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    HFS+文件系统概述

    编辑:ontrack     时间:2014-5-4    14:30:24  来自:道至道成都数据恢复

    The Hierarchical File System Plus (HFS+) was introduced in January 19, 1998 by Apple Computers as an enhancement of their HFS file system. 

    The maximum volume size supported by HFS+ is 8 Exabytes with a maximum file size of 8 Exabytes. The maximum file name length is 255 characters using UTF-16 characters.

    The directories and their contents are managed by a B-Tree file. The file allocation is stored in a bitmap file, while bad blocks are tracked by a B-Tree file.

    Stored dates can be in the range of January 1, 1904 to February 6, 2040 just like with HFS.

    HFS+ was introduced to replace the standard Hierarchical File System (HFS). HFS uses 16-bit integers for addressing limiting the maximum number of files. HFS+ uses a 32-bit integer for addressing, making the maximum number of files 4,294,967,295 on a volume.

    With Mac OS X 10.2.2, Apple updated HFS+ to include Journaling. Journaling was optional to use. With OS X 10.3, Journaling was enabled by default on HFS+ volumes. HFS+ with journaling is sometimes referred to as HFSJ. Journaling is a method to allow data/metadata to be recovered in case of an improper shutdown.

    Another option which came with OS X 10.3 was HFSX. This system allowed for optional case sensitivity with directory and file names. Also, the HFSX is identical to HFS+, but it does not need the HFS Wrapper. The wrapper is when the ROM contains support to allow an HFS+ volume to boot. HFSX did not need the ROM support to boot.

    Mac OS X 10.4 included HFS+ support to include Access Control List (ACL) permissions. ACLs allow permissions of not only who can access a file, but what they can do to the files such as read, write, modify, delete, etc.

    In Mac OS X 10.5, hard linking was added. Hard linking is the ability of a directory and file structure to use pointers. The pointers are associated with a file name for example, and points to the address of the physical file. When a copy is made of the file and the file is renamed, then two files point to the same address. For example, we create File1 in DirectoryA and then copy it to DirectoryB. File1 in DirectoryB is renamed to File2. In the B-Tree containing the file allocations, the physical address for File1 and File2 are the same. Two files on the HFS+ volume really only occupy the space of one file. Until one file is modified, this remains true. When one file is modified, it is written to another spot on the disk and the pointer is changed for the modified file.

    With the release of Mac OS X 10.6, HFS+ was updated by adding support forcompression. The addition of compression added the ability for more drive space by compressing files.

    For security, Mac OS X 10.7 added the ability of encryption. Files can now be secured from being accessed even when a hard disk is removed and attempted to be accessed in another system.

    编辑: ontrack
    标签:HFS  HFS PLUG;HFS;HFSX  
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