redologs store only the redo-information.
undo-information or the rollback information in stored in Rollback segment anyhow..

Before DBWn can write a modified buffer, all redo records associated with the changes to the buffer must be written 
to disk (the write-ahead protocol). If DBWn finds that some redo records have not been written, it signals LGWR to 
write the redo records to disk and waits for LGWR to complete writing the redo log buffer before 
it can write out the data buffers.   

When a user issues a COMMIT statement, LGWR puts a commit record in the redo log buffer and writes it to disk 
immediately, along with the transaction's redo entries. 
The corresponding changes to data blocks are deferred until it is more efficient to write them. 
This is called a fast commit mechanism. The atomic write of the redo entry containing the transaction's 
commit record is the single event that determines the transaction has committed. 
Oracle returns a success code to the committing transaction, 
even though the data buffers have not yet been written to disk.