On Sun, Jul 20, 2014 at 5:31 PM, Simon Riggs <si...@2ndquadrant.com> wrote: > The block comparison facility presented earlier by Heikki would not be > able to be used in production systems. ISTM that it would be desirable > to have something that could be used in that way. > > ISTM easy to make these changes > > * optionally generate a FPW for every WAL record, not just first > change after checkpoint > full_page_writes = 'always' > > * when an FPW arrives, optionally run a check to see if it compares > correctly against the page already there, when running streaming > replication without a recovery target. We could skip reporting any > problems until the database is consistent > full_page_write_check = on > > The above changes seem easy to implement. > > With FPW compression, this would be a usable feature in production. > > Comments?
This is an interesting idea, and it would be easier to use than what has been submitted for CF1. However, full_page_writes set to "always" would generate a large amount of WAL even for small records, increasing I/O for the partition holding pg_xlog, and the frequency of checkpoints run on system. Is this really something suitable for production? Then, looking at the code, we would need to tweak XLogInsert for the WAL record construction to always do a FPW and to update XLogCheckBufferNeedsBackup. Then for the redo part, we would need to do some extra operations in the area of RestoreBackupBlock/RestoreBackupBlockContents, including masking operations before comparing the content of the FPW and the current page. Does that sound right? -- Michael -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers