| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Peter Geoghegan <pg(at)heroku(dot)com> |
| Cc: | Stephen Frost <sfrost(at)snowman(dot)net>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Checksums by default? |
| Date: | 2017-01-24 02:01:26 |
| Message-ID: | 17496.1485223286@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Peter Geoghegan <pg(at)heroku(dot)com> writes:
> Perhaps I've missed the point entirely, but, I have to ask: How could
> there ever be false positives?
Bugs. For example, checksum is computed while somebody else is setting
a hint bit in the page, so that what is written out is completely valid
except that the checksum doesn't match. (I realize that that specific
scenario should be impossible given our implementation, but I hope you
aren't going to claim that bugs in the checksum code are impossible.)
Maybe this is a terminology problem. I'm taking "false positive" to mean
"checksum reports a failure, but in fact there is no observable data
corruption". Depending on why the false positive occurred, that might
help alert you to underlying storage problems, but it isn't helping you
with respect to being able to access your perfectly valid data.
regards, tom lane
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