Re: Postgres failover implementation

as a continuation to my post i found out that all these supervise tools
come packaged in a package called daemontools available at
http://cr.yp.to/daemontools.html.. Unfortunatly i am unable to access
these pages but some one could try
Thanx.. Hope this is of some help
Anand


On Thu, Dec 14, 2000 at 04:51:00PM +0530, Anand Raman wrote:
>hi all,
>the discussion on failover made me look into qmail installation i did a
>few months back..
>I think qmail uses something called svscan which supervises the qmail
>process and execs it again if it fails..
>Couldnt some such thing be done with postgresql implementation which
>checks if pg_ctl is alive and automatically restart it if it dies..
>Thanks
>Anand
>
>On Wed, Dec 13, 2000 at 08:30:31AM -0800, Schmidt, Peter wrote:
>>-----Original Message-----
>>From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
>>Sent: Tuesday, December 12, 2000 10:10 AM
>>
>>>Performance across an NFS mount will doubtless suck badly.
>>
>>It's a fact of life at this point. I'm hoping performance won't suck that
>>much with 1 GB ethernet and NAS/RAID. In any case, we can't run postmaster
>>on NFS mount machine.
>>
>>> Seems like this still means a single point of failure, ie the NFS box.  So
>>what's the point?
>>
>>The idea is to have a failover for postmaster itself. I realize you stated
>>that postmaster crashes are rare, but if the primary machine goes down we
>>will want a secondary to come up with postmaster and other processes
>>running.
>>
>>> You could remove that check, perhaps, but then you'd have to remove the
>>PID file manually anytime you had a postmaster crash.
>>
>>I don't want to touch postmaster.pid code, but I am working on similar code
>>for a seperate lockfile. From what I understand, one of the only options is
>>to use fcntl to lock a file on NFS mount. If I create the file, lock it, and
>>postmaster machine dies, I'm hoping the lock will go away and the secondary
>>will be able to lock it. That way I wouldn't need to manually remove it.
>>Which brings me to another question - does postgres use file locking for
>>isolation level or other database operations? If so, am I going to run into
>>problems if the database is on NFS mount?
>>
>>Thanks again for your comments.
>>Peter Schmidt
>>
>>
>>"Peter Schmidt" <peterjs@home.com> writes:
>>> My company is looking for a way to implement failover w/Postgres.
>>> I've determined that two postmasters running on different machines
>>(FreeBSD)
>>> can share a single $PGDATA directory(NFS mount) as long as only one
>>> postmaster is running at a time.
>>
>>Performance across an NFS mount will doubtless suck badly.  That might
>>be acceptable as an emergency backup mode of operation ... but if the
>>machine with the disk is up, you might as well be running the postmaster
>>there.
>>
>>It sounds like you intend to have both the primary and secondary
>>database servers access an NFS server.  Seems like this still means a
>>single point of failure, ie the NFS box.  So what's the point?
>>
>>> Originally I thought I might be able to use
>>> postmaster.pid to lock out the second postmaster, but the pid file is
>>> overwritten by the second postmaster when it starts.
>>
>>The lockfile code assumes that if the PID in the file doesn't belong to
>>a live process *on the local machine*, then it's left over from a
>>crashed postmaster.  You could remove that check, perhaps, but then
>>you'd have to remove the PID file manually anytime you had a postmaster
>>crash.  (However, postmaster crashes are rare, so this might be OK.)
>>
>>            regards, tom lane