Dump/Tape block question

[Glenn Dawson]

At 05:20 AM 7/25/2005, FreeBSD Questions wrote:
>Sorry if this is a repeat.  I posted this on the 21st and never saw it show up
>in the mailing list, so I'm assuming nobody ever received it.
>
>I have a question about dump and tape blocks.  When I do backups, dump
>tells me it uses a certain number of blocks for that particular
>backup.  I explicitly set my block size to 65K when I do a dump.  When
>I read the SCSI or hardware block location on the tape after the dump,
>it's nowhere near the same number as what dump said it used.

Assuming you're using the -b option, -b 64 will tell dump to write 64K at a 
time.

When dump is done, and it tells you how many blocks it wrote, multiply that 
by 65536 and that will tell you how many bytes were written.

-Glenn


>How big is each block?  I'd like to know about where on the tape I am
>when I do an 'mt rdhpos' command (but I don't know how to determine
>the reported block size), and I'd like to be able to calculate how
>much data was written for each backup based on the number of blocks
>dumped (I assume these blocks are the size I specify on the dump
>command line).  In fact, how can I calculate how many blocks my tape
>can hold?  (i.e. What is the biggest block number an 'mt rdhpos' or
>'mt rdspos' will report for my tape?)
>
>Of course, an obvious solution to the last question is to write enough
>data to fill the tape and then check the output of 'mt rdhpos'.  I'd
>rather not spend the hours needed and the unneeded wear on the tape to
>figure that out.
>
>Kevin
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