Re: kern/38643: [dM] st tape drive loses data

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

>> So I think what's happening is that physio is firing off sixteen 64k
>> reads, since PHYSIO_CONCURRENCY is 16 and MAXPHYS is 64k;
> Ooh.  Yes, that explains it very neatly: where the data is going and
> why exactly 15 records get lost.

As a quick test to see if this is what's wrong, I tried hacking
kern_physio to force concurrency to 0 (ie, effectively set
PHYSIO_CONCURRENCY to 1).  It reduces the problem but does not cure it;
it now appears to read two records, rather than 16, when the big read
is attempted (I didn't have a scratch tape handy today; this is based
on tape block numbers reported by "mt status").

I don't expect to be in a position to pick this up again until
Thursday; then, I should be able to try having st.c provide a buf to
physio....

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