Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!pasteur!ucbvax!AMHERST.BITNET!JWMANLY
From: JWMANLY@AMHERST.BITNET (John W Manly)
Newsgroups: comp.os.vms
Subject: (none)
Message-ID: <8804180213.AA04304@ucbvax.Berkeley.EDU>
Date: 12 Apr 88 17:22:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 44

Pardon if this is a repeat -- my mailer messed up.

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.

BTW, I want to make it clear that I am not talking in particular about the
PDDRIVER supplied with VMS, as I understand that it does character-by-character
data transfers and hence is rather slow.  I am talking about the supposedly
sleeker items like TurboDisk and more to the point about the theoretical
advantages of such virtual devices.

Basically, what is it that these items are supposed to get you?  "Improved
performance through faster access to data and programs" is the phrase I keep
hearing, but does this really help?  For images, what does this get you that
INSTALL doesn't?

Perhaps I don't understand INSTALL very well, but my impression was that when
you INSTALL something, a permanent global section is created for the readonly
image sections, which means that when someone wants to activate the image, they
just map to the global section, and thus only incur "soft" (memory to memory)
faults.  Certainly, the actual disk blocks must be read in once when the first
user invokes the image, but after that it is, in some sense, memory resident.

A RAMdisk, on the other hand, looks like a disk, so image activation proceeds
normally, generating page reads from memory to memory.  So it is not clear to
me what good the RAMdisk is doing.  Also, would there be any point in
installing an image on a RAMdisk?

But what about reading and writing data files?  I have never had occasion to,
so I am not sure what the results of installing such files are, so this might
be a place where the RAMdisk is a help, though presumably you eventually have
to copy the data from RAMdisk to real disk for permanent storage.

Also, to create a RAMdisk, a certain amount of memory must be dedicated to this
function. Is the theory of RAMdisks that VMS does not manage its memory very
well, and this dedication results in a performance win?  If this is true, is
VMS just not very good at memory management, or does it write things back to
disk too often (like image rundown)?

Anyone have any thoughts?

PHONE:  (413)-542-2526            - John W. Manly
BITNET: JWMANLY@AMHERST             System Manager
                                    Amherst College