Hbase & BigTable & RDBMS

[chico CHEN <ch...@gmail.com>]

Hi guys,
   I am a newer here. Recently, I am thinking about the data model of
Bigtable-like and that of  RDBMS. I feel confused about the difference of
the two. Take the blog-comment relation as the example. If we use RDBMS,
there will be three tables: one is blogs; one is comments; and the last one
is blog-comments relational table. If we use HBase or Bigtable, there will
be one table like:
blogID   timer   blog:content     blog:comments. If our tables in RDBMS is
denormal, we can put comments into blogs and combine them as one table.
  Is there something wrong in my example or my thoughts? Or else, where are
the differences in the two data models? And why did somebody say "RDBMS is
not suitable for cloud computing"?
  Thank you for taking some minutes to answer my questions.
-chico CHEN