NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/comp/sys/sun/misc/4155

Internet Message Format | 1992-09-09 | 3.0 KB

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From: [e-mail] (David Steffens)
Newsgroups: comp.sys.sun.misc,comp.sys.sun.hardware,comp.unix.solaris
Subject: Re: Adding to a TAR tape
Message-ID: <[e-mail]>
Date: 9 Sep 92 21:14:28 GMT
References: <[e-mail]> <[e-mail]> <[e-mail]>
Organization: Eaton-Peabody Lab, Boston, MA
Lines: 44

In article <[e-mail]>,
[e-mail] (Tom Karches) writes:
> Adding files to existing tar files is only possible on a 9 track tape
> drive. The reason is that other tape drives can not accurately position
> the write head to the exact spot on the tape where the additional files
> would be appended.

In article <[e-mail]>,
[e-mail] (Ruth Milner) writes:
> ... Exabytes using long filemarks (which Suns do by default... )
> are able to position between double EOFs to append another file.
> If it can position at the beginning of the second EOF, isn't this
> accurate enough to position at the beginning of the first EOF?

You have confused writing another file onto the logical end of a tape
with appending another file to an existing tar archive.  The former simply
requires positioning between the two file marks that indicate logical
end-of-tape and writing over the second one with the new tape file.
The latter is more complicated as it requires _updating_ the last record
of the tar archive itself in order to remove the logical end-of-archive
that tar has written while preserving whatever data may be present.
This process requires reading to logical end-of-archive, backspacing
one record, and rewriting said record with an updated version containing
the data from the old record plus the data from the head of the new file.

In article <[e-mail]>,
[e-mail] (Dominic Dunlop) writes:
> ...  There are two (512 byte) blocks full of nulls at
> the end of a tar archive, indicating end-of-archive.  (See also
> tar(5).)  If you want to extend the archive, you've got to update the
> contents of those two blocks.  Only block-structured devices (/dev/mt8
> and so on) are capable of doing updates in place...

Close, but not quite, since tar always uses the raw tape device not the
block tape device.  The tape driver and transport hardware must support
the magtape ioctl's that allow backspacing and rewriting the last record.
The old 1/4" cartridge tapes did not support this interface and so were
unable to update a tar archive in place.  Newer drives should be able to,
but perhaps the software has not been updated to reflect this change.
--
David Allan Steffens       | I believe in learning from past mistakes...
Eaton-Peabody Laboratory   | but why does a good education require so many?
Mass. Eye & Ear Infirmary, 243 Charles Street, Boston, MA 02114
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