[OpenAFS] fms (fwd)

[Derek Atkins]

I received the following message in private email, and am forwarding
it here with permission.

-derek

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Below is info from an old TRACS report that can be found in our old TRACS
query tool on our web site at:

      http://www.transarc.ibm.com/Search/tracs.html

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Problem Description:
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I've installed a new tape drive that I'm going to use for
backups. It's a Sony SDX500C/L, 8mm, AIT-2 Drive and I'm using
the Sony SDX2-36G AIT-2 cartridges (170m/557 ft.). These cartridges
hold 36GB native and 72GB compressed. The drive is set to run
in compressed mode. Can you give me some idea of what numbers
I should use in the tapeconfig file? "fms" won't run correctly due to the
compression.

Solution:
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Here's something from our Developer that specializes in the AFS backup
area:

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The fms command does two passes over the tape drive. The first pass
it writes nothing but data (16 K blocks of data) until it hits the EOT.
>From this, we know the total size of data will fit on the tape. Then it
rewinds the tape and writes 1 data block followed by 1 filemark over and
over until it hits the EOT. From this, it determines the size of the
filemark. The tape capacity and filemark size are put in tapeconfig to
determine how close we can get to the EOT without hitting it.

Of course, this fms calculation does not work for compressing tape
drives. Plus, fms takes a long time to run for tapes larger than 2 GB.
That combined with the fact that we support hitting the EOT for tapes
larger than 2 GB means we can put bogus numbers into the capacity and
filemark size fields of tapeconfig (skipping the need to run fms
altogether).

So, for a compressing tape drive and/or for tapes larger than 2GB, try
a tape capacity of 100G and a filemark size of 0K. This will force the
AFS dump to write data until it hits the EOT (or hits a
bad-spot/error). At which point, the dump will prompt for the next
tape.
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Regards,

--XXXXXXXXXXX

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-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord@MIT.EDU                        PGP key available