On Tue, Nov 30, 2010 at 11:22 AM, Robert Haas <robertmh...@gmail.com> wrote: > On Tue, Nov 30, 2010 at 11:16 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: >> How much is "quite a lot"? Do we have any real reason to think that >> this solution is unacceptable performance-wise? > > Well, let's imagine a 1GB insert-only table. It has 128K pages. If > you XLOG setting the bit on each page, you'll need to write 128K WAL > records, each containing a 12-byte relfilenode and a 4-byte block > offset, for a total of 16 bytes of WAL per page, thus 2MB of WAL. > > But you did just dirty a gigabyte of data.
Oh, but it's worse than that. When you XLOG a WAL record for each of those pages, you're going to trigger full-page writes for all of them. So now you've turned 1GB of data to write into 2+ GB of data to write. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
