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If the first extent of the attributes file (stored in the volume header) has zero allocation blocks, the attributes file does not exist. It is possible for a volume to have no attributes file. An attributes files has a variable length key and three data record types, which makes it roughly as complex as the catalog file. It is a special file, described by an HFSPlusForkData record in the volume header, with no entry in the catalog file. The attributes file is reserved for implementing named forks in the future.
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Each file created is assigned a catalog ID number. The nodes are grouped together in a linear fashion to add speed to the process. This catalog (a balanced tree) utilizes nodes to reference folders and files, and maintains the hierarchy of header, index, leaf and map nodes. The catalog file contains metadata about all the files and folders, including information on modified, access, and created times. The catalog file describes the folder and file hierarchy on a volume. This information is stored in a balanced tree format. The information recorded lists all extents used by a file and its’ allocated blocks in the proper order. This file tracks all allocation blocks that belongs to a file. The allocation file does not have to be stored contiguously within a volume.
#IOS 11 SYSTEM STORAGE FREE#
The file specifies whether an allocation block is free by storing this data in a bitmap, specifying a free allocation block with a “ clear bit” (zero). The purpose of the allocation file is to track which allocation blocks are used by the system or are free. The Volume Header stores a wide variety of data about the volume itself, for example the size of allocation blocks, a timestamp that indicates when the volume was created or the location of other volume structures such as the Catalog File or Extent Overflow File. A backup of the volume header can be found in the last 1024 bytes of the volume, primarily used for disk repair if the original header is damaged. The volume header is utilized to contain information about the structure of the HFS volume and is composed by the 1024 bytes after the reserved set of boot blocks on the partition. Sectors 0 and 1 of the volume are the boot blocks. The structures of this file system include a volume header, allocation file, extents overflow file, catalog file, attributes file and a startup file Volume Header