Re: pg_migrator issues

Robert Haas wrote:
> >> 4) ?I have implemented the ability to run pg_migrator --check on a live
> >> old server. ?However, pg_migrator uses information from controldata to
> >> check things, and it also needs xid information that is only available
> >> via pg_resetxlog -n(no update) to perform the migration. ?Unfortunately,
> >> pg_resetxlog -n cannot be run on a live server, so pg_migrator runs
> >> pg_controldata for --check and pg_resetxlog -n for real upgrades. ?It
> >> would simplify pg_migrator if I would run pg_resetxlog -n on a live
> >> server, but I can understand if people don't want to do that because the
> >> xid information reported on a live server is inaccurate.
> 
> I don't really have a specific thought on this issue, except that it
> sounds like you're launching a lot of shell commands, and I wonder
> whether it would be better to try to do this through either C code or
> by exposing the appropriate stuff at the SQL level.

I considered that but realize that pg_migrator has to read
pg_controldata in both the old and new servers, meaning it would need
access to both C structures, and considering they both have the same
structure names, that would require some odd C tricks.  Add to that you
don't know which version of Postgres you are migrating from/to during
compile and the idea of using C becomes even less attractive.

Doing this in C would require pg_migrator to track all changes in the
pg_controldata structure layout, which seems excessively
complex/error-prone.  Right now I only have to track changes to the
naming of the output fields.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + If your life is a hard drive, Christ can be your backup. +