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To: Jeff Roberson <jroberson@jroberson.net>
Subject: Re: Unmapped I/O
In-reply-to: <alpine.BSF.2.00.1212281217570.2005@desktop>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
References: <20121219135451.GU71906@kib.kiev.ua>
 <alpine.BSF.2.00.1212281217570.2005@desktop>
Date: Fri, 28 Dec 2012 22:43:15 +0000
Message-ID: <48513.1356734595@critter.freebsd.dk>
Cc: Konstantin Belousov <kostikbel@gmail.com>, arch@freebsd.org
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--------
In message <alpine.BSF.2.00.1212281217570.2005@desktop>, Jeff Roberson writes:

>3)  I find the NOTMAPPED negative flag awkward grammatically.  UNMAPPED 
>seems more natural.  Or a positive MAPPED flag would be better.  Minor 
>concern, bikeshedding, etc.

Given that down the road, MAPPED should be the exceptional case, I
think this should not be a negative option.

>4)  It would be better to have some wrapper functions around the bio 
>transient map and or sf buf handling.  I will need it to map unmapped cam 
>ccbs in device drivers.  We need to come to some agreement on this API. 
>There should be a fast page-by-page version and a potentially blocking 
>all-at-once linear version.

Do we have credible relevant use-cases for the "map all linear at
once" case ?

I think it would be better to leave it out, and force those obscure
(already pessimized) cornercases to deal with page by page, rather
than have them cramp our style WRT to max I/O size.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.