I wrote:
> Hm.  Currently sizeof(struct ControlFileData) = 296, at least on my
> machine.  Letting it grow past 512 would be problematic.  It's hard
> to see getting to that any time soon, though; we don't add fields
> there often.

I wonder if it'd be worth the trouble to stick something like this into
xlog.c:

        /*
         * For reliability's sake, it's critical that pg_control updates
         * be atomic writes.  That generally means the active data can't
         * be more than one disk sector, which is 512 bytes on common
         * hardware.  Be very careful about raising this limit.
         */
         StaticAssertStmt(sizeof(ControlFileData) <= 512,
                          "pg_control is too large for atomic disk writes");


                        regards, tom lane


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