Date: 20 Feb 2002 22:01:02 -0800 From: swear@blarg.net (Gary W. Swearingen) To: freebsd-questions@freebsd.org Subject: Blocks and compression and SCSI tape drives Message-ID: <ff1yffbcgx.yff@localhost.localdomain>
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I just got a HP SureStore 6000 external (Model C1529F) correctly (according to HP web site) reported as HP C1533A by the kernel. (It cost me 1O dollars -- the price of a tape! Seems to work too.) I've written a big file to it with the DCLZ compression (which it's supposed to have) enabled with "mt" and again with that disabled and "mt rdhpos" and "mt rdspos" show the same number of blocks written with compression on or off. Is this the way it's supposed to be? I would think that "hpos" ought to be blocks on the tape period, so that fewer blocks would be used with compression on, but either I'm not controlling compression or "blocks" are some kind of virtual thing and I'll have more tape blocks with compression on than off. What's the story? The "mt status" doesn't show block size. Do I have to experiment and fill the tape to determine what the block size is? What's with these "mt"-reported "hpos" and "spos" blocks? What's the diff, besides "hardware" and "software"? I guess I'm going to have to use these to position the tape, since the driver won't keep track of file position like it does with my TR-4 cartridge SCSI drive. What does "mt" mean by a "setmark". Of what use are they? Can I harm a DDS-2 DAT by doing "cat /dev/zero >/dev/tape"? I'm thinking of doing that and trying to see the block position at the end of the tape. If I've got the drive set for variable block size can I even use "cat", or must I use "dd" to limit the block size. What would that limit be? Is there any tradeoffs to using variable or fixed block sizes on the drive. Can "fixed" be used when using an archiver like "tar" which has it's own block size? Do they have to be equal or integer multiples? P.S. Know of any documentation on this stuff, other than the "mt" man page and the little handbook entry on 4mm drives? Thanks. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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