You've refreshed more of my memory. So you really could have more or less
volumes at different times, depending on how much data, blocksizes, etc.
that you have on them. But, I would think you need to define a static set
of volumes for normal processing, although I could see that at times such as
month end you might need more volumes.
I have a couple of questions. Does anyone have an RVA still? Is there any
current DASD that still works this way?
Eric
Eric Bielefeld
Sr. Systems Programmer
Milwaukee, Wisconsin
414-475-7434
----- Original Message -----
From: "Tom Marchant" <m42tom-ibmm...@yahoo.com>
Newsgroups: bit.listserv.ibm-main
To: <IBM-MAIN@bama.ua.edu>
Sent: Wednesday, April 01, 2009 9:28 AM
Subject: Re: "A foolish consistancy" or "3390 cyl/track architecture"
On Tue, 31 Mar 2009 17:50:08 -0500, Eric Bielefeld wrote:
... I read a lot about it the time, but in the end we didn't get
one. What you wrote below I remember, especially the compression, and
writing all new and updated data in a new location. BUT, you define so
many
volumes. Once you have them defined, and all of the space is allocated...
When you define volumes on an RVA, space is not allocated except for a
track
index. (I don't remember if that's what they called it, but that was its
function.) The track index has a pointer to the physical location of the
track and some flags. All mainframe access to data is through cache, and
cache is managed in full tracks. When a track is destaged from cache, the
entire track is written to a new location and the space required for that
track is allocated.
When Snapshot is used to copy a volume or a data set, the only thing that
is
copied is the relevant entries in the track index. At that moment, the
copied volume doesn't take up any space. As tracks are updated, the track
indexes diverge and space is used for the new volume or data set.
Somewhere, it also records the number of track indexes that point to each
track and when that count reaches zero, the space that the track occupied
becomes available for reuse.
We made extensive use of snapshot copy of full volumes when we were
upgrading from MVS 3.1.3 to OS/390 2.4. It is a big unsupported jump and
we
maintained a completely isolated copy of our entire DASD farm for the test
system. When a test was finished, we discarded the snapshot copies,
performed whatever maintenance was necessary and created new copies.
There is software on the mainframe that tells the RVA that tracks are no
longer needed. For example, when a data set is deleted. The tracks are
discarded and made available for reuse. When this is done, the NCL drops.
--
Tom Marchant
----------------------------------------------------------------------
For IBM-MAIN subscribe / signoff / archive access instructions,
send email to lists...@bama.ua.edu with the message: GET IBM-MAIN INFO
Search the archives at http://bama.ua.edu/archives/ibm-main.html
