| From: | Chapman Flack <chap(at)anastigmatix(dot)net> |
|---|---|
| To: | Bruce Momjian <bruce(at)momjian(dot)us> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: AdvanceXLInsertBuffer vs. WAL segment compressibility |
| Date: | 2016-08-02 19:43:45 |
| Message-ID: | 886a3377-9f2d-2176-15f7-b9ac1eb4dce7@anastigmatix.net |
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| Lists: | pgsql-hackers |
On 08/02/2016 02:33 PM, Bruce Momjian wrote:
> My guess is that the bytes are there to detect problems where
> a 512-byte disk sector is zeroed by a disk failure.
Does that seem plausible? (a) there is only one such header for
every 16 512-byte disk sectors, so it only affords a 6% chance of
detecting a zeroed sector, and (b) the header contains other
non-zero values in fields other than xlp_pageaddr, so the use
of a fixed value for _that field_ in zeroed tail blocks would
not prevent (or even reduce the 6% probability of) detecting
a sector zeroed by a defect.
-Chap
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