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

The following reply was made to PR kern/38643; it has been noted by GNATS.

From: der Mouse <mouse%Rodents-Montreal.ORG@localhost>
To: David Holland <>, Havard Eidnes 
        Manuel Bouyer <>
Subject: Re: kern/38643: [dM] st tape drive loses data
Date: Tue, 2 Sep 2008 23:46:26 -0400 (EDT)

 >> 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
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