06-04-2008
passthrough devices vs. named devices
I am having trouble understanding the difference between a passthrough device and a named device and when you would use one or the other to access equipment.
As an example, we have a tape library and giving the command
"camcontrol devlist" gives the following output:
akx[22]# camcontrol devlist
<SPECTRA 215 1014> at scbus3 target 3 lun 0 (pass4,ch0)
<SONY SDX-300C 04c7> at scbus3 target 8 lun 0 (pass5,sa0)
The SPECTRA 215 is the tape library the SONY SDX is the tape drive in the library.
To address the tape library a command such as "mtx -f /dev/pass4 command" is used, however to address the tape unit then /dev/sa0 would be used as in
"/sbin/dump -0uaf /dev/sa0 /dev/da4s1e".
Could someone give me a clue on when you address a passthrough device and when you address a named device, or at point me to some documentation that might clear this up?
Thanks
Thumper
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LEARN ABOUT FREEBSD
tcopy
TCOPY(1) BSD General Commands Manual TCOPY(1)
NAME
tcopy -- copy and/or verify mag tapes
SYNOPSIS
tcopy [-cvx] [-s maxblk] [src [dest]]
DESCRIPTION
The tcopy utility is designed to copy magnetic tapes. The only assumption made about the tape layout is that there are two sequential EOF
marks at the end. By default, the tcopy utility will print information about the sizes of records and files found on the /dev/sa0 tape, or
on the tape specified by the src argument. If a destination tape is also specified by the dest argument, a copy of the source tape will be
made. The blocking on the destination tape will be identical to that used on the source tape. Copying a tape will yield the same program
output as if just printing the sizes.
The following options are available:
-c Copy src to dest and then verify that the two tapes are identical.
-s maxblk Specify a maximum block size, maxblk.
-v Given the two tapes src and dest, verify that they are identical.
-x Output all informational messages to the standard error instead of the standard output. This option is useful when dest is given
as /dev/stdout.
SEE ALSO
mt(1), mtio(4)
HISTORY
The tcopy command appeared in 4.3BSD.
BUGS
Writing an image of a tape to a file does not preserve much more than the raw data. Block size(s) and tape EOF marks are lost which would
otherwise be preserved in a tape-to-tape copy.
End of data (EOD) is determined by two sequential EOF marks with no data between them. There used to be old systems which typically wrote
three EOF's between tape files. The tcopy utility will erroneously stop copying early in this case.
When using the copy/verify option -c, tcopy does not rewind the tapes prior to start. A rewind is performed after writing, prior to the ver-
ification stage. If one does not start at the beginning-of-tape (BOT) then the comparison may not be of the intended data.
BSD
December 20, 2006 BSD