Re: Block-level CRC checks

* Tom Lane <tgl@sss.pgh.pa.us> [081118 12:43]:
> Aidan Van Dyk <aidan@highrise.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@highrise.ca                                       command like a king,
http://www.highrise.ca/                                   work like a slave.