should be created. Configuration files for pseudo device drivers must identify the parent driver explicitly as pseudo, and must create an integer property called instance which is unique to this entry in the configuration file. Each entry in the configuration file creates a prototype devinfo node. Each node is assigned an instance number which is determined by the value of the instance property. This property is only applicable to children of the pseudo parent, and is required since pseudo devices have no hardware address from which to determine the instance number. See driver.conf(4) for further details of configuration file syntax. EXAMPLES
Example 1: A sample configuration file. Here is a configuration file called ramdisk.conf for a pseudo device driver that implements a RAM disk. This file creates two nodes called "ramdisk". The first entry creates ramdisk node instance 0, and the second creates ramdisk node, instance 1, with the additional disk-size property set to 512. # # Copyright (c) 1993, by Sun Microsystems, Inc. # #ident "@(#)ramdisk.conf 1.3 93/06/04 SMI" name="ramdisk" parent="pseudo" instance=0; name="ramdisk" parent="pseudo" instance=1 disk-size=512; SEE ALSO
driver.conf(4), ddi_prop_op(9F) Writing Device Drivers SunOS 5.10 15 Jun 1993 pseudo(4)
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