Macintosh: File System Specifications and Terms
Document 30344: "Mac OS: About Mac OS Extended Format"
Document 24601: "Mac OS Extended Format: Volume and File Limits"
Mac OS Standard Format Specifications
Before discussing hard drive block allocation, and the intricacies of determining allocation block sizes and such, let's first review the Macintosh Standard format, sometimes called HFS.
Below are some specifications and associated terminology used to describe the Macintosh Standard format structure.
A volume is either a full 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 under System 6 and System 7 is 2 GB. System 7.5 increased that limit to 4 GB and System 7.5.2 (and later) increases that limit to 2 TB on some computers, including:
|any computer that came with System 7.5.2 or newer.|
|any Macintosh computer with PCI slots.|
- The maximum number of volumes is theoretically limited to 173 due to the maximum number of open files in the Mac OS. Since there are other files that are opened when the computer boots up, the actual maximum will vary depending upon how many other files are open by the operating system.
- The maximum file size is 2 GB.
- 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 GB.
- The maximum size of the resource fork in a file is 16 megabytes.
Note that AppleShare, including Personal File Sharing, has its own volume size limitations. For more information, please see technical document 15460: "AppleShare File Server: Chart Of All Limitations"
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 hard disk. Take the volume size, divide it by 512 bytes, and you have the number of logical blocks.
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.
In both the Macintosh and DOS environments, the maximum number of blocks on a driver is 65,536 because both operating systems address the allocation blocks with a 16-bit address. Drives larger than 512 MB cannot use a block size of 8K or less because there just aren't enough addresses. Thus, if a 2 GB drive is one Macintosh partition, the smallest file size allowed is 32K. (If you save a SimpleText file with one character in it, it would take up 32K of disk space.) This means that the size of your hard drive determines the minimum size of each file.
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 1 allocation block long.
- 32,768 files, each 2 allocation blocks long.
- 2048 files, each 32 allocation blocks long.
- 1024 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 only display the first 32K.
A second element concerns the performance of the Finder when approaching 800 to 900 files per directory. Even simple operations, such as moving the icon of a file, tend to slow down considerably when this number of files exists in a directory. This limitation changes depending on the speed of the individual Macintosh model.
Determining Allocation Block Size
Follow these steps 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 MB of disk space).
2. Divide that number by 65,536, since 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. Here is an example using a 230 MB drive:
- 230 x 2000 / 65536 = 7.019
- 7.019 rounded up = 8
- 8 x 512 = 4096 bytes
Note: Since drives come from different manufacturers, it is possible to get different values. This is because a 500 MB drive is not exactly 500 MB but is usually somewhat larger. This could result in different logical block sizes than those listed in the chart (see Allocation Block Size Table below), but the difference should not be more than +/- 512 bytes. This difference is most noticeable on the larger drives. For example, the 500 MB drive actually calculates to 8192 bytes but the 500 MB drive in some Macintosh computers is actually 540 MB, which has a logical block size of 8704, as listed in the chart.
