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From: casey@gauss.llnl.gov (Casey Leedom)
Newsgroups: comp.arch
Subject: Re: Real disk FASTER than Ram disk
Message-ID: <60196@lll-winken.LLNL.GOV>
Date: 30 May 90 21:58:33 GMT
References: <641@sibyl.eleceng.ua.OZ> <136299@sun.Eng.Sun.COM> <1990May27.210935.8564@murdoch.acc.Virginia.EDU> <3402@auspex.auspex.com>
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Reply-To: casey@gauss.llnl.gov (Casey Leedom)
Organization: Lawrence Livermore National Laboratory
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  It's trivial to avoid the extra copy.  You just add an extra flag to the
buffer header that indicates whether the header points at a buffer pool
block or a block on the ram disk.  A read of a ram disk block not in the
buffer cache just caused a buffer header to be pointed at the ram disk block
and the special flag to be set.  A flush of a buffer cache block just caused
the buffer header to be repointed back into the buffer cache and the flag to
be cleared.  We did this for 2.10BSD and it worked very nicely.

  A better solution (as has been pointed out) is to allow for memory
resident file systems that are only written out to backing disk when
space is needed.  This works well for temporary file systems like /tmp
where a compiler pass may create a temporary files and a later pass reads
and deletes the file.  If there's enough space in memory, the files
blocks should never hit real disk.

Casey