Large Disks: How OS/2 Uses Them, Part 2

By: Walter metcalf
Date: 08/05/98

Last time we looked at large disk drives, the difficulties they inherently raise, and the techniques used by the computer to overcome them. This time we shall look at problems caused by software, originally designed for drives much smaller.

II.  Barriers Imposed by the Software

  1. FAT Filesystem

    1. FAT uses a 16-bit table to hold cluster numbers. (A cluster is a group of 1 to 64 sectors.) Because of this design the FAT filesystem only allows a maximum of 65,525 clusters. The largest cluster size for DOS, early versions of Windows 95, and OS/2 is 16K. This results in a maximum disk size of 1.0 GB. Under this configuration, even a 2-byte file occupies 16K resulting in a terrible waste of disk space!

    2. Later versions of Windows 95 (v4.00.95B or above) increased the maximum cluster size to 32K, increasing maximum supported disk space to 2.0GB. However the cost is even more wasted space. In fact under FAT 32, up to 40% of a disk's total space can be wasted!

  2. HPFS Filesystem

    1. OS/2 introduced a completely different filesystem called HPFS into the PC world. Because of a radical new design, the upper limit of disk space under HPFS is 2 terabytes (1 terabyte = 1024 GB). Although HPFS will be discussed in more detail later in this series, it's worth noting in passing a few of its characteristics here.

      1. HPFS scraps clusters altogether and allocates on a sector basis, minimizing wasted space.

      2. HPFS replaces the single FAT directory with zones spaced at 8MB intervals throughout the disk.

      3. HPFS locates files using a B-tree search algorithm instead of the simple sequential search used by FAT, resulting in much faster file access.

  3. Overcoming the Software-Imposed Barriers

    1. Partitions

      1. With technology available at the time of writing the only method for DOS, Windows 9x, and OS/2 systems to overcome the FAT software barriers discussed above is to divide the disk into smaller logical disks called partitions. FAT and HPFS both treat each logical partition exactly like a hard drive in its own right. This means that the above limitations of 2 GB and 2 terabytes now become the maximum sizes, not of the physical drive, but of the partition, or logical drive.

      2. Partitioning has several advantages besides simply being a work-around for maximum disk sizes:

        • It is often easier to manipulate (e.g. for backing up) several smaller logical disks than one huge physical disk.
        • Fewer directories and simpler pathnames are required in a smaller disk.
        • Several smaller disks often simplify organization of files and data.
        • In the case of FAT partitions, smaller clusters can be used resulting in less wasted space.

          These advantages often make it wise to partition a large disk even if you will only be using HPFS.

      3. Partitioning, however, has some drawbacks. These problems center around the fact that changing the partition structure of a drive once it contains data is very costly in terms of time. It involves backing up the whole drive, re-partitioning, re-formatting, and finally restoring the data! On a large disk this could easily take a day or more. Fortunately Powerquest invented a product called PartitionMagic that provides complete partition restructuring capability without endangering data contained on the drive!

      4. Tips on Partitioning

        • Install OS/2 on a partition by itself. Then if Warp becomes corrupt, you can simply do a full restore of that relatively small partition and be back online.
        • When sizing a partition be sure to leave adequate room for expansion.
        • Avoid FAT partitions unless you know they will be accessed by DOS or Windows.
        • If DOS or Windows is on your disk, logical drive C must be the first partition on the first physical drive.
        • Where possible put Win 95 on a separate drive from OS/2. This will reduce OS conflict problems.
        • Reserve primary partitions for operating systems.

    2. Journaled File System

      1. As this is being written, IBM is preparing to start beta testing a new version of Warp Server, code-named Aurora. Many people, including this writer, believe the release of Aurora will be followed by a new version of the client version of Warp. Aurora and the new Warp client, if released, will include the Journaled File System currently running on AIX, IBM premier UNIX system. Unlike some companies, IBM does not divulge a great deal of information on unreleased software, so details are sketchy, but IBM has said that, under JFS:

        The maximum file size is 2 terabytes;
        The maximum partition size is 2 terabytes;

      2. Since no one has built a 2 terabyte disk yet, JFS would appear to effectively eliminate the need for partitioning.

In the next issue we will look at the HPFS filesystem in some detail since it is the one native to OS/2 and is probably the one most commonly used by OS/2 users.

For further reading:

Hard Disk Barriers, PC Guide, © Charles M. Kozierok.
An Immediate Solution to the 528 MB Barrier, Quantum White Papers, © Quantum Corporation.
8.4 GB Barrier, Quantum White Papers, © Quantum Corporation.

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