Performance Analysis and Optimization of the Hurricane File System ...

CHAPTER 3. HFS ARCHITECTURE 17
Indirect File Index
(per disk)
section# => disk block #
0
1
n
File Indices
(per disk)
file # => disk block #
m
0
1
n
file # => disk block #
Figure 3.2: Indices.
Block Indices
(per file)
file block # => disk block #
0
1
n
0
1
n
start
block
num
blocks
etc...
Random−Access
Extent−Based
location of all file index blocks. The indirect file index resides in a meta-data structure called the superblock,
which occupies exactly one physical disk block at a well-known, fixed location.
The layout of the meta-data on disk is shown in Figure 3.2. To obtain the block index of file #N, the
following calculations are done. Using standard C and C library terminology:
• Let A = floor(N / # of file index blocks)
• Let B = N % # of file index blocks
By looking up the A th entry in the indirect file index, the appropriate file index block is found. By
looking up the B th entry in that file index block, the block index of file #N is obtained.
Under the “block indices” column of the diagram, two types of block indices are shown for two different
types of files, random-access and extent-based. An extent-based file allocates physical disk blocks in contigu-
ous groups called extents. An extent is specified by its starting block and the number of blocks it occupies.
HFS supports other file types and these all have their own unique block index structure. Other supported file
types, not shown in the figure, include sparse-data, small-data, fixed-sized record-store, striped, replicated,
distribution, checkpoint, parity, and application-specific distribution. Details of these file types can be found
in [34].