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| From: | Wayne Liu |
| Subject: | [Bug-tar] Spanning multiple hard disks using tar |
| Date: | Mon, 3 Jul 2006 10:04:02 -0600 |
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My company is developing a new tape-replacement technology
based on removable hard disks. While testing spanning an archive over multiple
removable HDs, I had some interesting observations that I couldn’t quite
explain. So I am writing to this mailing list, and hope to hear directly from
tar experts about what behind the behavior of tar I have seen as well as what
is the best way to use tar in my case. (Need to mention that I did some research on the web. But among
the references I have found, there are a few mentioning of Zip drive and
removable hard disks, but most seems to be projected out of past experience using
tape/floppy instead of real experience using removable disks.) First, some background information: My backup drive is
on /dev/sda, which, instead of taking tape cartridges, accepts removable hard disks.
The hard drive shows up in the system as a SCSI hard disk (/dev/sda). All
my removable hard disks have a single partition, and may be formatted to
different file systems (FAT16/32 or ext2/ext3). 1. I tested creating the archive by writing directly to the
partition handle directly, or to a tar file on the partition using the
following commands: Both command formats successfully wrote the archive to the
RDX cartridge, and read/extract it back. Note that the partition is not mounted
when using the partition handle as the tar destination since writing to the
partition handle destroys the file system. In both cases, if the archive is bigger than a single disk, tar
prompted for the second/third volumes after it has filled a cartridge. After I
ejected the cartridge and inserted a new one (which must be mounted at the same
point), tar completed the archive without error. 2. The following commands are used to extract contents of
the archive: - "tar -Mxvf /dev/sda1” In both cases, tar prompts for the second volume after it
has extracted the content from the first. However, after a new cartridge is
inserted (and mounted if necessary), tar always gives an error on the second
volume: "tar: This volume is out of
sequence". I checked the second partition (tar -t
..), and the second archive looks fine. I also tested it on different file
systems: FAT32, ext2, ext3. All failed the same way. Interestingly, the extraction worked
if the cartridge is formatted as FAT16! 3. I was able to get the extraction working by adding a
"-L N" option to the write pass, which tells tar to change volume
after N blocks. But there are a few catches here: - N must be smaller than the available blocks on the
partition (partition size minus the file system, etc). - Also, the archive must be written to a file on the
partition. If the archive is written to the partition handle, the write pass
works fine. But it seems tar couldn’t figure out the end of the volume on
the extraction pass and read right through it (-L does not work on the read
pass). The result is that tar exits with an error, such as invalid headers, etc. I would greatly appeciate if someone can shed some lights into
why tar works, or doesn’t work, as I have seen. I am also hoping
to find a way to NOT use the “-L N” option since it is always a
hassle to determine the value of N on the fly (especially when my HDs have
different capacities). Thanks in advance. _____________________________ ProStor Systems, Inc. (303) 545-2535 x236 |
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