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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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st_san_safe(5)							     OBSOLETED							    st_san_safe(5)

NAME
st_san_safe - determine whether access to tape device special files without the no-rewind-on-close option is allowed (OBSOLETED) VALUES
Failsafe Default Allowed values Recommended values or DESCRIPTION
Note: this tunable is obsoleted and has been renamed to beginning with release HP-UX 11i v3. This tunable notifies the tape driver whether it should allow access to device special files without the no-rewind-on-close option. When this tunable is on the tape driver will fail any open on a tape device special file without the no-rewind-on-close option (that is, or with a status of The tunable allows the safe sharing of tape devices in SAN tape backup solution configurations. The disabling of device special files without no-rewind-on-close in a SAN tape backup solution configuration prevents the accidental sending of a rewind command on close from HP-UX systems via standard tape commands like which would corrupt a backup in process. Who Is Expected to Change This Tunable? Anyone. Restrictions on Changing Changes to this tunable take effect immediately. When Should the Tunable Be Turned On? This tunable should be turned on if the system is going to be used in a SAN tape backup solution configuration. What Are the Side Effects of Turning the Tunable On? The tape driver will fail any open to a tape device special file that does not have the no-rewind-on-close option with a status of This behavior prevents the usage of any tape device special file without the no-rewind-on-close option. When Should the Tunable Be Turned Off? This tunable should be turned off if the system is not part of a SAN tape backup solution configuration or the system utilizes will not function unless it has access to device special files without the no-rewind-on-close option. What Are the Side Effects of Turning the Tunable Off? A rewind may be issued from this system accidentally on close possibly corrupting a backup being issued from another system on the SAN. What Other Tunables Should Be Changed at the Same Time? None. WARNINGS
All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its meaning changed in future releases of HP-UX. Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation, some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tun- able values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see at AUTHOR
was developed by Hewlett-Packard. SEE ALSO
norewind_close_disabled(5), scsi_tape(7), scsimgr_estape(7). Tunable Kernel Parameters st_san_safe(5)
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