Blobpage size considerations

Blobspaces are divided into units called blobpages. The database server retrieves simple large objects from a blobspace in blobpage-sized units. You specify the size of a blobpage in multiples of a disk page when you create the blobspace.

The optimal blobpage size for your configuration depends on the following factors:
  • The size distribution of the simple large objects
  • The trade-off between retrieval speed for your largest simple large object and the amount of disk space that is wasted by storing simple large objects in large blobpages

To retrieve simple large objects as quickly as possible, use the size of your largest simple large object rounded up to the nearest disk-page-sized increment. This scheme guarantees that the database server can retrieve even the largest simple large object in a single I/O request. Although this scheme guarantees the fastest retrieval, it has the potential to waste disk space. Because simple large objects are stored in their own blobpage (or set of blobpages), the database server reserves the same amount of disk space for every blobpage even if the simple large object takes up a fraction of that page. Using a smaller blobpage allows you to make better use of your disk, especially when large differences exist in the sizes of your simple large objects.

To achieve the greatest theoretical utilization of space on your disk, you can make your blobpage the same size as a standard disk page. Then many, if not most, simple large objects would require several blobpages. Because the database server acquires a lock and issues a separate I/O request for each blobpage, this scheme performs poorly.

In practice, a balanced scheme for sizing uses the most frequently occurring simple-large-object size as the size of a blobpage. For example, suppose that you have 160 simple-large-object values in a table with the following size distribution:
  • Of these values, 120 are 12 kilobytes each.
  • The other 40 values are 16 kilobytes each.
You can choose one of the following blobpage sizes:
  • The 12-kilobyte blobpage size provides greater storage efficiency than a 16-kilobyte blobpage size, as the following two calculations show:
    • 12 kilobytes
      This configuration allows the majority of simple-large-object values to require a single blobpage and the other 40 values to require two blobpages. In this configuration, 8 kilobytes is wasted in the second blobpage for each of the larger values. The total wasted space is as follows:
      wasted-space = 8 kilobtyes * 40 
                   = 329 kilobytes
    • 16 kilobytes

      In this configuration, 4 kilobytes is wasted in the extents of 120 simple large objects. The total wasted space is as follows:

      wasted-space = 4 kilobtyes * 120 
                   = 640 kilobytes
  • If your applications access the 16-kilobyte simple-large-object values more frequently, the database server must perform a separate I/O operation for each blobpage. In this case, the 16-kilobyte blobpage size provides better retrieval speed than a 12-kilobyte blobpage size.

The maximum number of pages that a blobspace can contain is 2147483647. Therefore, the size of the blobspace is limited to the blobpage size x 2147483647. This includes blobpages in all chunks that make up the blobspace.

Tip: If a table has more than one simple-large-object column and the data values are not close in size, store the data in different blobspaces, each with an appropriately sized blobpage.

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