On Fri, 12 May 2000, Ulrich Windl wrote:
> Hello,
>
> yesterday I read a DDS1 tape on my Linux system that was created on
> another Linux system. I noticed that the SCSI controller LED was
> constantly on and data thoughput was incredibly slow. However I could
> extract the tar archive with some MB.
>
> The syslog was full with messages like "Domain validation started",
> "... ended sucessfully". I think these messages should not be there.
> I don't understand the reason.
>
My guess is the following: the blocking factor you give tar does not match
the physical block size on the tape. The drive is in variable block mode.
For each block the drive responds with CHECK CONDITION and the sense data
is fetched from the drive. This data tells the tape driver what the block
length was and it can return proper data to tar. Tar is intelligent and it
can process the data even if the blocking factor was wrong. What is
happening here is perfectly OK.
The problem is that the aic7xxx driver interprets CHECK CONDITION as a
problem and performs domain validation (and logs this each time). This is
what kills your performance.
A short term fix for you is to find out what the physical block size on
the tape is and use the correct blocking factor with tar. The long term
fix is probably to modify the aic7xx driver so that it does not perform
domain validation when there is no apparent error condition (note that for
most devices CHECK CONDITION means something is wrong but for tapes there
are many situations where CHECK CONDITION is the correct response from the
drive).
Kai
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