Sbspace Structure

An sbspace is similar to a blobspace except that it holds smart large objects.

When an sbspace is created in a database, it contains an sbspace descriptor. Each sbspace chunk contains the following structures:
  • Sbspace chunk descriptors
  • Chunk free-page list
  • An sbspace metadata area (up to one for each chunk)
  • Reserved data areas (up to two for each chunk)
  • User-data areas (up to two for each chunk)

For best performance, it is recommended that the metadata area be located in the middle of the sbspace. The database server automatically places the metadata area in the correct location. However, to specify the location of the metadata area, specify the -Mo flag in the onspaces command.

If you do not specify the size of the metadata area in the -Ms flag of the onspaces command, the database server uses the value of AVG_LO_SIZE (defaults to 8 kilobytes) to calculate the size of the metadata area. For more information, see Creating an Sbspace with the -Df option.

Normally, you can let the system calculate the metadata size for you. If you want to estimate the size of the metadata area, see the chapter on table performance considerations in the HCL Informix® Performance Guide.

Figure 1 illustrates the chunk structure of an sbspace as it appears immediately after the sbspace is created. Each reserved area can be allocated to either the user-data or metadata area. Reserved areas are always within the user-data area of the chunk.
Figure 1. A Single Sbspace Chunk
begin figure description - The figure shows a single sbspace chunk as it looks immediately after it is created. The sbspace chunk is represented by a column divided into rows or sections; the sections are labeled: header pages (pages 0 through 52 compose the chunk header), User data areas 1 and 2, Reserved areas 1 and 2, and the metadata area, which is in the middle. - end figure description

Because the chunk in Figure 1 is the first in the sbspace, it contains an sbspace descriptor. The chunk descriptor tblspace in chunk one contains information about chunk one and all chunks added to the sbspace thereafter.


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