Re: Redundant databases/real-time backup

On Thu, Nov 16, 2000 at 11:14:15AM -0500, root wrote:
> It is necessary to create/alter the postgresql startup script. I have included
> a copy of mine. The database to be mirrored must start up with logging
> enabled:
>
> su -l postgres -c '/usr/bin/postmaster -i -D/home/postgres/data >/home/postgres/data/query_log 2>&1 &'

But why do you do 2>&1 in your startup-script? This will redirect data
on stderr to stdout - I don't think that is a good idea.

> Other points:
> The query_log can get large rather quickly. You cannot simply issue a rm -rf
> query_log, touch query_log and chmod. Even with the appropriate permissions
> the daemon will not write to a new file, for some reason you must restart
> postgres using the startup script. Perhaps one of the developers has an answer
> to this problem.....

This is becuase the postgres deamon keeps the file open, and will keep
on writing to the old file even after you delete it. Most deamons will
reopen their log-files if you send them a HUP signal, so a possible
solution is:

mv logfile logfile.1
kill -HUP <pid>
process logfile.1
rm logfile.1

The order is important to make sure you don't have a race-condition.

A different alternative is to use a FIFO instead of a normal file, and
process it continually istead of in batch-jobs.

A totally different approach would be to have a "sql-proxy" relay all
requests to two (or more) different servers to always keep them in sync.

--
Ragnar Kjørstad