Amit Kapila <amit.kapil...@gmail.com> writes: > While working on write-ahead-logging of hash indexes, I noticed that > this function allocates buckets in batches and the mechanism it uses > is that it initialize the last page of batch with zeros and expect > that the filesystem will ensure the intervening pages read as zeroes > too.
Yes. AFAIK that filesystem behavior is required by POSIX. > I think to make it WAL enabled, we need to initialize the page header > (using PageInit() or equivalent) instead of initializing it with > zeroes as some part of our WAL replay machinery expects that the page > should not be new as indicated by me in other thread . I don't really see why that's a problem. The only way one of the fill pages would get to be not-zero is if there is a WAL action later in the stream that overwrites it. So how would things become inconsistent? > Offhand, I don't see any problem with just > initializing the last page and write the WAL for same with > log_newpage(), however if we try to initialize all pages, there could > be some performance penalty on split operation. "Some" seems like rather an understatement. And it's not just the added I/O, it's the fact that you'd need to lock each bucket as you went through them to avoid clobbering concurrently-inserted data. If you weren't talking about such an enormous penalty, I might be okay with zeroing the intervening pages explicitly rather than depending on the filesystem to do it. But since you are, I think you need a clearer explanation of why this is necessary. regards, tom lane -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers