Hello Muthu,
For Enterprise Vault, a SP install is a full binary install, not an 'update'.
If you have to rollback, you will have to re-install 7.5 no service pack.
Proces:
Put tasks in report mode
restart them
make sure MSMQ's are all empty (might take a little)
backup SQL databases and data (indexes/stores)
make sure backup is ok
install sp6, follow guide.
check if ok. if ok, make another backup of sql. (do not overwrite 7.5 no sp databases!)
leave tasks running in report for 24 hours (if you can and not have journal archiving, and if you do, depends on how many mails go into journalmailbox for a day)
if eventlogs are ok, set tasks in normal, continue.
backup again
IF AND WHEN AN ERROR OCCURS after the upgrade, first check to see if it can be resolved (in forum/via support phone)
If not fixable. uninstall EV.
restore databases.
install EV7.5 no sp.
run configuration wizard, use existing directory.
good luck
Regards. Gertjan
