Question: I would like to backup a **single database** residing in a dbspace , while backup utilities in **Informix** such as `onbar` or `ontape` would provide whole system ,logs ,dbspaces backup only .Is there any way to do single database backup in **Informix** ? if there is then please tell me how to *backup* and *restore* a single database ?

Answer: 
If you need backup you could put database in its own dbspace.
Then use "warm restore" only for that dbspace.

If you want to migrate database from instance to instance
you can use dbexport/dbimport or onunload/onload but be careful
and read the manuals first. There are some drawbacks connected with both utilities.
Simply put, Informix does not have a database level backup and restore option. As you mentioned, onbar and ontape allow you to do backups and restore for the entire Informix instance. Having said that, if you are interested in having redundancy for a single database (residing in a dbspace) you can achieve it by turning on mirroring for that dbspace.