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From: "John S. Dyson" <toor@dyson.iquest.net>
Message-Id: <199607191431.JAA00152@dyson.iquest.net>
Subject: Re: RE:About buffer cache
To: heo@cslsun10.sogang.ac.kr (Heo Sung Gwan)
Date: Fri, 19 Jul 1996 09:31:05 -0500 (EST)
Cc: freebsd-fs@FreeBSD.ORG
In-Reply-To: <9607191338.AA05500@cslsun10.sogang.ac.kr> from "Heo Sung Gwan" at Jul 19, 96 10:38:44 pm
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> 
> And there are two possible approaches:
> 1. Modification of buffer cache to FIFO buffers dynamically allocated.
> 2. User-level process that manages that FIFO buffers that access files 
>  through raw disk without buffer cache.
>  
> 
> Thanks for your resposes.
> 
On some OSes there is the ability to tune the buffer cache/filesystem
to be warned of certain behavior.  Specifically, on sequential file
reads we will currently retain the old, sequentially read data,
non-preferentially.

We do have some code in our vfs_bio to remember if a buffer is being
accessed repeatedly, but I know that a large, multimedia read will still
flush the buffer cache.  Should we consider an fcntl or somesuch that
will send a hint to the buffer cache code to forget old sequentially
read data?  This would allow reuse of essentially the same buffer
space for multimedia apps.  So, instead of a multimedia read taking
up the entire cache with stale data, we could free the stale data.
And reuse the buffer space.  Shouldn't be too hard, but the keepers
of the API should tell me where/how to implement the system call
hooks...  Bruce?, Kirk?, David???, Anyone else have an idea as to
where to put the hints?

John