Difference between pages "HFS+" and "HFS"

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HFS+, or Hierarchical File System Plus, is the file system designed by Apple Computer[http://www.apple.com] to supersede HFS. First introduced with Mac OS 8.1, one of the biggest differences was the lower allocation block size of 4kb, which increased performance and lowered fragmentation [http://developer.apple.com/technotes/tn/tn1121.html#HFSPlus]. It also implemented Unicode (rather than Mac proprietary formats) for naming files.
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#REDIRECT [[HFS+]]
 
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There are structurally many differences between HFS and HFS+, which are listed below[http://developer.apple.com/technotes/tn/tn1150.html#HFSPlusBasics]:
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<br><br>
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<CENTER><TABLE Border=1 cellpadding=2 cellspacing=0 width=75%>
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            <TR>
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              <TD>
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                  <P><B>Feature</B></p>
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              </TD><TD>
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                  <P><B>HFS</B></p>
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              </TD><TD>
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                  <P><B>HFS Plus</B></p>
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              </TD><TD>
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                  <P><B>Benefit/Comment</B></p>
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              </TD></TR>
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            <TR>
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              <TD>
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                  <P>User visible name</p>
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              </TD><TD>
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                  <P>Mac OS Standard</p>
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              </TD><TD>
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                  <P>Mac OS Extended</p>
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              </TD><TD>
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                  <P></p>
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              </TD></TR>
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            <TR>
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              <TD>
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                  <P>Number of allocation blocks</p>
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              </TD><TD>
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                  <P>16 bits worth</p>
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              </TD><TD>
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                  <P>32 bits worth</p>
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              </TD><TD>
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                  <P>Radical decrease in disk space used on large
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                  volumes, and a larger number of files per volume.</p>
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              </TD></TR>
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            <TR>
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              <TD>
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                  <P>Long file names</p>
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              </TD><TD>
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                  <P>31 characters</p>
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              </TD><TD>
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                  <P>255 characters</p>
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              </TD><TD>
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                  <P>Obvious user benefit; also improves
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                  cross-platform compatibility</p>
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              </TD></TR>
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            <TR>
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              <TD>
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                  <P>File name encoding</p>
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              </TD><TD>
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                  <P>MacRoman</p>
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              </TD><TD>
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                  <P>Unicode</p>
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              </TD><TD>
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                  <P>Allows for international-friendly file names,
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                  including mixed script names</p>
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              </TD></TR>
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            <TR>
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              <TD>
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                  <P>File/folder attributes</p>
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              </TD><TD>
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                  <P>Support for fixed size attributes (FileInfo and
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                  ExtendedFileInfo)</p>
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              </TD><TD>
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                  <P>Allows for future meta-data extensions</p>
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              </TD><TD>
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                  <P>Future systems may use metadata for a richer
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                  Finder experience</p>
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              </TD></TR>
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            <TR>
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              <TD>
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                  <P>OS startup support</p>
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              </TD><TD>
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                  <P>System Folder ID</p>
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              </TD><TD>
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                  <P>Also supports a dedicated startup file</p>
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              </TD><TD>
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                  <P>May help non-Mac OS systems to boot from HFS
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                  Plus volumes</p>
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              </TD></TR>
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            <TR>
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              <TD>
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                  <P>catalog node size</p>
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              </TD><TD>
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                  <P>512 bytes</p>
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              </TD><TD>
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                  <P>4 KB</p>
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              </TD><TD>
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                  <P>Maintains efficiency in the face of the other
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                  changes. (This larger catalog node size is due to
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                  the much longer file names [512 bytes as opposed to
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                  32 bytes], and larger catalog records (because of
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                  more/larger fields)).</p>
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              </TD></TR>
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            <TR>
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              <TD>
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                  <P>Maximum file size</p>
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              </TD><TD>
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                  <P>2<SUP>31</SUP> bytes</p>
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              </TD><TD>
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                  <P>2<SUP>63</SUP> bytes</p>
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              </TD><TD>
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                  <P>Obvious user benefit, especially for multimedia
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                  content creators.</p></td>
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                  </tr>
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</table></CENTER>
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<br>
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An HFS+ volume contains five special files:
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<ol>
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<li>
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Catalog file - Describes the folder and file hierarchy of the volume. It is organized as a "balanced tree" for fast and efficient searches
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</li>
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<li>Extents overflow file - Additional extents (contiguous allocation blocks allocated to forks) are stored in a b-tree in this file
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</li>
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<li>
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Allocation file - Specifies whether an allocation block is free (similar to $Bitmap in NTFS). This is stored in a bitmap, specifying a free allocation block with a "clear bit"
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</li>
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<li>Attributes file - Contains attribute information regarding files or folders
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</li>
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<li>
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Startup file - Allows computers to boot that do have built in support for HFS+ file systems
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</li>
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</ol>
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<br>
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HFS+ also implements journaling, which allows fast recovery in the case of a crash or power outage. According to Apple, "The purpose of the journal is to ensure that when a group of related changes are being made, that either all of those changes are actually made, or none of them are made."[http://developer.apple.com/technotes/tn/tn1150.html#Journal]
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Apple technical notes are available for the HFS+ file system from their [http://developer.apple.com/cgi-bin/search.pl?q=HFS+&num=10&site=default_collection website].
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== External Links ==
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* [http://web.archive.org/web/20090530120010/http://developer.apple.com/technotes/tn/tn1121.html Internet Archive Copy of: Technical Note TN1121]]
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[[Category:Disk file systems]]
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Latest revision as of 19:25, 28 November 2006