On Tue, Jan 12, 2021 at 01:15:44PM -0500, Bruce Momjian wrote: > On Tue, Jan 12, 2021 at 01:11:29PM -0500, Stephen Frost wrote: > > I don't think there's any doubt that we need to make sure that the IV is > > distinct and advancing the LSN to get a new one when needed for this > > case seems like it's probably the way to do that. Hint bit change > > visibility to users isn't really at issue here- we can't use the same IV > > multiple times. The two options that we have are to either not actually > > update the hint bit in such a case, or to make sure to change the > > LSN/IV. Another option would be to, if we're able to make a hole to put > > the GCM tag on to the page somewhere, further widen that hole to include > > an additional space for a counter that would be mixed into the IV, to > > avoid having to do an XLOG NOOP. > > Well, we have eight unused bits in the IV, so we could just increment > that for every hint bit change that uses the same LSN, and then force a > dummy WAL record when that 8-bit counter overflows --- that seems > simpler than logging hint bits.
Sorry, I was incorrect. The IV is 16 bytes, made up of the LSN (8 bytes), and the page number (4 bytes). That leaves 4 bytes unused or 2^32 values for hint bit changes before we have to generate a dummy LSN record. -- Bruce Momjian <br...@momjian.us> https://momjian.us EnterpriseDB https://enterprisedb.com The usefulness of a cup is in its emptiness, Bruce Lee
