Greg Stark wrote: > I think double buffering solves the torn page problem but not the lack > of wal logging. Alvarro solved the wal logging by deferring the wal > logs. But I'm not sure how confident we are that it's logging enough. >
Right now, it's WAL-logging HeapTupleHeader hint bits (infomask and infomask2), and ItemId (line pointer) flags. Page pd_flags are skipped in the CRC checksum -- this is easy to do because they are in a constant offset in the page and I'm just skipping those bytes in CRC_COMP(). So what I'm missing is: - btree hint bits - bgwriter calls XLogInsert during shutdown, to WAL-log the hint bits of unwritten pages. This causes a PANIC to trigger about concurrent WAL activity during checkpoint. (The easy solution to this problem is just to remove the check; another idea is to flush the buffers before grabbing the final address to watch for at shutdown.) > I'm beginning to think just excluding the hint bits would be simpler and > safer. If we're double buffering then it might be possible to do that > pretty cheaply. Copy the whole buffer with memcpy then loop through the > line pointers unsetting the hint bits. Then do the crc. Though that would > prevent us from doing "zero-copy" crc by doing it in the copy. This can probably be made to work, and it solves the problem that bgwriter calls XLogInsert during shutdown. I would create new routines to clear hint bits in all involved modules (heap_resethintbits, btree_%, item_%, page_%), and call them on a copy of the page. The downside to this idea is that we need to create a copy of the page and call those routines when we read the page in, too. -- 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
