HFSX

HSFX is a file system developed by Apple Inc in order to replace HFS – Hierarchical File System that is the main file system used in Mac computers or other various PC. The iPod digital media player also uses this file system. HFSX file system is another version of HFS Plus. They are almost similar to HFS+ volumes, except that the HFS Wrapper, which is typical of HFS Plus volumes will never surrounds them and they support case sensitivity for the names given to all the files and folders. These volumes are easily recognized by a value of HX in the signature field with two entries in the Volume Header.

Architecture of HSFX

In HSFX file system, all the volumes are divided into sectors i.e. logical blocks, size of each block is 512 bytes. These sectors again grouped into allocation blocks that comprise one or more sectors. In HFSX, the number of allocation blocks is depended upon total size of the partition. In HSFX, address allocation blocks uses a larger value compared to HFS i.e. 32 bits instead of 16 bits (it can access 232 allocation blocks rather than 216 allocation blocks available to HFS.

Following information which describes the structure of typical HFSX volume

The sectors 0 and 1 of the volume are similar to the boot blocks as in an HFS volume and these are part of HFS wrapper.

Volume Header is contained in sector 2 that is well equivalent with the Master Directory Blocks in an HFS volume. It stores wide range of files regarding volume itself i.e. the size of allocation blocks, locality of other volume such as the Extent Overflow File or Catalog File.

Lists of allocation blocks, which are in use, and the list of blocks, which are free, are keeping tracked by the Allocation File. In HFSX, each allocation blocks are symbolized by one bit i.e. a zero bit means the block is in free and a one means the block is in use. In addition, the Allocation File can also vary size and there is no restriction to store contiguously within a volume.

All records of files are stored in the volume are contained in the Catalog file. A record in the HFS catalog file is 512 bytes in size but in HFSX catalog files are 4 KB in Mac OS and 8 KB in OS X. Fields in HFSX the size can differ according to the size of data they store.

The Catalog File is just like a B-tree that keeps record of all allocation blocks that allocated to each file as extents. A file recorded in Catalog File is competent to recording about 8 extents for each fork of a file. Files that are used other extents are recorded in the Extent Overflow File.

The Attribute File can store three types of 4 KB records, which are not similar such as

  • Inline Data Attribute records- stores small features which are easily get fitted in the record list itself
  • Fork Data Attribute records- contains reference to a maximum of eight extents that can store larger attributes
  • Extension Attributes – used to expand a Fork Data Attribute record when its eight extent records are used previously

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