Specifying an Offset

When you allocate a chunk of disk space to the database server, you might want to specify an offset for one of the following two purposes:

Many computer systems and some disk-drive manufacturers keep information for a physical disk drive on the drive itself. This information is sometimes referred to as a volume table of contents (VTOC) or disk label. (For convenience, it is referred to here as the VTOC.) The VTOC is commonly stored on the first track of the drive. A table of alternate sectors and bad-sector mappings (also called revectoring table) might also be stored on the first track.

If you plan to allocate partitions at the start of a disk, you might need to use offsets to prevent the database server from overwriting critical information required by the operating system. For the exact offset required, refer to your disk-drive manuals.

If you are running two or more instances of the database server, be extremely careful not to define chunks that overlap. Overlapping chunks can cause the database server to overwrite data in one chunk with unrelated data from an overlapping chunk. This overwrite effectively destroys overlapping data.

Specifying an Offset for the Initial Chunk of Root Dbspace

For the initial chunk of root dbspace and its mirror, if it has one, specify the offsets with t he ROOTOFFSET an d MIRROROFFSET parameters, respectively. For more information, see the chapter on configuration parameters in the IBM Informix: Administrator's Reference.

Specifying an Offset for Additional Chunks

To specify an offset for additional chunks of database server space, you must supply the offset as a parameter when you assign the space to the database server with the onutil utility.

For more information on specifying an offset for chunks of database server space, see Creating a Dbspace.

Using Offsets to Create Multiple Chunks

You can create multiple chunks from a disk partition, disk device, or file, by specifying offsets and assigning chunks that are smaller than the total space available. The offset specifies the beginning location of a chunk. The database server determines the location of the last byte of the chunk by adding the size of the chunk to the offset.

For the first chunk, assign any initial offset, if necessary, and specify the size as an amount that is less than the total size of the allocated disk space. For each additional chunk specify the offset to include the sizes of all previously assigned chunks, plus the initial offset, and assign a size that is less than or equal to the amount of space remaining in the allocation.