oracle redo log files do not dynamically grow when more
space is needed for redo entries; they have a fixed size (on
SAP systems,typically 50MB). When the current online redo
log file becomes full, the log writer process closes this
file and starts writing into the next one. This is called a
log switch.

oraarch
Offline redo log files are written to the oraarch directory;
their names are specified with help of Oracle instance
configuration parameters, so the name <DBSID>arch1_<LSN>.dbf
is just an example. Moreover, offline redo log files should
be stored on a mirrored disk to prevent loss of redo
information. A RAID system can be used for this purpose. 
If u loose a disk containing offline redo logs and data files,
complete recovery is no longer possible. Therefore, offline redo
logs and data files should be on different disks!

oraarch
Offline redo log files are written to the oraarch directory;
their names are specified with help of Oracle instance
configuration parameters, so the name <DBSID>arch1_<LSN>.dbf
is just an example. Moreover, offline redo log files should
be stored on a mirrored disk to prevent loss of redo
information. A RAID system can be used for this purpose. 
If u loose a disk containing offline redo logs and data 
files,
complete recovery is no longer possible. Therefore, offline 
redo
logs and data files should be on different disks!