On 19/03/17 17:02, Jim Richardson wrote:

>    I am not interested in the IBM driver if I can get the ST to work.

I can understand why, but....


There would be significant advantage in using the IBMtape driver over 
the generic ST driver if Bacula could be modified to handle its oddity 
on forward/back spacing.


The IBMtape driver supports multipathing to tape drives and robots 
(Character and Generic devices) which the linux standard ST and SG 
drivers don't have. For 99.9% of installations that's irrelevant, but if 
you have a multipathed fabric (iscsi, FC or SAS) then it provides a 
_major_ leap in robustness and gets rid of the issue of a single drive 
or changer showing up as multiple /dev/st* units

The reason for this being a nuisance under ST driver is when you're 
using udev and pointing to the drive using /dev/tape/by-id (which is the 
only reliable way to get to a drive on a fabric), because on any fabric 
disturbance udev may repoint that symbolic link to a different /dev/nst* 
- at which point things start breaking badly.

Remember, drives have to be unlocked by the initiator WWID that locked 
them and all locks are additive, so what happens is that when the 
secondary FC controller issues an unlock and eject command, the drive 
doesn't remove the lock that's been set by the primary FC controller, 
thern fails to eject and generates an error. The robot will then throw 
another error ("removal prevented") which may (or may not) require 
manual acknowledgement before it can access other drives and as a reult 
the night's backup sequence comes to a an early halt.

(This is also the usual cause of "My tapes won't eject" problems in 
robots on SAN fabrics)

The IBMtape driver also has a lot more debugging, logging and monitoring 
capablities than the generic ST driver - which is rather long in the 
tooth, to say the least.... :)

As far as I've been able to determine, the IBMtape driver doesn't care 
if the drives it's talking to are actually IBM or HP ones (they're the 
only 2 LTO drive makers left now), so if Bacula could use it, the driver 
is a worthwhile addition to any LTO-based backup system.





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