Posted: Wed May 11, 2005 5:36 am Post subject: Memory management in z-OS, OS/390
Hi
Can anybody help me in understanding the fundamentals in the memory management of OS/390. Below are the points I would like to focus on.
1. What is DASD, TRACK, VOLUME etc... How they are related to each other and how they are physically attached to the mainframe server.
2. I observed "MIGRATE" status for some datasets. What actually happens for those datasets physically. When I say "HRECALL", it says "RECALLING FROM DASD". What it actually means and how is it managed by mainframe.
3. To my knowledge, PC memory management is fully controlled by operating system. How is it in mainframe.
Any links or any kind of material ..... please help me....
Joined: 26 Nov 2002 Posts: 12378 Topics: 75 Location: San Jose
Posted: Wed May 11, 2005 8:36 am Post subject:
Psridhar,
Quote:
1. What is DASD, TRACK, VOLUME etc... How they are related to each other and how they are physically attached to the mainframe server.
DASD stands for Direct Access Storage Device which is basically a storage device in which the access time is independent of the location of the data--for example, a disk or diskette.
Check this link which explains in detail about the tracks and cylinders
Volume is the unit of physical storage. Volume is the storage space on DASD, tape, or optical devices, which is identified by a volume label. Originally the volume equated to a single disk or tape, but logical volumes are more the norm today, especially with most current DASD devices emulating previous products and VTS doing volume stacking on tape.
Quote:
2. I observed "MIGRATE" status for some datasets. What actually happens for those datasets physically. When I say "HRECALL", it says "RECALLING FROM DASD". What it actually means and how is it managed by mainframe.
Migration is the process of moving unused data to lower cost storage in order to make space for high-availability data. If you wish to use the data set, it must be recalled.
Migration occurs to either of two levels: migration level 1 or migration level 2. Migration level 1 (ML1) volumes are always DASD. Migration level 2 (ML2) volumes can be either DASD or tape. Your computing center controls which volumes are to be used as migration volumes.
The time frame that a dataset is moved to migration level 1 is set by your storage management folks and it varies from shop to shop.
Quote:
3. To my knowledge, PC memory management is fully controlled by operating system. How is it in mainframe.
The term you are looking for is Multitasking which is an ability of an operating system to run multiple programs concurrently. Basically, the operating system uses a hardware clock to allocate "time slices" for each currently running process. If the time slices are small enough _________________ Kolusu
www.linkedin.com/in/kolusu
I want to clear a point over here. Asd you mentioned that volume is a stogae space on DASD. And each volume has a "volume label". Also each DASD have a unique number associated with it. My point is can we have more than one volume on a single DASD?
Also I didn't get what u meant by "access time is independent of data location" Can u throw more light on this.
Joined: 03 Dec 2002 Posts: 579 Topics: 1 Location: Iowa, USA
Posted: Wed May 18, 2005 8:28 am Post subject:
1. The volume label is the same as volume serial number (VOLSER). Each DASD volume has a different volser.
2. When you say "each DASD has a unique number" are you referring to the MVS unit address? If so, there can only be one volume on each unit address.
3. By "access time independent of data location" I believe kolusu was referring to the advantage of DASD over tape. You may need to spin through a tape to reach a record near the end, but on DASD all records are quickly available. _________________ Regards,
Bill Dennis
Disclaimer: My comments on this foorum are my own and do not represent the opinions or suggestions of any other person or business entity.
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