On 2018-01-12 17:24:54 -0500, Tom Lane wrote: > Andres Freund <and...@anarazel.de> writes: > > On 2018-01-12 10:45:54 -0500, Tom Lane wrote: > >> The xl_prev field is our only way of detecting that we're looking at > >> old WAL data when we cross a sector boundary. > > > Right. I wonder if it be reasonable to move that to a page's header > > instead of individual records? To avoid torn page issues we'd have to > > reduce the page size to a sector size, but I'm not sure that's that bad? > > Giving up a dozen or two bytes out of every 512 sounds like quite an > overhead.
It's not nothing, that's true. But if it avoids 8 bytes in every record, that'd probably at least as much in most usecases. > Also, this'd mean that a much larger fraction of WAL records > need to be split across page boundaries, which I'd expect to produce a > performance hit in itself --- a page crossing has to complicate figuring > out how much space we need for the record. It does increase the computation a bit, see XLogBytePosToRecPtr(). I'd guess that more of the overhead would come from the xlog buffer management though. Greetings, Andres Freund