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
0-32 MB 1 logical block 0.5K
33-64 MB 2 logical blocks 1.0K
65-96 MB 3 logical blocks 1.5K
97-128 MB 4 logical blocks 2.0K
129-160 MB 5 logical blocks 2.5K
161-192 MB 6 logical blocks 3.0K
193-224 MB 7 logical blocks 3.5K
225-256 MB 8 logical blocks 4.0K
257-288 MB 9 logical blocks 4.5K
289-320 MB 10 logical blocks 5.0K
321-352 MB 11 logical blocks 5.5K
353-384 MB 12 logical blocks 6.0K
385-416 MB 13 logical blocks 6.5K
417-448 MB 14 logical blocks 7.0K
449-480 MB 15 logical blocks 7.5K
481-512 MB 16 logical blocks 8.0K
513-544 MB 17 logical blocks 8.5K
545-576 MB 18 logical blocks 9.0K
577-608 MB 19 logical blocks 9.5K
609-640 MB 20 logical blocks 10.0K
641-672 MB 21 logical blocks 10.5K
673-704 MB 22 logical blocks 11.0K
705-736 MB 23 logical blocks 11.5K
737-768 MB 24 logical blocks 12.0K
769-800 MB 25 logical blocks 12.5K
801-832 MB 26 logical blocks 13.0K
833-864 MB 27 logical blocks 13.5K
865-896 MB 28 logical blocks 14.0K
897-928 MB 29 logical blocks 14.5K
929-960 MB 30 logical blocks 15.0K
961-992 MB 31 logical blocks 15.5K
993-1024 MB 32 logical blocks 16.0K
1025-1056 MB 33 logical blocks 16.5K
1057-1088 MB 34 logical blocks 17.0K
1089-1120 MB 35 logical blocks 17.5K
1121-1152 MB 36 logical blocks 18.0K
1153-1184 MB 37 logical blocks 18.5K
1185-1216 MB 38 logical blocks 19.0K
1217-1248 MB 39 logical blocks 19.5K
1249-1280 MB 40 logical blocks 20.0K
1281-1312 MB 41 logical blocks 20.5K
1313-1344 MB 42 logical blocks 21.0K
1345-1376 MB 43 logical blocks 21.5K
1377-1408 MB 44 logical blocks 22.0K
1409-1440 MB 45 logical blocks 22.5K
1441-1472 MB 46 logical blocks 23.0K
1473-1504 MB 47 logical blocks 23.5K
1505-1536 MB 48 logical blocks 24.0K
1537-1568 MB 49 logical blocks 24.5K
1569-1600 MB 50 logical blocks 25.0K
1601-1632 MB 51 logical blocks 25.5K
1633-1664 MB 52 logical blocks 26.0K
1665-1696 MB 53 logical blocks 26.5K
1697-1728 MB 54 logical blocks 27.0K
1729-1760 MB 55 logical blocks 27.5K
1761-1792 MB 56 logical blocks 28.0K
1793-1824 MB 57 logical blocks 28.5K
1825-1856 MB 58 logical blocks 29.0K
1857-1888 MB 59 logical blocks 29.5K
1889-1920 MB 60 logical blocks 30.0K
1921-1952 MB 61 logical blocks 30.5K
1953-1984 MB 62 logical blocks 31.0K
1985-2016 MB 63 logical blocks 31.5K
2017-2048 MB 64 logical blocks 32.0K
2049-2080 MB 65 logical blocks 32.5K
2081-2112 MB 66 logical blocks 33.0K
2113-2144 MB 67 logical blocks 33.5K
2145-2176 MB 68 logical blocks 34.0K
2177-2208 MB 69 logical blocks 34.5K
2209-2240 MB 70 logical blocks 35.0K
2241-2272 MB 71 logical blocks 35.5K
2273-2304 MB 72 logical blocks 36.0K
2305-2336 MB 73 logical blocks 36.5K
2337-2368 MB 74 logical blocks 37.0K
2369-2400 MB 75 logical blocks 37.5K
2401-2432 MB 76 logical blocks 38.0K
2433-2464 MB 77 logical blocks 38.5K
2465-2496 MB 78 logical blocks 39.0K
2497-2528 MB 79 logical blocks 39.5K
2529-2560 MB 80 logical blocks 40.0K
2561-2592 MB 81 logical blocks 40.5K
2593-2624 MB 82 logical blocks 41.0K
2625-2656 MB 83 logical blocks 41.5K
2657-2688 MB 84 logical blocks 42.0K
2689-2720 MB 85 logical blocks 42.5K
2721-2752 MB 86 logical blocks 43.0K
2753-2784 MB 87 logical blocks 43.5K
2785-2816 MB 88 logical blocks 44.0K
2817-2848 MB 89 logical blocks 44.5K
2849-2880 MB 90 logical blocks 45.0K
2881-2912 MB 91 logical blocks 45.5K
2913-2944 MB 92 logical blocks 46.0K
2945-2976 MB 93 logical blocks 46.5K
2977-3008 MB 94 logical blocks 47.0K
3009-3040 MB 95 logical blocks 47.5K
3041-3072 MB 96 logical blocks 48.0K
3073-3104 MB 97 logical blocks 48.5K
3105-3136 MB 98 logical blocks 49.0K
3137-3168 MB 99 logical blocks 49.5K
3169-3200 MB 100 logical blocks 50.0K
