On Friday, May 10, 2013 6:09 PM Greg Stark wrote: > On Fri, May 10, 2013 at 7:44 AM, Simon Riggs <si...@2ndquadrant.com> > wrote: > >> Having one corrupt record followed by a valid record is not an > >> abnormal situation. It could easily be the correct end of WAL. > > > > I disagree, that *is* an abnormal situation and would not be the > > "correct end-of-WAL". > > > > Each WAL record contains a "prev" pointer to the last WAL record. So > > for the next record to be valid the prev pointer would need to be > > exactly correct. > > Well then you're wrong. The OS makes no guarantee that blocks are > written out in order. When the system crashes any random subset of the > blocks written but not synced might have been written to disk and > others not. There could be megabytes of correct WAL written with just > one block in the middle of the first record not written. If no xlog > sync had occurred (or one was in progress but not completed) then > that's the correct end of WAL.
In the case where one block is missing, how can it even reach to next record to check "prev" pointer. I think it can be possible when one of the record is corrupt and following are okay which I think is the case in which it can proceed with warning as suggested by Simon. With Regards, Amit Kapila. -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
