Large Disks: How OS/2 Uses Them, Part 1

By: Walter Metcalf
Date: 07/22/98


PC's did not always come with built-in hard disks. Microcomputers originally ran exclusively on floppy drives whose capacity was at most 720K. Although the simple file indexing system developed for diskettes by Microsoft known as FAT was flawed in design, it worked reasonably well for disks of such small capacity. The FAT system is flawed in two ways:

  1. If even one bit in the small FAT table is corrupted, then the whole disk(ette) can be lost!

  2. The size of the FAT table is restricted. Therefore as disks get bigger, each bit in the FAT table is required to represent more than one sector of disk space (known as a cluster), leading to the possibility of small files taking up much more space on the disk than they should and wasting disk space. On a 2 GB drive, up to 40% of the drive can be wasted in this way.

As long as disk storage was limited to floppies, or very small hard drives, these problems really didn't matter too much. With the advent of very large disks, however, the FAT system is clearly becoming unworkable, and IBM and Microsoft are developing new technologies to handle the larger drives.


There are several limitations, or barriers, you may encounter when using hard drives. These barriers can usually be crossed as long as careful attention is paid to certain details. Before dealing with these barriers, we need to briefly look at a new file system introduced by OS/2, the High Performance File System or HPFS.

HPFS was designed to overcome the limitations and inefficiencies of the FAT system. With HPFS the cluster size is always 512 bytes, regardless of the disk size. (Throughout Part 1 of this series, I will assume for the sake of simplicity the hard disk contains only one partition. I'll deal with partitioning later on.) In addition HPFS uses a far more robust indexing system which prevents the massive loss of data when a small table is corrupted. Finally, under HPFS, when files are written, empty spaces are deliberately left at the end and sometimes in the middle of the files to allow room for expansion. This greatly reduces the fragmentation that so plagues FAT drives. There are commercial HPFS "defraggers" and I've used them occasionally; but I always come away feeling the result wasn't worth the time involved! Although OS/2 supports the older FAT system, HPFS is preferred and should be used except where there is a compelling reason for DOS or Windows to access the OS/2 files.

Now let's look at each of the barriers in turn.

  1. Barriers Imposed by the Hardware

    1. IDE and EIDE drives

      The 512MB Barrier

      1. By default your computer accesses data on a disk drive by specifying a cylinder number, head number, and sector number. (Technically this is known as "logical geometry," which differs from the internal or physical geometry used by your drive.) Like several other computer schemes, logical geometry is inherited from older hardware designs, in this case IDE drives. This geometry is limited to 1024 cylinders, 16 heads, and 64 sectors resulting in a maximum specification of 1,048,576 sectors or 536,870,912 bytes (512 MB). (Each sector contains 512 bytes.) This is the infamous 512 MB barrier. Sometimes this barrier is "rounded off" to cylinders and spoken of as the 1024 cylinder or 504 MB limit.

        Since it's difficult to even buy a disk this "small" today, we need a different access method.

      2. The 512 MB barrier is crossed by selecting the Large Block Addressing (LBA) feature available on practically all modern hard drive controllers. This is a much a simpler scheme in which the first sector on the drive is number 0, the second is number 1, and so on until the last physical sector of the drive. Another important feature of LBA is that the sector number can be 32 bits long. This enables your computer to access up to 2 terabytes of data. (1 terabyte = 1024 GB.) Note that the FAT system imposes additional limitations so that the 2 terabyte limit can only be realized if the drive is formatted as HPFS. These FAT limitations will be discussed under "Barriers Imposed by the Software" in Part 2 of this series. See the Quantum white paper entitled, Breaking the 2.1 Gigabyte Barrier, for more information.

        1. To access the LBA feature, reboot your computer; then watch for and follow the instructions for entering Setup. On my computer, the LBA setting is located under "Standard CMOS Setup." (The wording on your computer may be slightly different.) Follow the on-screen directions for changing the disk access mode to LBA. (I suggest that for the sake of simplicity you set all your drives to LBA and leave them that way.)

          • Important note: Selecting or deselecting LBA mode will DESTROY all data on the affected drive. So before changing the LBA setting, be sure to back up your drive. If this drive contains your operating system you should first reboot from floppies or from a maintenance partition in order to get a complete backup.

      3. Occasionally when installing a disk with LBA on an older computer, you may find that you are unable to select LBA or that LBA is ineffective. It's possible that the motherboard itself is incapable of handling large drives. Check your motherboard manual carefully for any restrictions on hard disk size. If you find yourself in this position, you can install Ontrack's Disk Manager software. This software may be available for free download from your disk manufacturer's web site. Otherwise you will have to purchase a copy from Ontrack. Ontrack Disk Manager remaps the disk structure at boot time so that you can access the entire disk. Another less expensive product designed for the same purpose is EZ-Drive from Microhouse. Unfortunately, however, I haven't had the opportunity to test it.

    2. SCSI Disks

      The 1 Gigabyte Barrier

      (Note: The above limit is based on the Adaptec 2940W adapter. It may be different with your adapter. )

      SCSI disk drives are handled somewhat differently from IDE, but the basic results are the same. Here is a summary of the main differences:

      1. There is no LBA option in the CMOS; the disk settings under Basic CMOS Setup are for IDE drives only;

      2. Instead of an LBA in the CMOS there is an option in your SCSI adapter to enable large drives. On the Adaptec 2940W I have the setting is labelled "Extended BIOS translation for DOS drives > 1 GB." The wording is likely to vary from one manufacturer to another, but you should be able to recognize it easily.

      3. Warning: Like the LBA setting, changing this setting will DESTROY data, but in this case it will destroy the data on ALL drives connected to adapter! Be sure you back up all those disks, first rebooting from floppies or a maintenance partition if necessary.

My next feature will continue this series by looking at "Barriers Imposed by Software." Stay tuned!

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