Re: Block-level CRC checks

* Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> [081118 12:43]:
> Aidan Van Dyk <aidan(at)highrise(dot)ca> writes:
> > But why can't you wal-log the hint bits from the "buffered" page. then your
> > consitent. At least as consistent as the original write was.
>
> > So you're CRC ends up being:
> > Buffer the page
> > Calculate CRC on the buffered page
> > WAL (in bulk) the hint bits (and maybe CRC?)
> > write buffered page
>
> The trouble here is to avoid repeated WAL-logging of the same hint bits.
>
> (Alvaro's patch tried to do that by depending on another hint bit in the
> page header, but that seems unsafe if hint bit setters aren't taking
> exclusive lock.)

And I know it's extra IO. That's why I started the whole thing with a question
along the lines of "how much extra IO are people going to take" for the sake of
"guarenteeing" we read exactly what we wrote.

a.

--
Aidan Van Dyk Create like a god,
aidan(at)highrise(dot)ca command like a king,
http://www.highrise.ca/ work like a slave.