Re: Checksums by default?

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Stephen Frost <sfrost(at)snowman(dot)net>, Peter Geoghegan <pg(at)heroku(dot)com>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, 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: 2017-01-26 03:01:03
Message-ID: CA+TgmobBchynDJr1TLyBuqHN7HpFgQzhO5j9spCsKhAt=d7G6Q@mail.gmail.com
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On Wed, Jan 25, 2017 at 7:37 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2017-01-25 19:30:08 -0500, Stephen Frost wrote:
>> * Peter Geoghegan (pg(at)heroku(dot)com) wrote:
>> > On Wed, Jan 25, 2017 at 3:30 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
>> > > As it is, there are backup solutions which *do* check the checksum when
>> > > backing up PG. This is no longer, thankfully, some hypothetical thing,
>> > > but something which really exists and will hopefully keep users from
>> > > losing data.
>> >
>> > Wouldn't that have issues with torn pages?
>>
>> No, why would it? The page has either been written out by PG to the OS,
>> in which case the backup s/w will see the new page, or it hasn't been.
>
> Uh. Writes aren't atomic on that granularity. That means you very well
> *can* see a torn page (in linux you can e.g. on 4KB os page boundaries
> of a 8KB postgres page). Just read a page while it's being written out.

Yeah. This is also why backups force full page writes on even if
they're turned off in general.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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