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OS Unit ID question

 
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Christopher Nichols
Occasional Visitor

OS Unit ID question

I've read in other posts that the OS Unit ID is used when the lun is presented to create the device name.

Is the OS Unit Id for the VD used after the LUN is presented? If it is, what impact would it have if the OS Unit Id changed while the LUN is presented?

In other words, can the OS Unit ID change while the LUN is presented, or does the LUN need to be unpresented, then changed, then present again?

Once the LUN is presented is the OS Unit ID used as a mechanism to identify which device name goes with what LUNS?

Sorry if this seems like a simple question, but I don't know anything about openVMS.
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Christopher Nichols
Occasional Visitor

Re: OS Unit ID question

I suppose I should of mentioned this is on a hp eva8000 using commandview software
Uwe Zessin
Honored Contributor
Solution

Re: OS Unit ID question

The OS Unit ID is only relevant for OpenVMS and Tru64 Unix. Its value is reflected in a vendor-specific SCSI page, so - yes: it is definitely used after the virtual disk is presented.

I am pretty sure it can be changed while the virtual disk is presented, but I would not do that while the disk is mounted, because a subsequent SCSI inquire will suddenly give a different value, which can confuse the host operating system and can lead to data corruption - best is if you dismount the disk before the change and reboot the server after the change. It might be possible to avoid a reboot and work around it with a series of SYSMAN> commands, but I have never tried it.

The OS Unit ID is used to generate a user-assigned device name on the server console level and within the OpenVMS operating system, but there are 3 so-called "IDs" that play together when identifying a device name / LUN adress:

- the "OS Unit ID"

- the LUN address (sometimes also called LUN ID) which maps a virtual disk into the target address space for a server. It is not very visible within OpenVMS and, but then it is not very important for identification purposes anyway.

- the "WWID" or "LUN WWN" or whatever it is called at some obscure space - this is a 128-bit unique number in SCSI page 83(16) which is used to identify multiple paths to a virtual disk, because all its LUNs have the same value. This is the most relevant value to match an OpenVMS device name to a virtual disk on the EVA.


These mechanisms are the same within the entire EVA family. Other storage arrays (HSG-based, MSA, XP) use similar mechanisms, but their terminology is - of course - different.
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Re: OS Unit ID question

The following is for OpenVMS. (I don't know if it also holds true for Tru64 Unix, although I strongly suspect it does.)
With the EVAs, the "OS Unit ID" directly maps to the Device name. For example, an OS Unit ID of 42 becomes $1$DGA42, and an OS Unit ID of 5678 becomes $1$DGA5678.
For the HSGs, the "IDENTIFIER" is the name. Again, "IDENTIFIER = 42" from a "Show unit full" command becomes $1$DGA42.
I have no experience with the MSAs, although given their history, I assume they have a LUN parameter that works exactly like the EVA's OS Unit ID and the HSG's IDENTIFIER.
The above is why it is critical to ensure multiple LUNs do not have the same OS Unit ID. At best, VMS would get very confused and simply stop accessing the disks in question if it 'sees' disks with different LUN WWIDs and the same OS Unit ID.

XPs, at least all the XP1024s I have seen, are a completely different creature (which is no surprise, given that the EVA, HSG and MSA all came from DEC/Compaq, and the XP has its roots in 'classic' HP). On the XP, LUNs are designated by their starting Controller Unit and LDEV numbers (CU:LDEV), which are written in hexadecimal notation. Combined together and converted to decimal, the CU:LDEV again directly maps to the VMS Device name. The LDEV is the low unsigned byte, and the CU number is the high unsigned byte.
Keeping the same examples as before, a CU:LDEV of 0:2a becomes $1$DGA42 (hex 02a = dec 42), and 16:2e becomes $1$DGA5678 (hex 162e = dec 5678).
Remember, the XP's CU and LDEV numbers are in hex.

Note: While I am an HPE Employee, all of my comments (whether noted or not), are my own and are not any official representation of the company

Accept or Kudo

Uwe Zessin
Honored Contributor

Re: OS Unit ID question

Tru64 Unix does a persistent binding based on the 128-bit LUN WWN to a device name (e.g. dsk17) whose number is increased for every new device. The "OS Unit ID" can be displayed within Tru64 Unix, but is otherwise only used during AlphaServer boot support.

The MSA1000 has a similar mechanism:
CLI> set unit_id 5 17
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Christopher Nichols
Occasional Visitor

Re: OS Unit ID question

Thanks, that helps me understand this a bit.