Devices - introduction

To separate database objects from physical server components all databases are build on logical devices. Logical devices seat between a database and physical disk structures. Having database build on devices it is quite easy to migrate database to another server. All you need are just devices with the same names on the another system!

A device can be a file (common solution on Windows implementation) or a raw device (usually used in Unix implementation).


One database can be splitted among many devices and one device can be used by many databases. Looking into internals of a relation between a database and device, there is additional level between them. It is called "fragment". Fragment is a contiguous allocation on a disk. Reasuming relation between device and database may look like:

disk/volume/partition <--> device (file or raw device) <--> fragments <--> database <--> database objects (tables, indexes...)