Re: Offline enabling/disabling of data checksums

Hi,

Am Mittwoch, den 26.12.2018, 19:43 +0100 schrieb Fabien COELHO:
> > It adds an (now mandatory) --action parameter that takes either verify,
> > enable or disable as argument.
> 
> I'd rather have explicit switches for verify, enable & disable, and verify 
> would be the default if none is provided.

I changed that to the switches -c/--verify (-c for check as -v is taken,
should it be --check as well? I personally like verify better), 
-d/--disable and -e/--enable.

> About the patch: applies, compiles, "make check" ok.
>
> There is no documentation.

Yeah, I'll write that once the CLI is settled.

> In "scan_file", I would open RW only for enable, but keep RO for verify.

OK, I've changed that.

> Also, the full page is rewritten... would it make sense to only overwrite
> the checksum part itself?

So just writing the page header? I find that a bit scary and don't
expect much speedup as the OS would write the whole block anyway I
guess? I haven't touched that yet.

> It seems that the control file is unlinked and then rewritten. If the
> rewritting fails, or the command is interrupted, the user has a problem.
>
> Could the control file be simply opened RW? Else, I would suggest to
> rename (eg add .tmp), write the new one, then unlink the old one, so that
> recovering the old state in case of problem is possible.

I have mostly taken the pg_rewind code here; if there was a function
that allowed for safe offline changes of the control file, I'd be happy
to use it but I don't think it should be this patch to invent that.

In any case, I have removed the unlink() now (not sure where that came
from), and changed it to open(O_WRONLY) same as in Michael's code and
pg_rewind.

V2 attached.

Michael

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