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Operating System - OpenVMS
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Device Status

 
Vineet Deshpande
Frequent Advisor

Device Status

When I run the command "show device" I get the status of the devices as "online" or "mounted"

What is the difference between a mounted device and an online device ?

If a device is just online does it mean that it’s not currently in use? Should a device be compulsorily mounted i.e. have volumes mounted on it for it to be used for storage etc?

Do Magnetic tapes need to have volumes mounted on them for the OS to use them?

Please help

3 REPLIES
Steven Schweda
Honored Contributor

Re: Device Status

> What is the difference between a mounted
> device and an online device ?

A "mounted" device has been mounted.

HELP MOUNT

The Mount command (MOUNT) is used to make a disk or magnetic tape
available for processing.
[...]

> Do Magnetic tapes need to have volumes
> mounted on them for the OS to use them?

I'd say that's normally true. Some programs
(like BACKUP) can mount a tape automatically
if needed.

http://h71000.www7.hp.com/doc/index.html
http://h71000.www7.hp.com/doc/os83_index.html
RBrown_1
Trusted Contributor

Re: Device Status


"Device online" means that the device can communicate with the host. For some devices, terminals for example, this is sufficient. User activities cause I/O requests to be sent to the device driver, which interacts directly with the device.

Other devices such as disks have another piece of software, called an ACP, between the device and the user. MOUNTing the device associates the device with the ACP. In the case of disks, this extra layer handles the file system, which allows the user to work with files instead of disk blocks. User activities cause I/O requests to be sent to the ACP which sends I/O requests to the device driver.

So:

1. not all devices need to be mounted

2. disks need to be mounted to be used for storage

3. tapes need to be mounted to be used, but if you are using BACKUP you will find that it does the MOUNT for you
Hoff
Honored Contributor

Re: Device Status

1: The literal answer to your question is that the device driver has decided the device is accessible, and that the device driver and an ancillary control process (ACP; or extended $qio processor; XQP) have decided a device is accessible.

2: no. Mounting means a file system has been associated with the device. But if you want to use the assistance of a file system with the the volume, you do usually have an ACP or XQP around; the device is mounted. But that's not a requirement.

3: see 2.

There's a short introduction to the OpenVMS I/O stack:

http://64.223.189.234/node/114

And a link from there over to a discussion of ACPs.

Lower-level details of the OpenVMS device driver and I/O stack can be interpolated from the Internals and Data Structures Manual (IDSM; a book that's separately available for purchase) or in the device driver documentation (older versions are in the manual set; newer versions are in a book that's available for purchase). And from parts of the operating system documentation, including the I/O User's Reference Manual. The OpenVMS manuals are here:

http://www.hp.com/go/openvms/doc

The IDSM and the Driver book are not AFAIK on-line and are usually acquired separately.

Pointed or specific questions tend to get you specific replies. More general questions or the inclusion of some background information or a more general goal can often elicit different or alternative approaches or answers.