On Tuesday 01 December 2009 14:38:26 marcin mank wrote: > On Mon, Nov 30, 2009 at 9:27 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. > > What if we treated the hint bits as all-zeros for the purpose of CRC > calculation? This would exclude them from the checksum. That sounds like doing a complete copy of the wal page zeroing specific fields and then doing wal - rather expensive I would say. Both, during computing the checksum and checking it...
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