Robert Haas wrote: > On Mon, Nov 30, 2009 at 3:27 PM, Heikki Linnakangas > <heikki.linnakan...@enterprisedb.com> wrote: > > Simon Riggs wrote: > >> Proposal > >> > >> * We reserve enough space on a disk block for a CRC check. When a dirty > >> block is written to disk we calculate and annotate the CRC value, though > >> this is *not* WAL logged. > > > > Imagine this: > > 1. A hint bit is set. It is not WAL-logged, but the page is dirtied. > > 2. The buffer is flushed out of the buffer cache to the OS. A new CRC is > > calculated and stored on the page. > > 3. Half of the page is flushed to disk (aka torn page problem). The CRC > > made it to disk but the flipped hint bit didn't. > > > > You now have a page with incorrect CRC on disk. > > This is probably a stupid question, but why doesn't the other half of > the page make it to disk? Somebody pulls the plug first?
Yep, the pages are 512 bytes on disk, so you might get only some of the 16 512-byte blocks to disk, or the 512-byte block might be partially written. Full page writes fix these on recovery. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + If your life is a hard drive, Christ can be your backup. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers