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 [email protected] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
