RE: Laymans explaination for single level store?

[CWilt@xxxxxxxxxxxx]

David,

"Single-level store is not about a large address space; it's about sharing."

"It may seem obvious to some that every server designed to support a
multi-user, multi-application environment should use an addressing model
designed for sharing.  As I stated earlier, this is a foreign concept to
hardware and operating system designers."

"Virtual memory evolved to support time-sharing by giving each user a
separate address space.  One users memory space was isolated from another
users, thereby providing a degree of protection between them."

"Worse yet, the designers of these time-sharing systems decided to keep the
file system outside virtual memory. They created two places to store data
and programs: the virtual memory and the file system.  With this design, the
data and programs can be used or changed only when they are in virtual
memory.  This means anything in the file system must be moved into virtual
memory before it can be used or changed."

"A simple, familiar example for this process is using a word processor on a
PC.  We first open the file, which contains the document we want to use, and
we watch the hard disk light blink as the document is read into memory.
Actually, it is first being read into our virtual memory and then part of
the document is read into memory.  When we originally configured our PC
operating system, we told it how much disk space should be reserved for
virtual memory.  In the PC world, this is called the application swap file.
As we scroll through the document, we notice the hard drive light again
blinking.  As needed, new parts of the document are being read into memory
from this reserved space.  

        The act of opening the file has created a second copy of our
document.  The original copy of the document still exists unchanged on our
hard drive.  The second copy is in the disk space reserved for virtual
memory. ... There are actually three copies of some parts of the document,
if we count the copy in memory."

[Single-Level Virtual Memory]
"The programmer in the virtual memory implementation just described sees and
managers two levels of storage; the file system and virtual memory are
separate.  This two-level store also creates overhead in the system.
Opening a file causes a disk write to the swap area and closing a file
requires a disk write back to the permanent location.  An alternative
approach is to have only one copy of the file.

        Not having two separate copies means we don't need to reserve disk
space for a swap file.  With this approach, the entire file system becomes
part of virtual memory."

"The open and close operations no longer must physically copy the entire
file from its permanent location on the disk.  Only the portion (or record)
you're reading or working on is copied to a memory buffer.  We often
describe this by saying files are always used "in place," thereby improving
overall system performance."

"As with a two-level virtual memory, the memory is still used as a buffer.
Processors can operate directly only on data in memory, not on the disk.
The difference with only one level is that memory is now a cache for all the
disk storage, rather than for only a reserved area on the disk.  Also, when
one user makes a change to a file, the change is instantly available to any
other user who shares the file."


(note to self...make sure I save this somewhere so I don't have to type it
in again ;-)


HTH,

Charles Wilt
iSeries Systems Administrator / Developer
Mitsubishi Electric Automotive America
ph: 513-573-4343
fax: 513-398-1121
 

> -----Original Message-----
> From: David Gibbs [mailto:david@xxxxxxxxxxxx]
> Sent: Tuesday, November 23, 2004 7:49 PM
> To: Midrange Systems Technical Discussion
> Subject: Laymans explaination for single level store?
> 
> 
> Can anyone point me to a laymans explanation of what single 
> level store 
> is all about.
> 
> I'm trying to educate some folks in our office and some of 
> them are not 
> geeks like me.
> 
> Thanks!
> 
> david
> --
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