I mean, it depends on whether your pmem data is a bunch of copy-on-write persistent data structures like HAMTs; or maybe packed data structures like Vector<Foo>s where you can't easily rewrite one Foo to be a different size without rewriting the whole vector; etc.
If 1. the new format is just like the old format except for one little difference to one struct, and 2. structs point to other structs, rather than containing them; then it's just a matter of calling your within-mmap(2)ed-arena malloc(2)-equivalent function to get a new chunk of the pmem arena of the right size for the new version of the struct; and then rewriting the pointer in the other struct to point to it; and then calling your free(2)-equivalent on the old version of the struct.
If you change the structure of some fundamental primitive type like how strings are represented, then you're probably going to have to rewrite your whole pmem arena.
Though, also, you can just make your code deal with both old and new versions of the struct, and only migrate structs when they're getting modified anyway. (This is equivalent to the way you'd avoid an RDBMS migration rewrite an entire table, by instead adding a trigger that makes the migration happen to a row on UPDATE, and then ensuring that your business-layer can deal with both migrated and un-migrated versions of the row.)
> If you change the structure of some fundamental primitive type like how strings are represented, then you're probably going to have to rewrite your whole pmem arena.
That's part of the reason why I was thinking something like Cap'n Proto or Protocol Buffers might make sense for a lot of structures. You pay a bit of cost for writing but get to gracefully handle upgrades to the structure if you do it right. I'd imagine you want to use something higher level just above them to organize the records. But this is all a really new area of thinking about this so I'm probably being a bit obtuse about it.
If 1. the new format is just like the old format except for one little difference to one struct, and 2. structs point to other structs, rather than containing them; then it's just a matter of calling your within-mmap(2)ed-arena malloc(2)-equivalent function to get a new chunk of the pmem arena of the right size for the new version of the struct; and then rewriting the pointer in the other struct to point to it; and then calling your free(2)-equivalent on the old version of the struct.
If you change the structure of some fundamental primitive type like how strings are represented, then you're probably going to have to rewrite your whole pmem arena.
Though, also, you can just make your code deal with both old and new versions of the struct, and only migrate structs when they're getting modified anyway. (This is equivalent to the way you'd avoid an RDBMS migration rewrite an entire table, by instead adding a trigger that makes the migration happen to a row on UPDATE, and then ensuring that your business-layer can deal with both migrated and un-migrated versions of the row.)