Re: CRCs (was Re: [GENERAL] Re: Loading optimization)

Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
>> Oh. I thought we'd agreed that a CRC on each stored disk block would
>> be a good idea as well. I take it you didn't do that.

> No, I thought we agreed disk block CRC was way overkill. If the CRC on
> the WAL log checks for errors that are not checked anywhere else, then
> fine, but I thought disk CRC would just duplicate the I/O subsystem/disk
> checks.

A disk-block CRC would detect partially written blocks (ie, power drops
after disk has written M of the N sectors in a block). The disk's own
checks will NOT consider this condition a failure. I'm not convinced
that WAL will reliably detect it either (Vadim?). Certainly WAL will
not help for corruption caused by external agents, away from any updates
that are actually being performed/logged.

regards, tom lane