[20] in Info-AFS_Redistribution
[Liz_Hines@transarc.com: Change to backup system for tape sizes]
daemon@ATHENA.MIT.EDU (daemon@ATHENA.MIT.EDU)
Thu Dec 6 13:04:35 1990
Date: Thu, 6 Dec 90 13:04:22 -0500
To: info-afs@MIT.EDU
From: Richard Basch <probe@MIT.EDU>
------- Forwarded Message
Resent-Date: Thu, 6 Dec 1990 11:27:59 -0500 (EST)
Resent-From: Liz_Hines@transarc.com
Resent-To: AFS_Contacts@transarc.com
Date: Thu, 6 Dec 1990 10:56:15 -0500 (EST)
From: Liz_Hines@transarc.com
To: AFS_Contacts@transarc.com
Subject: Change to backup system for tape sizes
AFS Site Contacts:
Several of our sites have complained about not being able to put 2 Gbytes
worth of data on an Exabyte tape drive with our backup system. We
know what the problem is and we have a solution. The problem is that
our backup system puts end-of-file marks on the tape after each volume
dump and it turns out that the size of the EOF marks varies with the
tape drives, operating system, controllers, etc. UNIX's tape handling
routines do not provide a way to find out what the size of the EOF mark
is. In addition, most UNIX systems do not provide a way to gracefully
handle hitting the end of the tape. Our backup system keeps a running
count of how much data it put on the tape and when it believes it has
run out of tape (based on the size specified on the tape label), it will
shift to a new tape. Because it was not using a large enough size for
the EOF mark, it would sometimes run into a hard EOT error.
One solution is to survey the different tape drives and use the maximum
EOF mark size in the backup system. But we have discovered that the EOF
marks can range from 2 Kbytes to 2 Mbytes (yes, megabytes!). Exabyte
tapes (at least the ones we have here) seem to use 2 Mbyte EOF marks, based
on our testing. What we have done is to add a field to the tapeconfig
file for the size of the EOF mark for that particular device on that
system. We have recommendations for these values. This means that before
sites run our
latest backup system (this change is not in the beta release), they must
edit their tapeconfig file. Also, realize that the more volumes put on
a tape, the less "real data" that will fit on a tape. So sites with
many small volumes will not be able to fit as much volume data on tapes
as sites with larger volumes.
Using the EOF marks after volume dumps allows the backup system to quickly
find the desired volume on restore. We are looking into doing away with
the EOF marks, which may slow down restores. Since this change would make
the backup tape format incompatible, we are moving cautiously. Removing
EOF marks may go into AFS 4.0 (or DCE) - we are still investigating it.
If you would like to use this new change to the backup system, please
contact your Product Support Representative. Note that as with the
original AFS 3.1 beta release, there are compatibility issues between the
AFS 3.0 and AFS 3.1 backup systems.
Liz Hines
Product Support Manager, File Systems
Transarc Corporation
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