On Fri, 2006-10-27 at 10:54 +0200, Zeugswetter Andreas ADI SD wrote: > >> In the WAL we just need to be able to detect torn pages and stop > >> reading WAL at that point. That's easier and doesn't really need a > >> CRC. We could just adopt the Sybase strategy of storing a unique id > >> number every 512 bytes throughout the WAL page. If those numbers > don't > >> match then we have a torn page; the system crashed at that point and > we should stop reading WAL pages. > > > I've looked into this in more depth following your > > suggestion: I think it seems straightforward to move the > > xl_prev field from being a header to a trailer. That way when > > we do the test on the back pointer we will be assured that > > there is no torn page effecting the remainder of the xlrec. > > That would make it safer with wal_checksum = off. > > I do not think we can assume any order of when a block is written to > disk. > > I think all this can only be used on OS and hardware, that can guarantee > that what is written by one IO call (typically 8k) from pg is safe. > Those combinations do exist, so I think we want the switch.
OK, good. ... but from the various comments I'll not bother moving xl_prev to be a trailer; I'll just leave that where it is now. > Putting xl_prev to the end helps only for direct IO WAL sync methods, > else we would need it on every page. [There is already an XLogRecPtr on each 8k page.] -- Simon Riggs EnterpriseDB http://www.enterprisedb.com ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster
