1. A host, comprising: an interface configured to exchange data with a storage device which stores a multi-level hash table comprising directory entries of each directory of files stored in the storage device; and a file system module configured to receive a file lookup command designating a target directory and a target filename, to calculate a hash value which reflects the target filename and a lookup level, and to search for a directory entry which comprises the target filename in a bucket corresponding to the hash value from among buckets at the lookup level which are included in a multi-level hash table of the target directory, wherein when the searching for the directory entry at the lookup level fails, the file system module increases the lookup level and performs again the calculating of the hash value based on the increased lookup value and the searching for the directory entry which comprises the target filename.