Re: [HACKERS] AdvanceXLInsertBuffer vs. WAL segment compressibility

On 03/27/18 20:09, Tomas Vondra wrote:
> Not sure what's up with gitweb, but git finds it without any issue:
>
> https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=2dd9322ba6eea76800b38bfea0599fbc459458f2

Thanks, that worked.

On 03/27/18 22:10, Michael Paquier wrote:
> Here you go for one example:
> https://sourceforge.net/projects/pglesslog/

So far, I have been able to study the commit pertaining to
XLP_BKP_REMOVABLE. For again some odd reason, I am striking out on
finding pglesslog code to study. Using the clone URL offered by sourceforge:

$ git clone https://git.code.sf.net/p/pglesslog/code pglesslog-code
Cloning into 'pglesslog-code'...
warning: You appear to have cloned an empty repository.
Checking connectivity... done.

and there's a Files tab, but it tells me This project has no files.

I can find 1.4.2 beta on pgFoundry, but that predates the BKP_REMOVABLE
commit.

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."

In the only case where this patch will zero a header--in the unused space
following the switch record in a segment--there are no "WAL records
beginning in this page". There will not be another WAL record of any kind
until the next valid page (with valid xlp_magic xlp_tli xlp_pageaddr),
which will be at the start of the next segment, and that page will have
XLP_BKP_REMOVABLE if it ought to, and that will tell the reader what it
needs to know.

Am I overlooking something?

-Chap