From: David Steele <david@pgmasters.net>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Magnus Hagander <magnus@hagander.net>
To: Michael Banck <michael.banck@credativ.de>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: pgsql: Validate page level checksums in base backups
Date: Tue, 10 Apr 2018 17:45:12 -0400
Message-ID: <a701f950-30a9-d9b9-d231-82c917420040@pgmasters.net> (raw)
In-Reply-To: <f23e92ec-118d-e6ea-0c81-876ad62a588a@2ndquadrant.com>
References: <20180404121930.GE20852@nighthawk.caipicrew.dd-dns.de>
<20180404134732.tsxx2osvooolxy24@alvherre.pgsql>
<CABUevEwrCF7TzAyD=4WPfTyzH+5xaFYf+-=ATW58XLi7b4dURA@mail.gmail.com>
<20180404182247.GG20852@nighthawk.caipicrew.dd-dns.de>
<CABUevExp8BOo3ZjEXM5xkz5P0zrjZT4_eM7QM4DLyPorDVUQUQ@mail.gmail.com>
<20180405124113.GH20852@nighthawk.caipicrew.dd-dns.de>
<CABUevEzZ55qgLVtMxW=hXhzjHV3oU-gChpeKtufyf_siL-rBWg@mail.gmail.com>
<f23e92ec-118d-e6ea-0c81-876ad62a588a@2ndquadrant.com>
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@pgmasters.net
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To: pgsql-committers@postgresql.org
Cc: david@pgmasters.net, tomas.vondra@2ndquadrant.com, magnus@hagander.net, michael.banck@credativ.de, alvherre@alvh.no-ip.org, tgl@sss.pgh.pa.us, andres@anarazel.de, pgsql-hackers@postgresql.org
Subject: Re: pgsql: Validate page level checksums in base backups
In-Reply-To: <a701f950-30a9-d9b9-d231-82c917420040@pgmasters.net>
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