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Date:      Tue, 08 Jun 1999 00:34:01 -0600
From:      Warner Losh <imp@harmony.village.org>
To:        Zhihui Zhang <zzhang@cs.binghamton.edu>
Cc:        freebsd-hackers@FreeBSD.ORG
Subject:   Re: The choice of MAXPHYS 
Message-ID:  <199906080634.AAA03429@harmony.village.org>
In-Reply-To: Your message of "Thu, 03 Jun 1999 10:40:10 EDT." <Pine.GSO.3.96.990603101914.28480A-100000@sol.cs.binghamton.edu> 
References:  <Pine.GSO.3.96.990603101914.28480A-100000@sol.cs.binghamton.edu>  

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In message
<Pine.GSO.3.96.990603101914.28480A-100000@sol.cs.binghamton.edu>
Zhihui Zhang writes: 
: The value of MAXPHYS is chosen to be 64K for the maximum raw I/O transfer
: size. I am wondering why it is not set larger.

I don't think that it is possible to guarantee that you can do a
larger write than 64k on a aha-1542 card given the worst case scatter
gather list you can have in FreeBSD.  I think that this used to be a
common limit.  I thought CAM raised the limit somewhat on cards that
could support it.

Warner


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