Fast skip to end of tape
Posted: Mon Jan 25, 2021 12:52 pm
DLT (and LTO) tapes maintain a catalog of the files contained on the tape (not the filenames, ANSI or otherwise, just the position where a set of data records start - or, equivalently, the position of the tape mark separating each set of data records). My question is, does the DCL command SET MAGTAPE /SKIP=END_OF_TAPE use this data to skip directly to right place, thus avoiding traversing the many serpentine data tracks on the tape? And if so what are the constraints (which is the oldest VMS version that supports it, etc.?)
I see in the help for Set Mag (OpenVMS/Alpha V8.3) that there is a $ set mag/fast_skip=always command that sounds useful. Not sure how that would be used, and neither the help (examples) nor the DCL Dictionary says much about it. Would one for instance do $ set mag/fast=always once in your session, and from then on all $ set mag/skip=end commands go straight to the doubled TMs at the end of the last data written to the tape?
This might be moot, though, in light of my underlying concern. We use SLS to backup VMS 5.5-2 and 6.2 systems (among others). First off, the /fast_skip qualifier isn't present on V6.2. Second, it is "for use on local SCSI tape drives only" which seems to make things awkward for SLS, unless I could figure out how to get the SLS tape-server to position the tape at the end before making it available to the client. That is also dependent, though, on how the RMT driver interacts with the MKDRIVER.
The problem I'm trying to solve is timeouts in SLS jobs that occur when a remote LTO-3 tape is "Seeking" to get to the end of tape before writing the saveset.
I see in the help for Set Mag (OpenVMS/Alpha V8.3) that there is a $ set mag/fast_skip=always command that sounds useful. Not sure how that would be used, and neither the help (examples) nor the DCL Dictionary says much about it. Would one for instance do $ set mag/fast=always once in your session, and from then on all $ set mag/skip=end commands go straight to the doubled TMs at the end of the last data written to the tape?
This might be moot, though, in light of my underlying concern. We use SLS to backup VMS 5.5-2 and 6.2 systems (among others). First off, the /fast_skip qualifier isn't present on V6.2. Second, it is "for use on local SCSI tape drives only" which seems to make things awkward for SLS, unless I could figure out how to get the SLS tape-server to position the tape at the end before making it available to the client. That is also dependent, though, on how the RMT driver interacts with the MKDRIVER.
The problem I'm trying to solve is timeouts in SLS jobs that occur when a remote LTO-3 tape is "Seeking" to get to the end of tape before writing the saveset.