On Apr17, 2013, at 18:15 , Bruce Momjian <br...@momjian.us> wrote:
> On Wed, Apr 17, 2013 at 05:28:06PM +0200, Florian Pflug wrote:
>> However, you're right that time's running out. It'd be a shame though
>> if we'd lock ourselves into CRC as the only available algorithm essentially
>> forever.  Is there any way we can change the checksum algorithm in 9.4
>> *without* breaking pg_upgrade? Maybe pd_pagesize_version could be used
>> for that - we could make version 5 mean "just like version 4, but with
>> a different checksum algorithm". Since the layout wouldn't actually
>> chance, that'd be far easier to pull off than actually supporting multiple
>> page layouts. If that works, then shipping 9.3 with CRC is probably
>> the best solution. If not, we should see to it that something like Ants
>> parallel version of FNV or a smallget into 9.3 if at all possible,
>> IMHO.
> I was going to ask about the flexibility of pg_upgrade and checksums. 
> Right now you have to match the old and new cluster checksum modes, but
> it seems it would be possible to allow pg_upgrade use from checksum to
> no-checksum servers.  Does the backend look at the pg_controldata setting,
> or at the page checksum flag?  If the former, it seems pg_upgrade could
> run a a no-checksum server just fine that had checksum information on
> its pages.  This might give us more flexibility in changing the checksum
> algorithm in the future, i.e. you only lose checksum ability.

AFAIK, there's currently no per-page checksum flag. Still, being only
able to go from checksummed to not-checksummed probably is for all
practical purposes the same as not being able to pg_upgrade at all.
Otherwise, why would people have enabled checksums in the first place?

best regards,
Florian Pflug

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