Jun 22, 2022
The Storage Hierarchy in an Oracle Database- Files-2
The relationship between segments, extents, and blocks is shown in Figure 3-1.
Figure 3-1. Segments, extents, and blocks
A segment is made up of one or more extents, and an extent is a logically contiguous allocation of blocks. A database may have up to six different block sizes in it.
Note This feature of multiple block sizes was introduced for the purpose of making transportable tablespaces usable in more cases. The ability to transport a tablespace allows a DBA to move or copy the already formatted datafiles from one database and attach them to another—for example, to immediately copy all of the tables and indexes from an Online Transaction Processing (OLTP) database to a data warehouse (DW). However, in many cases, the OLTP database might be using a small block size, such as 2KB or 4KB, whereas the DW would be using a much larger one (8KB or 16KB). Without support for multiple block sizes in a single database, you wouldn’t be able to transport this information. Tablespaces with multiple block sizes should be used to facilitate transporting tablespaces; they are not generally used for anything else.
There will be the database default block size, which is the size specified in the initialization file during the CREATE DATABASE command. The SYSTEM tablespace will have this default block size always, but you can then create other tablespaces with nondefault block sizes of 2KB, 4KB, 8KB, 16KB, and, depending on the operating system, 32KB. The total number of block sizes is six if and only if you specified a nonstandard block size (not a power of two) during database creation. Hence, for all practical purposes, a database will have at most five block sizes: the default size and then four other nondefault sizes.
Any given tablespace will have a consistent block size, meaning that every block in that tablespace will be the same size. A multisegment object, such as a table with a LOB column, may have each segment in a tablespace with a different block size, but any given segment (which is contained in a tablespace) will consist of blocks of exactly the same size.
Most blocks, regardless of their size, have the same general format, which looks something like Figure 3-2.
Figure 3-2. The structure of a block
Exceptions to this format include LOB segment blocks and hybrid columnar compressed blocks in Exadata storage, for example, but the vast majority of blocks in your database will resemble the format in Figure 3-2. The block header contains information about the type of block (table block, index block, and so on), transaction information when relevant (only blocks that are transaction managed have this information—a temporary sort block would not, for example) regarding active and past transactions on the block, and the address (location) of the block on the disk.
The next two block components are found on the most common types of database blocks, those of heap-organized tables. We’ll cover database table types in much more detail in Chapter 10, but suffice it to say that most tables are of this type.
The table directory, if present, contains information about the tables that store rows in this block (data from more than one table may be stored on the same block). The row directory contains information describing the rows that are to be found on the block.
This is an array of pointers to where the rows are to be found in the data portion of the block. These three pieces of the block are collectively known as the block overhead, which is space used on the block that is not available for your data, but rather is used by Oracle to manage the block itself.
The remaining two pieces of the block are straightforward: there may be free space on a block, and then there will generally be used space that is currently storing data.
Now that you have a cursory understanding of segments, which consist of extents, which consist of blocks, let’s take a closer look at tablespaces and then at exactly how files fit into the big picture.
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