The Macintosh Hierarchical File System (HFS)

Article translations Article translations
Article ID: 130437 - View products that this article applies to.
This article was previously published under Q130437
Expand all | Collapse all

On This Page


This article covers:
  • Hierarchical File System Specifications
  • Determining Allocation Block Size
  • Allocation Block Size Table
  • HFS Volume Organization
  • Decreasing The File Size On Large Hard Drives
This article also explains why different HFS structures on different hard drives can cause small files to take up more space on a larger drive than they do on a smaller drive.

Hierarchical File System Specifications

The following sections cover specifications and associated terminology used to describe the Macintosh HFS structure. The following numbers apply to both System 6 and System 7.


A volume is either an entire disk, or a section of a disk, partitioned into separate parts. If you partition a single drive then each partition is considered a volume.
  • The maximum volume size is 2 gigabytes. Under System 7.5, the maximum is 4 gigabytes.
  • The maximum number of volumes is limited only by the amount of available system memory.
  • The maximum file size is 2 gigabytes.
  • The maximum number of files on a volume is 65,536.
  • The maximum number of files in a folder is 32,767.
  • The maximum size of the data fork in a file is 2 gigabytes.
  • The maximum size of the resource fork in a file is 16 megabytes.

Logical block

A logical block is a unit of drive space composed of up to 512 bytes. A logical block is numbered from 0 to n, n being the last block on the volume (not necessarily the last block on the hard disk). To figure out the number of logical blocks, divide the volume size by 512 bytes.

Allocation block

An allocation block is a unit of storage on a volume, composed of one or more logical blocks. The larger the volume, the more logical blocks comprise one allocation block. The maximum number of allocation blocks per volume is 65,536; most volumes have slightly less.

The size of your hard drive determines the minimum size of each file. In both the Macintosh and MS-DOS environment, you can have up to 65,536 blocks on a drive (because both operating systems address the allocation blocks with 16-bit addressing). Drives larger than 512 megabytes cannot use 8K blocks because there are not enough addresses. Thus, if a 2 gigabyte drive is one Macintosh partition, the smallest file size allowed is 32K. If you save a TeachText file with one character in it, it would take up 32K of disk space.

A non-empty file fork always occupies at least one allocation block, no matter how many bytes of data the file fork contains. If the data fork and resource fork of a file contain data, the file uses two allocation blocks. If all the files are only one allocation block long, then there can be as many files on the volume as there are allocation blocks for the volume. However, when a file is longer than one allocation block, the total number of possible files decreases. For example, on a volume with 65,535 allocations block, you can have:
   65,536 files, each one allocation block long.
   32,768 files, each two allocation blocks long.

    2,048 files, each 32 allocation blocks long.
    1,024 files, each 64 allocation blocks long.

All of the numbers above refer to the system's record structures. Other elements place tighter restrictions on the actual number of usable files in the root directory. The Standard File dialog uses the List Manager. The List Manager has a limit of 32K of data, which is somewhere between 800 and 900 files. Though more files can be handled in a directory, the List Manager will not display the first 32K.

A second element concerns the performance of the Finder when you have approximately 800 to 900 files per directory. Even simple operations, such as moving the icon of a file, tend to slow down considerably when this many files exist in a directory. This limitation changes depending on the speed of the individual Macintosh model.

Determining Allocation Block Size

To calculate the allocation block size:
  1. Take the size of the drive in megabytes and multiply it by 2000. (There are 2000 disk blocks in 1 megabyte of disk space).
  2. Divide that number by 65,536. (Because the Macintosh cannot have more than 65,536 allocation blocks).
  3. Round this number up and multiply it by 512.
The result is the number of bytes in the allocation block. For example, if you do the math with a 230 MB drive:
   230 x 2000 / 65536 = 7.019
   7.019 rounded up = 8
   8 x 512 = 4096 bytes

So larger hard drives mean larger allocation blocks, and consequently, more wasted space for small files. If you have a large drive with a lot of small files, the hard drive space is being used less efficiently than if most of your files average 32K in size.

NOTE: Since drives come from different manufacturers, it is possible to get different values. For example, 500 megabyte drive might be slightly larger than 500 megabytes. This would result in different logical block sizes than those listed in the Allocation Block Size Table below. The difference is typically not more than 512 bytes. The difference is most noticeable on the larger drives. For example, a 500 megabyte drive has a logical block size of 8192 bytes. But the 500 MB drive in some Macintosh computers is actually 540 MB, which has a logical block size of 8704.

