Planned and Unplanned Downtime 
You need to distinguish between planned and unplanned downtime.
Planned Downtime
Planned downtime is the time for scheduled maintenance and upgrade during which a system cannot be used for normal productive operations. This time is used for a variety of purposes so that a system can function optimally and reliably.
Some of the possible causes of planned downtime are as follows:
Hardware maintenance:
Upgrades to new releases of SAP application components, database, or operating system (databases are listed in alphabetical order):
Database reorganization (listed in alphabetical order):
Database backup (listed in alphabetical order):
Note
Archiving of SAP business objects (that is, logical backup) does not itself normally cause downtime. Indeed, it tends to minimize planned downtime in other areas by minimizing data storage in the SAP system.
You can perform Data Archiving in online mode.
Unplanned Downtime
Unplanned downtime is the time during which a system cannot be used for normal productive operations due to unforeseen failure in hardware or software components, or operator mistakes.
Unplanned downtime can be costly to an organization. The source of unplanned downtime can be in any of the layers that make up the complete software and hardware environment:
Front-end and middleware services for connection to the web, including the Internet Transaction Server (ITS), the SAP NetWeaver Application Server and the SAP Enterprise Portal.
SAP system services of the individual application components and the SAP kernel.
Underlying hardware and software services, such as the database services, network and operating system services, and various hardware services, including servers, disks, memory, and uninterruptible power supply (UPS).