There are various challenges with RDBMS such as RDBMS are not able to handle huge data volumes of Terabytes & Peta bytes. Even if you have Redundant Array of Independent/Inexpensive Disks (RAID) & data shredding, it does not scale well for huge volume of data. You require very expensive hardware.
The major difference between the two is the way they scales.
RDBMS follow vertical scalability. It means if the data increases for storing then we have to increase the particular system configuration.(like RAM and memory space)
While Hadoop follows horizontal scalability. It means we just have to add one more node to the cluster if there is any requirement for the increase in data volumes as a result of which Hadoop becomes a fault tolerant. There is no single point of failure. Due to the presence of more machines in the cluster, you can easily recover data irrespective of the failure of one of the machines.