3201-3232 MB 101 logical blocks 50.5K
3233-3264 MB 102 logical blocks 51.0K
3265-3296 MB 103 logical blocks 51.5K
3297-3328 MB 104 logical blocks 52.0K
3329-3360 MB 105 logical blocks 52.5K
3361-3392 MB 106 logical blocks 53.0K
3393-3424 MB 107 logical blocks 53.5K
3425-3456 MB 108 logical blocks 54.0K
3457-3488 MB 109 logical blocks 54.5K
3489-3520 MB 110 logical blocks 55.0K
3521-3552 MB 111 logical blocks 55.5K
3553-3584 MB 112 logical blocks 56.0K
3585-3616 MB 113 logical blocks 56.5K
3617-3648 MB 114 logical blocks 57.0K
3649-3680 MB 115 logical blocks 57.5K
3681-3712 MB 116 logical blocks 58.0K
3713-3744 MB 117 logical blocks 58.5K
3745-3776 MB 118 logical blocks 59.0K
3777-3808 MB 119 logical blocks 59.5K
3809-3840 MB 120 logical blocks 60.0K
3841-3872 MB 121 logical blocks 60.5K
3873-3904 MB 122 logical blocks 61.0K
3905-3936 MB 123 logical blocks 61.5K
3937-3968 MB 124 logical blocks 62.0K
3969-4000 MB 125 logical blocks 62.5K
4001-4032 MB 126 logical blocks 63.0K
4033-4064 MB 127 logical blocks 63.5K
4065-4096 MB 128 logical blocks 64.0K
Volume Organization of Macintosh Standard Format
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. Contained in the allocation blocks are the catalog, extents and data files.
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" doesn't 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 (such as where to find file, and how many extents a file is divided into) about the data files you have created is stored. Any given file you create may be broken up into multiple extents. When extents are linked together, behind the scenes, with information from the extents tree file, the appearance is 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.
What You Can Do
So, can you do anything to decrease the file size on a large hard drive? One solution is to partition larger drives into smaller partitions, or logical drives, each with a maximum of 65,536 blocks.
Apple's Drive Setup can be used to partition qualified Apple hard drives and some removable devices as listed in the Drive Setup Guide file. Unsupported hard drives will show up in the device list by name, however, if you select an unsupported hard drive you will get the message, "Cannot modify a disk in an unsupported drive."
Drive Setup is supported on all of the Power Macintosh computers. This includes the original Power Macintosh computers and the new PCI-based Power Macintosh computers. Drive Setup is also compatible with 68040 and 68LC040 processor-based systems that have IDE drives installed. Drive Setup is not supported with the Power Macintosh Upgrade Card. Drive Setup 1.0.2 is available from online services.
There are third parties that offer a solution for creating multiple Macintosh partitions on Macintosh systems not supported by Drive Setup. A few are:
- Hard Disk Toolkit (HDT) by FWB Software, Inc.
- Silverlining by La Cie Ltd.
- Micronet Utility by MicroNet Technology
- MicroTech Utility by MicroTech International
- Drive 7 by Casa Blanca Works, Inc.
Mac OS Extended Format
In 1998, concurrent with the release of Mac OS 8.1, Apple released a new file system format known as Mac OS Extended Format (sometimes referred to as HFS Plus). For more information on Mac OS Extended Format, please see the following technical documents:
30344: "Mac OS 8.1: About Mac OS 8.1 Extended Format"
24601: "Mac OS Extended Format: Volume And File Limits"
This document provides information about a non-Apple product. Apple, Inc. is not responsible for its content. Please contact the vendor for additional information. Document 17159: "Locating Vendor Information" can help you search for a particular vendor's address and phone number.