Quick File Access (QFA) for tape devices in FreeBSD?

[Stefan A. Deutscher]

Hi,

     I've got another question regarding tape access in FreeBSD. I am
running 5.1Release on the box in question, but didn't see anything else
on 4.8Release either.

I find that, when using tar or dump, finding and restoring some file
from a large archive on tape takes often a long time:
- tar just appears to access the tape sequentially, reading it until it
  has encountered and restored the desired file. Which, when it is near
  the end of a 12 GB DDS-3 tape takes from minutes to hours.

- restore is pretty fast to _find_ the tape index, but -- even though it
  says it would be quick to skip over parts of a volume to the file to
  be restored, it does take quite a bit of time to complete as well.

'A long time' here means, significantly longer than the 40 seconds or so
that modern DAT hardware needs to access locations on tape using quick
file access (QFA): The tape device forwards/rewinds with increased speed
to position the head (well, actually: the tape) right above the block of
interest, rather than working its way through the whole 400+ feet.
For this to work, there needs to be some sort of index file that holds
for each file on tape the physical and/or logical position.

The OS/2 port of GNU tar (GNU tar version 1.10 - AK 2.58), does offer
such a feature, and it is activated with a -D option and works pretty
fast:

-D      --tape-directory [M,]F  use/maintain tape directory in file F
                                mode M=abs : use both absolute and relative
                                             addressing when available (def)
                                     M=rel : use relative addressing only
                                M=rel requires volume labels


I have not found anything like that in the versions of tar and dump on
FreeBSD 5.1, don't know about bsdtar though.

Is it me or is it not implemented? 
If not, are there any other tools available to us which do allow QFA?
And, if not, should I submit this as a suggestion/PR via send-pr for tar
and dump, or to the gnu-tar folks for the tar part of it?


 Cheers, Stefan