From: | David Steele <david(at)pgmasters(dot)net> |
---|---|
To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Michael Banck <michael(dot)banck(at)credativ(dot)de> |
Cc: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: pgsql: Validate page level checksums in base backups |
Date: | 2018-04-10 21:45:12 |
Message-ID: | a701f950-30a9-d9b9-d231-82c917420040@pgmasters.net |
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Lists: | pgsql-committers pgsql-hackers |
On 4/10/18 5:24 PM, Tomas Vondra wrote:
>
> I think there's a bug in sendFile(). We do check checksums on all pages
> that pass this LSN check:
>
> /*
> * Only check pages which have not been modified since the
> * start of the base backup. Otherwise, they might have been
> * written only halfway and the checksum would not be valid.
> * However, replaying WAL would reinstate the correct page in
> * this case.
> */
> if (PageGetLSN(page) < startptr)
> {
> ...
> }
>
> Now, imagine the page is new, i.e. all-zeroes. That means the LSN is 0/0
> too, and we'll try to verify the checksum - but we actually do not set
> checksums on empty pages.
>
> So I think it should be something like this:
>
> if ((!PageIsNew(page)) && (PageGetLSN(page) < startptr))
> {
> ...
> }
>
> It might be worth verifying that the page is actually all-zeroes (and
> not just with corrupted pd_upper value. Not sure it's worth it.
>
> I've found this by fairly trivial stress testing - running pgbench and
> pg_basebackup in a loop. It was failing pretty reliably (~75% of runs).
> With the proposed change I see no further failures.
Good catch, Tomas!
I should have seen this since I had the same issue when I implemented
this feature in pgBackRest.
Anyway, I agree that your fix looks correct.
Thanks,
--
-David
david(at)pgmasters(dot)net
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