The data file stores the last checkpoint SCN.
Online and offline data files
A data file is either online or offline.
The data in an offline data file cannot be accessed until it is brought online.
Reasons to take a data file offline:
Oracle will automatically bring a data file offline if it cannot write to it anymore.
Alloction unit blocks
The
data file is a
physical file. When Oracle needs to allocate more space for a data file, that is, to grow the physical file, it does so by allocating that space in the
logical units of
data blocks.
The maximum number of data blocks that can be allocated for a data file is operating system dependent, typically, it is 222-1 blocks.