Re: Checksums by default?

From: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Peter Geoghegan <pg(at)heroku(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, Robert Haas <robertmhaas(at)gmail(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 00:26:53
Message-ID: CAB7nPqSRZjRRd=ht+EoYpjdbTKAFLv=L6SfP6m6Zjzc8yH1DKg@mail.gmail.com
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On Thu, Jan 26, 2017 at 9:22 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2017-01-26 09:19:28 +0900, Michael Paquier wrote:
>> On Thu, Jan 26, 2017 at 9:14 AM, 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?
>>
>> Why? What do you foresee here? I would think such backup solutions are
>> careful enough to ensure correctly the durability of pages so as they
>> are not partially written.
>
> That means you have to replay enough WAL to get into a consistent
> state...

Ah, OK I got the point. Yes that would be a problem to check this
field on raw backups except if the page size matches the kernel's one
at 4k.
--
Michael

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