Re: 16-bit page checksums for 9.2

On Mon, Feb 6, 2012 at 10:02 AM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> On 06.02.2012 10:05, Simon Riggs wrote:
>>
>> On Mon, Feb 6, 2012 at 7:51 AM, Heikki Linnakangas
>> <heikki(dot)linnakangas(at)enterprisedb(dot)com>  wrote:
>>>
>>> On 06.02.2012 05:48, Bruce Momjian wrote:
>>>>
>>>>
>>>> Thanks.  Clearly we don't need 16 bits to represent our page version
>>>> number because we rarely change it. The other good thing is I don't
>>>> remember anyone wanting additional per-page storage in the past few
>>>> years except for a checksum.
>>>
>>>
>>> There's this idea that I'd like to see implemented:
>>> http://archives.postgresql.org/pgsql-hackers/2010-05/msg01176.php
>>
>>
>> As you'll note, adding that field will change the page format and is
>> therefore ruled out for 9.2.
>>
>> ISTM there is a different way to handle that anyway. All we need to do
>> is to record the LSN of the last wraparound in shared memory/control
>> file. Any block with page LSN older than that has all-frozen rows.
>> That doesn't use any space nor does it require another field to be
>> set.
>
>
> Good idea. However, the followup idea to that discussion was to not only
> avoid the I/O needed to mark tuples as frozen, but to avoid xid wraparound
> altogether, by allowing clog to grow indefinitely. You do want to freeze at
> some point of course, to truncate the clog, but it would be nice to not have
> a hard limit. The way to do that is to store an xid "epoch" in the page
> header, so that Xids are effectively 64-bits wide, even though the xid
> fields on the tuple header are only 32-bits wide. That does require a new
> field in the page header.

We wouldn't need to do that would we?

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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