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Re: 16-bit page checksums for 9.2

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Kevin Grittner <kevin(dot)grittner(at)wicourts(dot)gov>, david <david(at)fetter(dot)org>, aidan <aidan(at)highrise(dot)ca>, stark <stark(at)mit(dot)edu>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: 16-bit page checksums for 9.2
Date: 2012-02-29 19:29:26
Message-ID: 4F4E7C96.3070707@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 29.02.2012 21:18, Alvaro Herrera wrote:
> Excerpts from Heikki Linnakangas's message of miƩ feb 29 16:09:02 -0300 2012:
>> I thought my view on how this should be done was already clear, but just
>> in case it isn't, let me restate: Enlarge the page header to make room
>> for the checksum. To handle upgrades, put code in the backend to change
>> the page format from old version to new one on-the-fly, as pages are
>> read in. Because we're making the header larger, we need to ensure that
>> there's room on every page. To do that, write a utility that you run on
>> the cluster before running pg_upgrade, which moves tuples to ensure
>> that. To ensure that the space doesn't get used again before upgrading,
>> change the old version so that it reserves those N bytes in all new
>> insertions and updates (I believe that approach has been discussed
>> before and everyone is comfortable with backpatching such a change). All
>> of this in 9.3.
>
> Note that if we want such an utility to walk and transform pages, we
> probably need a marker in the catalogs somewhere so that pg_upgrade can
> make sure that it was done in all candidate tables -- which is something
> that we should get in 9.2 so that it can be used in 9.3.

In the simplest form, the utility could just create a magic file in the 
data directory to indicate that it has run. All we need is a boolean 
flag, unless you want to be fancy and make the utility restartable. 
Implemented that way, there's no need to have anything in the catalogs.

> Such a marker would also allow us get rid of HEAP_MOVED_IN and
> HEAP_MOVED_OUT.

True.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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Subject: Re: 16-bit page checksums for 9.2
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