passthrough devices vs. named devices Post: 302202416

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Top Forums UNIX for Dummies Questions & Answers passthrough devices vs. named devices Post 302202416 by Perderabo on Wednesday 4th of June 2008 09:13:48 PM

06-04-2008

Administrator Emeritus

They are both the same type of objects. I don't know what os you're using so I talk talk specificly to that. But imagine a version of unix with device files like /dev/mt0 to talk to a tape drive. Any device file has a major and a minor number. The major number identifies which driver is being used. The minor number is passed to the driver for its use. It the case of a typical mt driver, it ids the particular tape drive. So you can open /dev/mt0 and read or write to the tape drive via it. You can also invoke the ioctl system call for special purposes like rewinding the tape. This is the typical type a tape driver that existed for years and it is what you call a "named device".

Now someone invents a new type of tape drive that can hold 5 tapes. How to control that? The mt ioctl doesn't handle stuff like that. Well one answer is to redefine the rewind or the unload command to now mean "go to the next tape". A lot of juke box style drives do just that and call it stacker mode. But we want something more... we want to just jump to tape number 4 (as an example) regardless of where we were. The tape drive can do that but the mt driver doesn't have a way to send the right command. All we need is some quick a dirty way to send a particular scsi command to the tape drive. This is where the pass-though driver comes in. It does not know it is controlling a tape drive. But you can give it a scsi command and it can send it to the device. And it can even return a status code. But that is it. It just passes simple commands to a device. Now a real smart program figures out what scsi command is needed to jump to tape number 4, it uses the pass though driver to send it, and it gets a status code back. So now we are using a secondary driver to access extra device features that the primary driver can't control.

But the pass through device is a special file too with a major and minor number. You don't use it to tranfer large blocks of data though. Just special commands.

In this case both special files referred to the same device. But often there will be a collection of tape drives in a juke box with a single pass-though device for the box itself.

We would use the mt driver where we can. And we use the pass-though driver only when we must. Drivers like mt have a man page that describes which ioctl command it can do. Pass though drivers usually have sparse man pages because you can't document all possible scsi commands.

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amtape

AMTAPE(8)						      System Manager's Manual							 AMTAPE(8)

NAME

       amtape - user interface to Amanda tape changer controls

SYNOPSIS

       amtape config command [ command options ]

DESCRIPTION

       Amtape performs tape changer control operations.  It uses the underlying tape changer script defined by the tpchanger option for a particu-
       lar Amanda configuration as specified by the config argument.

       Tape changers maintain a notion of the current and next slot for each configuration.  These may or may not correspond to an actual physical
       state  of  the device, but do tend to minimize searching through the tape storage slots.  If the desired tape is in the current slot, it is
       likely the next tape needed is in the next slot rather than at some random position in the storage slots.

       See the amanda(8) man page for more details about Amanda.

COMMANDS

       reset  Reset the tape changer to a known state.	The current slot is set to the first slot.  Other device-specific side effects may  occur.
	      Some  gravity stackers need to be reset to the top position by hand.  This command notifies Amanda the stacker is back in that posi-
	      tion.

       eject  If a tape is loaded in the drive, it is ejected and returned to the slot from which it was loaded.

       clean  If a cleaning tape is defined for the changer, it is used to clean the drive.

       show   Show the contents of all slots.  This can be slow.

       label label
	      Search for and load the Amanda tape with label label.

       taper  Perform the taper scan algorithm.  Load the next tape in the configuration's tape sequence, or a fresh tape with a suitable label.

       device Display the name of the current tape device on stdout.

       current
	      Display the current slot.

       update Update the changer label database, if it has one, to match the tapes now available.

       slot slot
	      Eject any tape in the drive and put it away, then load the tape from slot slot and reset current.

       slot current
	      Eject any tape in the drive and put it away, then load the tape from the current slot.

       slot prev
	      Eject any tape in the drive and put it away, then load the tape from the previous slot and reset current.

       slot next
	      Eject any tape in the drive and put it away, then load the tape from the next slot and reset current.

       slot first
	      Eject any tape in the drive and put it away, then load the tape from the first slot and reset current.

       slot last
	      Eject any tape in the drive and put it away, then load the tape from the last slot and reset current.

       slot advance
	      Eject any tape in the drive and put it away.  Advance current to the next tape, but do not load it.

	      This is useful with non-gravity stackers to unload the last tape used and set up Amanda for the next run.  If you  just  use  eject,
	      the  current  tape will be mounted again in the next run, where it will be rejected as being still in use, ejected and the next tape
	      requested.  Using slot next followed by eject does an unnecessary mount.

       Note: most changers optimize the slot commands to not eject the loaded tape if it is the one being requested.

AUTHOR

       James da Silva <jds@cs.umd.edu>
       University of Maryland, College Park

SEE ALSO

       amanda(8)

																	 AMTAPE(8)

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