On Fri, Mar 28, 2008 at 9:22 AM, Mail Administrator mailadmin@baladia.gov.kw wrote: <snip>
now i would like to have another server like mirroring this server .. so incase there is any problem with this server the other server is always online and the problem server could be fixed without our users gettin affected
apprecite if anybody can help giving some clues or is there any software avaliable. also is the hardware in both servers have to be identical or it can be different
That is indeed, the holy grail of server administration and is not easily achieved.
For DNS, you can define a master/slave server setup and have both online at the same time. I'm not sure what MailScanner needs as far as real-time data storage. Perhaps it can just be set up in parallel and either machine could filter messages.
Mail message storage is your primary problem, as the data constantly changes.
The simplest way might be to house your mail store on a third server and mount it with NFS. Then if your primary goes down, simply boot up the secondary, mount the live data files and away you go with only a momentary service interruption. One problem with this setup is that whenever some configuration detail changes, you must change it on another machine. And you need to be concerned with the redundancy of the file server. There is also the issue of user accounts for mail. Are they local to the mail server or do they reference an external directory server?
You could rsync the mail files between servers to have a near-real-time copy, but any resulting inconsistencies could be a problem for your mail software. The only safe way to rsync a mail server is to do it while the mail services are stopped. You could stop the mail server, take a file system snapshot, then restart the mail server which would only take a few seconds. Then rsync from the snapshot to the backup and delete the snapshot when done.
More advanced options are clustering and drbd but those are toys I've never played with. However, as search terms on Google, they will get you started in the right direction. But they are probably overkill.
Judging by your machine specs, I'm guessing this is a pretty small scale operation. Your best bet might simply be to do nightly backups and have spare hardware at the ready. The most likely point of failure is the hard disk, so get another one and set up raid 1. Other than that, your hardware will probably run for years without issue. We all want 100% uptime, but you have to weigh the cost against the actual need.
-- Jeff