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,

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.

  Bruce Momjian  <br...@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +

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