BillyBoyo wrote:A member of a PDS is just a sequential file. You can have as many duplicates as you want. ...
Maybe while doing your testing with bulk data, just use a plain sequential file, not a member.
I'm not quite sure what BillyBoyo means by "duplicates" here. There can be just one member xxx in one PDS or PDSE at a time, though member yyy can have the exact same data as member xxx.
As BillyBoyo says, when you add a member to a PDS it is placed at the end. If the new member replaces an existing member, the old member is removed from the directory and the directory is updated to point to the new member. A PDS "compress" just recovers the space.
As BillyBoyo says, running two jobs in parallel that attempt to add a member to the same PDS is a recipe for disaster, even if the new members have different names. The Linkage Editor and Binder have a method to avoid a disaster. ISPF uses a similar (but different) method. However, regular batch utilities like IEBGENER and IEBCOPY do not use these methods.