On Sat, Jan 13, 2018 at 03:40:01PM +0000, Simon Riggs wrote:
> The new two byte value is protected by CRC. The 2 byte value repeats
> every 32768 WAL files. Any bit error in that value that made it appear
> to be a current value would need to have a rare set of circumstances.
If you use the two lower bytes of the segment number, then this gets
repeated every 64k segments, no? In terms of space this represents
500GB worth of WAL segments with a default segment size. Hence the more
PostgreSQL scales, the more there is a risk of collision, and I am
pretty sure that there are already deployments around allocating
hundreds of gigs worth of space for the WAL partition. There are no
problems of this class if using the 8-byte field xl_prev. It seems to
me that we don't want to take any risks here.
--
Michael