Re: Online enabling of checksums

Greetings,

* Tomas Vondra (tomas(dot)vondra(at)2ndquadrant(dot)com) wrote:
> On 09/29/2018 02:19 PM, Stephen Frost wrote:
> > * Tomas Vondra (tomas(dot)vondra(at)2ndquadrant(dot)com) wrote:
> >> While looking at the online checksum verification patch (which I guess
> >> will get committed before this one), it occurred to me that disabling
> >> checksums may need to be more elaborate, to protect against someone
> >> using the stale flag value (instead of simply switching to "off"
> >> assuming that's fine).
> >>
> >> The signals etc. seem good enough for our internal stuff, but what if
> >> someone uses the flag in a different way? E.g. the online checksum
> >> verification runs as an independent process (i.e. not a backend) and
> >> reads the control file to find out if the checksums are enabled or not.
> >> So if we just switch from "on" to "off" that will break.
> >>
> >> Of course, we may also say "Don't disable checksums while online
> >> verification is running!" but that's not ideal.
> >
> > I'm not really sure what else we could say here..? I don't particularly
> > see an issue with telling people that if they disable checksums while
> > they're running a tool that's checking the checksums that they're going
> > to get odd results.
>
> I don't know, to be honest. I was merely looking at the online
> verification patch and realized that if someone disables checksums it
> won't notice it (because it only reads the flag once, at the very
> beginning) and will likely produce bogus errors.
>
> Although, maybe it won't - it now uses a checkpoint LSN, so that might
> fix it. The checkpoint LSN is read from the same controlfile as the
> flag, so we know the checksums were enabled during that checkpoint. Soi
> if we ignore failures with a newer LSN, that should do the trick, no?
>
> So perhaps that's the right "protocol" to handle this?

I certainly don't think we need to do anything more.

Thanks!

Stephen