Tuesday, September 30, 2008

Re: [HACKERS] Block-level CRC checks

Alvaro Herrera wrote:
> Initially I'm aiming at a CRC32 sum
> for each block. FlushBuffer would calculate the checksum and store it
> in the CRC fork; ReadBuffer_common would read the page, calculate the
> checksum, and compare it to the one stored in the CRC fork.

There's one fundamental problem with that, related to the way our hint
bits are written.

Currently, hint bit updates are not WAL-logged, and thus no full page
write is done when only hint bits are changed. Imagine what happens if
hint bits are updated on a page, but there's no other changes, and we
crash so that only one half of the new page version makes it to disk (=
torn page). The CRC would not match, even though the page is actually valid.

Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com

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