RE: Buffering and single level store (was RE: chain problem)

Thanks for the link.  Very good info.

As I understand SLS, one of the key ideas is that the object ( or particular
page of the object ) only exists in one place at a time.  It's either on
DASD or in main storage but not both.

Dr. Soltis book gave the following example:
Say you've got a Word document.  The file exists on disk.  When you open
that document, it is copied into memory.  Now the document exists in two
places, on disk and in memory.  Now take into account the fact that memory
can be swapped out to the swap file on disk and the document can exist in
THREE places, on disk, in memory, and in the swap file.

In contrast, SLS means when a page is read in from DASD, it in effect
doesn't exist on the DASD anymore.  Obviously, at a low level its still
there but as far as the OS is concerned it's not.

Here's an interesting section of the link you posted:
"The system reads a block of records into the system buffer (located in the
Open Data Path (ODP) for that file) when the first read in the user's
program is run and then moves the first of these records into the program
buffer so the program can process that data. When the program next performs
a read, the next record in the block is moved into the program buffer by the
program (no system calls are made)."

In effect, the data exists in two places. The page in main storage and the
system buffer.  Since the system buffer is in the ODP, I wonder what effect
sharing the ODP has on the buffer and the problems of blocking? Let's see,
shared ODP are shared across jobs, so you'd need a single job that had for
example, one program doing blocked writes, and a second doing reads.  Would
the second see the records in the buffer?  Seems like the answer should be
yes, but how the heck would that work?  Particularly if the second is doing
blocked reads.  I'm guess blocking is turned off if the ODP is shared.


Charles



> -----Original Message-----
> From: DeLong, Eric [mailto:EDeLong@xxxxxxxxxxxxxxx]
> Sent: Friday, August 13, 2004 2:49 PM
> To: 'RPG programming on the AS400 / iSeries'
> Subject: RE: Buffering and single level store (was RE: chain problem)
> 
> 
> Charles,
> 
> I found some decent info on IBM's site:
> http://www-1.ibm.com/support/docview.wss?uid=nas1d6738e1cd37e1
> f33862565c2007
> cef79&rs=110
> 
> Regarding buffering and SLS, my understanding of this is that 
> SLS is just a
> big memory map that encompases all storage within the box.  
> The fact that a
> database file is associated with a particular memory address 
> does not impart
> any real performance benefit.... It just makes the management of those
> resources simpler.  When you access a file that is stored on 
> DASD, the OS
> must still copy that data from the device into main storage.  
> From main
> storage, then, it gets sent to the application in discreet 
> chunks.  Record
> blocking sends larger chunks, saving on overhead.
> 
> 
> Eric DeLong
> Sally Beauty Company
> MIS-Project Manager (BSG)
> 940-898-7863 or ext. 1863
> 
> 
>