On 07/29/2010 06:26 PM, Thompson, Steve wrote: > -----Original Message----- > From: IBM Mainframe Discussion List [mailto:[email protected]] On > Behalf Of Frank Swarbrick > Sent: Thursday, July 29, 2010 5:53 PM > To: [email protected] > Subject: ALTER of open VSAM cluster > > It appears we can ALTER FREESPACE while a file is open to CICS (or > anywhere else, I assume). Is this an OK thing to do, so the next time > the file is reloaded it will use the new freespace values? > <SNIP> > > The ALTER modifies the definition info in the Catalog (as I recall). So > the effect is rather immediate. What this will do is allow you to LOAD > with x y for freespace, and then when running change to m n for > freespace. > > So, let us say that you LOAD a KSDS with FREESPACE(0 0) that has 50 CYLs > of data. Once the load is complete you ALTER it to FREESPACE(25 25). You > know that the majority of the adds to the file will start in the last 2 > cylinders of data (as loaded) and go from there. So the CA/CI splits > will start at that point. > > Regards, > Steve Thompson ... Steve, If you are implying that changing FREESPACE of a loaded file will affect future adds at the high end of the key range, this is not true. Changing FREESPACE has no effect on a VSAM file already containing records. FREESPACE is only used when the file is in a reset or initial empty state with no records and an initial file load is done. I am also certain that if you could ALTER the VSAM catalog entry while the file is still OPEN for that initial load process, that the running program doing the load is at that point only using in-memory control blocks for the VSAM file with values that were derived at OPEN time, and only future OPENS would see any change.
After that intial load, adding a record than doesn't fit in an existing CI results in a CI split with about half the CI records going to a new CI, and if there are no free CI's in that CA, a CA-split is first done to put about half of that CA's CI's in a new CA. The FREESPACE is totally ignored in that process. Even if all the future adds have ascending key values, the add logic is the same. Unless you define the VSAM file with REUSE and have an application that resets the file to empty and reloads all records at some later time without deleting and redefining the dataset, changing FREESPACE after initial load would have no effect. CI splits are relatively cheap, so using CI FREESPACE to minimize CI-splits after initial load has minimal benefit and in cases where a large percent of records must be read may actually reduce performance by requiring reading of more CIs to get the same data. CA-splits are relatively expensive, so there may be special cases where CA-FREESPACE can improve update response consistency after an initial load. But even with FREESPACE(0,0), if updates are clustered by key, performance will improve after the initial CI and CA splits from earlier updates introduce free space at the precise points in the file undergoing repeated changes. -- Joel C. Ewing, Fort Smith, AR [email protected] ---------------------------------------------------------------------- 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
