Mysql Alter table add column with ALGORITHM=INPLACE, LOCK=NONE
Found out the issue. Ran out of storage space on RDS while running Alter table query. 

For some reason it did not throw error disk space issue.

But when i tried without ALGORITHM=INPLACE like the query below, it throws out error of disk space issue.

Alter table dbname.tablename 
add column column1 varchar(50),
add column column2 text , 
LOCK=NONE;

I increased 100G to see if that suffices the problem, it worked.

But i still did not understand the fact why does the alter query take 60mins to add two columns when i run the Alter statement above.

While Alter statement to add single column takes 30mins to complete.

How can we improvise the execution time for Alter statement, any suggestions would help?