Allocation Block Size Table

Here is a complete list of allocation block sizes for the various volume sizes, based on the formula described above:
   Volume Size    Allocation Block Size       Minimum file size
   ------------   ------------------------    ------------------
        0-32MB       1 logical block           0.5K
       33-64MB       2 logical blocks          1.0K
       65-96MB       3 logical blocks          1.5K
      97-128MB       4 logical blocks          2.0K
     129-160MB       5 logical blocks          2.5K
     161-192MB       6 logical blocks          3.0K
     193-224MB       7 logical blocks          3.5K
     225-256MB       8 logical blocks          4.0K
     257-288MB       9 logical blocks          4.5K
     289-320MB      10 logical blocks          5.0K
     321-352MB      11 logical blocks          5.5K
     353-384MB      12 logical blocks          6.0K
     385-416MB      13 logical blocks          6.5K
     417-448MB      14 logical blocks          7.0K
     449-480MB      15 logical blocks          7.5K
     481-512MB      16 logical blocks          8.0K
     513-544MB      17 logical blocks          8.5K
     545-576MB      18 logical blocks          9.0K
     577-608MB      19 logical blocks          9.5K
     609-640MB      20 logical blocks         10.0K
     641-672MB      21 logical blocks         10.5K
     673-704MB      22 logical blocks         11.0K
     705-736MB      23 logical blocks         11.5K
     737-768MB      24 logical blocks         12.0K
     769-800MB      25 logical blocks         12.5K
     801-832MB      26 logical blocks         13.0K
     833-864MB      27 logical blocks         13.5K
     865-896MB      28 logical blocks         14.0K
     897-928MB      29 logical blocks         14.5K
     929-960MB      30 logical blocks         15.0K
     961-992MB      31 logical blocks         15.5K
    993-1024MB      32 logical blocks         16.0K
   1025-1056MB      33 logical blocks         16.5K
   1057-1088MB      34 logical blocks         17.0K
   1089-1120MB      35 logical blocks         17.5K
   1121-1152MB      36 logical blocks         18.0K
   1153-1184MB      37 logical blocks         18.5K
   1185-1216MB      38 logical blocks         19.0K
   1217-1248MB      39 logical blocks         19.5K
   1249-1280MB      40 logical blocks         20.0K
   1281-1312MB      41 logical blocks         20.5K
   1313-1344MB      42 logical blocks         21.0K
   1345-1376MB      43 logical blocks         21.5K
   1377-1408MB      44 logical blocks         22.0K
   1409-1440MB      45 logical blocks         22.5K
   1441-1472MB      46 logical blocks         23.0K
   1473-1504MB      47 logical blocks         23.5K
   1505-1536MB      48 logical blocks         24.0K
   1537-1568MB      49 logical blocks         24.5K
   1569-1600MB      50 logical blocks         25.0K
   1601-1632MB      51 logical blocks         25.5K
   1633-1664MB      52 logical blocks         26.0K
   1665-1696MB      53 logical blocks         26.5K
   1697-1728MB      54 logical blocks         27.0K
   1729-1760MB      55 logical blocks         27.5K
   1761-1792MB      56 logical blocks         28.0K
   1793-1824MB      57 logical blocks         28.5K
   1825-1856MB      58 logical blocks         29.0K
   1857-1888MB      59 logical blocks         29.5K
   1889-1920MB      60 logical blocks         30.0K
   1921-1952MB      61 logical blocks         30.5K
   1953-1984MB      62 logical blocks         31.0K
   1985-2016MB      63 logical blocks         31.5K
   2017-2048MB      64 logical blocks         32.0K
   2049-2080MB      65 logical blocks         32.5K
   2081-2112MB      66 logical blocks         33.0K
   2113-2144MB      67 logical blocks         33.5K
   2145-2176MB      68 logical blocks         34.0K
   2177-2208MB      69 logical blocks         34.5K
   2209-2240MB      70 logical blocks         35.0K
   2241-2272MB      71 logical blocks         35.5K
   2273-2304MB      72 logical blocks         36.0K
   2305-2336MB      73 logical blocks         36.5K
   2337-2368MB      74 logical blocks         37.0K
   2369-2400MB      75 logical blocks         37.5K
   2401-2432MB      76 logical blocks         38.0K
   2433-2464MB      77 logical blocks         38.5K
   2465-2496MB      78 logical blocks         39.0K
   2497-2528MB      79 logical blocks         39.5K
   2529-2560MB      80 logical blocks         40.0K
   2561-2592MB      81 logical blocks         40.5K
   2593-2624MB      82 logical blocks         41.0K
   2625-2656MB      83 logical blocks         41.5K
   2657-2688MB      84 logical blocks         42.0K
   2689-2720MB      85 logical blocks         42.5K
   2721-2752MB      86 logical blocks         43.0K
   2753-2784MB      87 logical blocks         43.5K
   2785-2816MB      88 logical blocks         44.0K
   2817-2848MB      89 logical blocks         44.5K
   2849-2880MB      90 logical blocks         45.0K
   2881-2912MB      91 logical blocks         45.5K
   2913-2944MB      92 logical blocks         46.0K
   2945-2976MB      93 logical blocks         46.5K
   2977-3008MB      94 logical blocks         47.0K
   3009-3040MB      95 logical blocks         47.5K
   3041-3072MB      96 logical blocks         48.0K
   3073-3104MB      97 logical blocks         48.5K
   3105-3136MB      98 logical blocks         49.0K
   3137-3168MB      99 logical blocks         49.5K
   3169-3200MB     100 logical blocks         50.0K
   3201-3232MB     101 logical blocks         50.5K
   3233-3264MB     102 logical blocks         51.0K
   3265-3296MB     103 logical blocks         51.5K
   3297-3328MB     104 logical blocks         52.0K
   3329-3360MB     105 logical blocks         52.5K
   3361-3392MB     106 logical blocks         53.0K
   3393-3424MB     107 logical blocks         53.5K
   3425-3456MB     108 logical blocks         54.0K
   3457-3488MB     109 logical blocks         54.5K
   3489-3520MB     110 logical blocks         55.0K
   3521-3552MB     111 logical blocks         55.5K
   3553-3584MB     112 logical blocks         56.0K
   3585-3616MB     113 logical blocks         56.5K
   3617-3648MB     114 logical blocks         57.0K
   3649-3680MB     115 logical blocks         57.5K
   3681-3712MB     116 logical blocks         58.0K
   3713-3744MB     117 logical blocks         58.5K
   3745-3776MB     118 logical blocks         59.0K
   3777-3808MB     119 logical blocks         59.5K
   3809-3840MB     120 logical blocks         60.0K
   3841-3872MB     121 logical blocks         60.5K
   3873-3904MB     122 logical blocks         61.0K
   3905-3936MB     123 logical blocks         61.5K
   3937-3968MB     124 logical blocks         62.0K
   3969-4000MB     125 logical blocks         62.5K
   4001-4032MB     126 logical blocks         63.0K
   4033-4064MB     127 logical blocks         63.5K
   4065-4096MB     128 logical blocks         64.0K

