Aidan Van Dyk wrote: > > I think I'm missing something... > > In this patch, I see you writing WAL records for hint-bits (bufmgr.c > FlushBuffer). But doesn't XLogInsert then make a "backup block" record > (unless > it's already got one since last checkpoint)?
I'm not causing a backup block to be written with that WAL record. The rationale is that it's not needed -- if there was a critical write to the page, then there's already a backup block. If the only write was a hint bit being set, then the page cannot possibly be torn. Now that I think about this, I wonder if this can cause problems in some filesystems. XFS, for example, zeroes out during recovery any block that was written to but not fsync'ed before a crash. This means that if we change a hint bit after a checkpoing and mark the page dirty, the system can write the page. Suppose we crash at this point. On recovery, XFS will zero out the block, but there will be nothing with which to recovery it, because there's no backup block ... -- Alvaro Herrera http://www.CommandPrompt.com/ PostgreSQL Replication, Consulting, Custom Development, 24x7 support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
