title: how HDFS allocates storage on disk
body: <p>I am very curious to to know how HDFS reserves and allocates storage space on a Data node.</p>
  
  <p>Say if I have 500GB hard disk in one of the Data node, out of it 400GB is allocated for /data partition and hadoop was said to sit on it.</p>
  
  <p>Here how much space out of 400GB will be taken by hadoop daemons for the storage of the input splits(blocks).</p>
  
  <p>Does it reserve all the storage space priorly or does it allocate on demand?.</p>
  
  <p>and also wanted to know whether formatting Name node while setting up the cluster has anything to do with this. </p>
  
  <p>Many Thanks...</p>