Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!wuarchive!uunet!virtech!cpcahil
From: cpcahil@virtech.uucp (Conor P. Cahill)
Newsgroups: comp.unix.internals
Subject: Re: RAM disk.
Message-ID: <1990Oct09.121447.3336@virtech.uucp>
Date: 9 Oct 90 12:14:47 GMT
References: <18560@rpp386.cactus.org> <143359@sun.Eng.Sun.COM> <18574@rpp386.cactus.org> <1850@necisa.ho.necisa.oz>
Reply-To: cpcahil@virtech.UUCP (Conor P. Cahill)
Organization: Virtual Technologies Inc., Sterling VA
Lines: 24

In article <1850@necisa.ho.necisa.oz> boyd@necisa.ho.necisa.oz (Boyd Roberts) writes:
>When I hear `ram disk' I reach for my revolver.  Now, repeat after me...
>    What is the buffer cache? -- A ram disk.
>Increase NBUF and throw tmpfs away.

This is not true.  The ram disk could be considered a buffer cache for
a particular portion of the disk, but it is not a buffer cache for
the entire disk.  Depending upon the configuration of the system and 
the application mixture it may be more advantagious to have a ram disk
as opposed to increasing the buffer cache.

On a system that was running near 90% utilization (i.e. very little CPU 
left) we doubled the number of NBUF entries and system performance
*dropped* significantly.  This was probably due to the extra time spent
searching through the buffer cache looking to see if a block was there.

This is an example of why performance tuning is magic.  There are no
simple answeres for all cases. What seems at first examination to be an
obvious performance gain, may turn out to be a loss.

-- 
Conor P. Cahill            (703)430-9247        Virtual Technologies, Inc.,
uunet!virtech!cpcahil                           46030 Manekin Plaza, Suite 160
                                                Sterling, VA 22170