>>>>> "Tom" == Tom Lane <[EMAIL PROTECTED]> writes:
Tom> Postgres has a bad habit of becoming very confused if the Tom> page header of a page on disk has become corrupted. In Tom> particular, bogus values in the pd_lower field tend to make I haven't read this piece of pgsql code very carefully so I apologize if what I suggest is already present. One "standard" solution to handle disk page corruption is the use of "consistency" bits. The idea is that the bit that starts every 256th byte of a page is a consistency bit. In a 8K page, you'd have 32 consistency bits. If the page is in a "consistent" state, then all 32 bits will be either 0 or 1. When a page is written to disk, the "actual" bit in each c-bit position is copied out and placed in the header (end/beginning) of the page. With a 8K page, there will be one word that contains the "actual" bit. Then the c-bits are all either set or reset depending on the state when the page was last read: if on read time the c-bits were set, then on write time they are reset. So when you read a page, if some of the consistency bits are set and some others are reset then you know that there was a corruption. This is of course based on the assumption that most disk arms manage to atomically write 256 bytes at a time. -- Pip-pip Sailesh http://www.cs.berkeley.edu/~sailesh ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster