Path: utzoo!mnetor!uunet!husc6!bloom-beacon!mit-eddie!ll-xn!ames!pioneer!pstevens
From: pstevens@pioneer.arpa (Paul Stevens RCE Sterling)
Newsgroups: comp.os.vms
Subject: Re: What good are VMS RAMdisks anyway?
Message-ID: <7544@ames.arpa>
Date: 18 Apr 88 17:10:30 GMT
References: <8804180208.AA04234@ucbvax.Berkeley.EDU>
Sender: usenet@ames.arpa
Reply-To: pstevens@pioneer.UUCP (Paul Stevens RCE Sterling)
Organization: NASA Ames Research Center, Moffett Field, Calif.
Lines: 57

In article <8804180208.AA04234@ucbvax.Berkeley.EDU>  (John W Manly) writes:
>Hi everyone.  I have a few questions about some software that I have seen
>advertised recently and would like to hear from anyone who has anything to say
>on the subject of RAMdisks for VMS.

>  I am talking about the supposedly
>sleeker items like TurboDisk and more to the point about the theoretical
>advantages of such virtual devices.
>

Interesting you should ask about RAM disks and Turbodisk in particular.
We are just doing some studies and tests using Turbodisk to boost
performance on one of the heaviest use nodes on our cluster here:
An 8650 with 60-80 users on average and up to 100 at peak times.

Now about, INSTALL vs RAM disks,  If you INSTALL with OPEN/HEADER
all that has been done is to keep header information available in
memory for locating the image from the file.  This means that one
less disk I/O needs to be done at image activation when the page tables
are mapped.  If you go all the way and (for a re-entrant routine) also
make it SHARED then *IF* the image is already in memory then all subsequent
processes will map to those shared pages.  However, this does not mean
that the image will always be in memory.  This will depend upon how
often the image is invoked, wether the entire image gets mapped (i.e. If
it pages within itself) and how much memory you have.

Turbodisk helps you to make the right decisions in this area by use of
something they call recording mode.  This alows you to collect statistics
about disk activity (not image activity) for all image files on your
system.  Therefore, you can find out which of your files (even those
declared OPEN/HEADER/SHARED are generating lots of disk I/O, for whatever
reason.  These are the good candidates for RAM disk.  I believe DEC has
something called HOTFILES which does essentially the same thing, but
someone here claims Turbodisk's version is better (I don't really know
either way).

Turbodisk does not save anything back to disk, so putting something
like SYSUAF.DAT, though a high activity file, has the disadvantage of
the possibility of losing data (like everyone's password change for the
last N days since the RAM disk was installed :-( !)

As far as memory goes, yes, RAM disks in general are of greatest benefit
when your system is not thrashing due to lack of memory.  It's hard to
give any real figures, but I would guess that even for a small system
there would be a optimum (though small) size for a RAM disk that would
still give a performance improvement.

Sorry, for the longwinded talk, but I thought others might be interested too.
If you want some details about our results (when they're ready) email
to me and I'll send a quick summary.  I am not endorsing Turbodisk, merely
stating what we're doing with it; other RAM disks may be equally good or
better.
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Paul Stevens - NASA Ames Research Center Moffet Field CA
               Mail Stop 233-10
               (415)694-4887
	       pstevens@ames-pioneer.arpa