| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Chapman Flack <chap(at)anastigmatix(dot)net> |
| Cc: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Stephen Frost <sfrost(at)snowman(dot)net>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Robert Haas <robertmhaas(at)gmail(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: [HACKERS] AdvanceXLInsertBuffer vs. WAL segment compressibility |
| Date: | 2018-03-30 18:18:46 |
| Message-ID: | 12915.1522433926@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
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.
regards, tom lane
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