Is it good idea to use RAID 1 mirroring to back up database content I'm working on web hosting project. The planned system structure is going to be like this:


one main hard-drive will contain the binary of mysql.
A second hard-drive will contain the datadir 
the third hard-drive should contain a backup of this mentioned datadir.


I can only think of two options here either replication using 2 instances of MySQL servers or just using RAID 1 mirroring only the datadir. 

I would go with mirroring but I'm kind of suspicious of using this method and my main frightening thought is what if I lost the main hard-drive that contain the binary of mysql does setting new version will cause my a problem .. any ideas suggestions will be great full thanks    

I should mention that using INNODB is a constraint
Relying on RAID 1 as a backup is not a great idea. If you had data corruption in the database, it would likely appear on both drives - certainly if it were caused by faulty SQL commands being executed. RAID 1 will only protect you from physical failure of one of the two drives, not corruption caused by a duff application or end-users.

In addition, backups that are stored on the same machine is also asking for trouble. You should always store backups off the source machine, and use something like mysqldump or the Percona Xtrabackup tool to perform these backups.

If you can mount an external volume onto the machine through NFS (or CIFS if Windows), then use Xtrabackup to backup the database content onto the additional volume, which is physically on a separate machine, then if you lost the machine, you could rebuild and restore from the backup.

Ensure you also backup your configuration, such as OS users, MySQL configuration files, etc. as without these, restoration may prove trickier.

Hope that all makes sense?

Dave