On Fri, Jan 12, 2018 at 8:43 PM, Claudio Freire <klaussfre...@gmail.com> wrote:
> > > On Sat, Dec 30, 2017 at 7:32 AM, Simon Riggs <si...@2ndquadrant.com> > wrote: > >> So we can't completely remove xl_prev field, without giving up some >> functionality. But we don't really need to store the 8-byte previous >> WAL pointer in order to detect torn pages. Something else which can >> tell us that the WAL record does not belong to current WAL segno would >> be enough as well. I propose that we replace it with a much smaller >> 2-byte field (let's call it xl_walid). The "xl_walid" (or whatever we >> decide to call it) is the low order 16-bits of the WAL segno to which >> the WAL record belongs. While reading WAL, we always match that the >> "xl_walid" value stored in the WAL record matches with the current WAL >> segno's lower order 16-bits and if not, then consider that as the end >> of the stream. >> >> For this to work, we must ensure that WAL files are either recycled in >> such a way that the "xl_walid" of the previous (to be recycled) WAL >> differs from the new WAL or we zero-out the new WAL file. Seems quite >> easy to do with the existing infrastructure. >> > > Or, you can use the lower 16-bits of the previous record's CRC > > > Sorry, missed the whole point. Of the *first* record's CRC