HFS Volume Organization

The first two logical blocks (labeled 0 and 1) of a volume are the boot blocks. This is where the information for mounting the volume is stored.

The third logical block (labeled 2) is the Master Directory Block, or MDB for short. This block contains part of the data structure of a flat directory volume. It contains the volume information and the volume allocation block map. This block is where the information for the hard disk, such as number of files in the directory and the last time the drive was initialized, is stored.

Logical blocks 3 to x (see the following table) contain the volume bitmap. This block is a data structure containing a sequence of bits, one bit for each allocation block. The volume bitmap stores a reference to every piece of data that is in the allocation blocks and indicates whether the block is allocated or free for use. Volume bitmaps exist both on hierarchical directory volumes and in memory.

Volumes may have as few as 32,768 allocation blocks and as many as 65,536 allocation blocks. This table defines what logical block the volume bitmap ends on based on the number of allocation blocks:
   Allocation Blocks            Last Volume Bitmap Logical Block
   > 61,440                     18
   > 57,344                     17
   > 53,248                     16
   > 49,152                     15
   > 45,056                     14
   > 40,960                     13
   > 36,864                     12
   > 32,768                     11

Allocation blocks begin after the volume bitmap. The catalog, extents and data files are contained in the allocation blocks.

The catalog file is a list of all files and folders stored in a volume. The catalog file maintains the relationships between the files and directories on a hierarchical directory volume. It corresponds to the file directory on a flat directory volume. The catalog file is organized and accessed using a B-tree structure. The files are arranged in alphabetical order evenly balanced on the tree so that finding a Z does not take any longer than finding an A. This structure is the glue that keeps the catalog file together. The extents tree file contains the locations of all the files on a volume. (An extent is a series of contiguous allocation blocks). The extents tree file is where the information about the data files you have created is stored (such as where to find file, and how many extents a file is divided into). Any file you create may be broken up into multiple extents. The extents are linked together, behind the scenes, with information from the extents tree file to give the appearance of one data file.

The next, and largest, section of the volume contains all the actual data files and applications which are referenced using the above files.

The next to last block on the hard disk contains the alternate master directory. This alternate is a backup to the MDB kept on logical block 2. It is used when the file manager determines that the MDB is corrupt and needs to be rewritten with the correct information.

The very last block is empty. It is used to check for bad sections of the hard disk.

Decreasing The File Size On Large Hard Drives

To decrease the file size on a large hard drive, partition larger drives into smaller partitions or logical drives, each with a maximum of 65,536 blocks. However, Apple does not offer a tool for creating multiple Macintosh partitions for System 7.1. Future versions of the operating system and HD SC Setup will support smaller file sizes on larger hard drives.

The following third-party utilities offer a solution for creating multiple Macintosh partitions:
  • Hard Disk Toolkit (HDT) by FWB Software, Inc.
  • SilverLining by LaCie Ltd.
  • Micronet Utility by MicroNet Technology
  • MicroTech Utility by MicroTech International
  • Drive 7 by Casa Blanca Works, Inc.


For additional information, please click the article number below to view the article in the Microsoft Knowledge Base:
162468 Volume Creation Date Is Not Reported Correctly from WinNT Server


Article ID: 130437 - Last Review: November 1, 2006 - Revision: 2.1
  • Microsoft Windows NT Advanced Server 3.1
  • Microsoft Windows NT Server 3.5
  • Microsoft Windows NT Server 3.51
  • Microsoft Windows NT Server 4.0 Standard Edition
kbnetwork KB130437

Give Feedback


Contact us for more help

Contact us for more help
Connect with Answer Desk for expert help.
Get more support from