Re: [HACKERS] AdvanceXLInsertBuffer vs. WAL segment compressibility

Greetings,

* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> Chapman Flack <chap(at)anastigmatix(dot)net> writes:
> > On 03/27/18 22:10, Michael Paquier wrote:
> >> Here you go for one example:
> >> https://sourceforge.net/projects/pglesslog/
>
> > In any case, from my study of the commit, it is hard for me to see an issue.
> > The code comment says: "mark the header to indicate that WAL records
> > beginning in this page have removable backup blocks."
>
> Yeah, that commit just moved a flag from individual WAL records to page
> headers, arguing that it was okay to assume that the same flag value
> applies to all records on a page. If there are no records in the page,
> it doesn't matter what you think the flag value is.
>
> A potentially stronger complaint is that WAL-reading tools might fail
> outright on a page with an invalid header, but I'd say that's a robustness
> issue that they'd need to address anyway. There's never been any
> guarantee that the trailing pages of a WAL segment are valid.

Agreed, I don't buy off that tools which fall apart when reading a page
with an invalid header should block this from moving forward- those
tools need to be fixed to not rely on trailing/unused WAL pages to be
valid.

Thanks!

Stephen