1. Harvard University Oracle Database Administration Session 5 Data Storage

2. Harvard University Data Storage  Oracle allocates logical database space for all data in a database  The units of this allocation are data blocks, extents and segments  The lowest level of granularity that stores data are data blocks, also know as logical blocks, pages or Oracle blocks

3. Harvard University Data Storage  The next level of storage is the extent  The extent is a specific number of contiguous data blocks  The level of storage above the extent is the segment  A segment is a set of extents, allocated to a specific data structure

4. Harvard University Data Storage  Each table’s data is stored in it’s own data segment  Each index’s data is stored in its own index segment  If a table or index is partitioned, each partition is stored in its own segment

5. Harvard University Data Storage

6. Harvard University Data Storage  Oracle allocates space for each segment in units of one extent  When existing extents of a segment are full, Oracle allocates another extent to that segment  This means that the extents of a segment may or may not be contiguous on the disk

7. Harvard University Data Storage  The segment and all it’s extents are stored in one tablespace  Inside a tablespace, a segment can include extents from more than one datafile. That means segments can span datafiles  However, each extent can contain data from only one datafile

8. Harvard University Data Storage  All the rows for a particular index are stored in one index segment  If a table has 3 indexes, each one has it’s own index segment  The purpose of this segment is to look up the location of rows in a table, based on a specified key

9. Harvard University

10. Segments  There are many types of segments – Table – Table Partition – Index – Undo – Temporary – Large Object (LOB)

11. Harvard University Segments  A table is the only method of storing data in a database  The data in a table segment is stored in no particular order  We have no control over the location of the rows in the blocks, inside the segment  All data in a table segment must be stored in one tablespace

12. Harvard University Segments  Tables can be stored in many partitions, each of which are in a different tablespace  If a table is partitioned, each partition is a segment with it’s own storage parameters  To use partitions, we must have installed the Partitioning Option

13. Harvard University The Storage Clause  A storage clause can be specified at the segment level to control how extents are allocated to that segment  The storage parameter setting at the segment level overrides the corresponding option at the tablespace level  If parameters are not set at the segment level, they default to the tablespace level  If they are not specified at the tablespace level, the Oracle Server sets default values

14. Harvard University Create Table (statement) Create table [schema.] table (column datatype [, column datatype] …) [TABLESPACE tablespace] [pctfree integer] [pctused integer] [initrans integer] [maxtrans integer] [storage clause] [logging | nologging] [cache | nocache]

15. Harvard University The Storage Clause Storage (initial bytes, next bytes, minextents integer, maxextents integer, pctincrease integer)

16. Harvard University The Storage Clause  If these parameters are changed after the segment has been created, the new options apply only to the new extents not yet allocated  Some parameters cannot be set at the tablespace level. These need to be specified at the segment level  A minimum extent size is set at the tablespace level. This will apply to all extents that are allocated for segments in that tablespace

17. Harvard University The Storage Clause  Extents are allocated when the segment is – Created – Extended – Altered  Extents are deallocated when the segment is – Dropped – Altered – Truncated – Automatically resized ( manual rollback segments only)

18. Harvard University The Storage Clause  When a tablespace is created, the associated datafile(s) contain the following – A header block, the first block in the file – One extent consisting of the remaining part of the file  As segments are created they allocate space from the free space  As segments release space, these extents are given back to the pool of free space

19. Harvard University The Storage Clause  The frequent allocation and deallocation of extents can lead to the fragmentation of space within the data files of a tablespace

20. Harvard University Data Blocks  A data block is the smallest unit of I/O used by a database  Each Operating System has a block size. Solaris uses 512 Bytes or one half kilobyte  Oracle blocks are multiples of this block size  Oracle requests data in multiples of Oracle data blocks

21. Harvard University Data Block Format  The data block can be broken into the following sections – Header – Table Directory – Row Directory – Free space – Row Data

22. Harvard University Data Block Format

23. Harvard University The Header  The block header contains the block address and the type of segment it belongs to.  It is also used to hold the transaction slots used when changes are made to rows in the block. Block headers grow from the top down

24. Harvard University Table Directory and Row Directory  The Table Directory contains information about the table that has rows of data in this block  The Row Directory contains information about the actual rows in the block. It holds an address for each row piece in the row data area  The data block header, table directory and the row directory are referred to collectively as block overhead

25. Harvard University Table Directory and Row Directory  On average, the fixed and variable portions of the 8k data block, overhead can total between 84 and 107 bytes  This portion of each data block, in every extent and segment, is taken up as overhead  The rest of the block is used to store data  Rows can span data blocks, this is known as row chaining.

26. Harvard University Free Space and Row Data  Free space is used to insert new rows or update existing rows  To help us manage the use of free space in a block, we use the PCTFREE and PCTUSED parameters  The values of these parameters help to decide if a row in a block can or will be updated in that block.  It may have to be updated in another block

27. Harvard University Free Space and Row Data  The PCTFREE and PCTUSED parameters are specified when creating or altering a table  The PCTFREE parameter can also be specified when creating or altering an index  The PCTFREE parameter sets the minimum percentage of a data block, to be reserved as free space, for possible updates to existing rows

28. Harvard University PCTFREE  If we set the parameter in the CREATE TABLE statement, as follows – PCTFREE 20  This states that 20% of each data block in this table’s data segment, will be kept free for possible updates, of existing rows, in the future  The default value is 10

29. Harvard University PCTFREE  This means that new rows can be added to the block and the associated overhead can grow.  Up to the point where the total gets to 80% of the total block size

30. Harvard University PCTUSED  The PCTUSED parameter sets the minimum percentage of a block that can be used for row data plus overhead, before new rows will be added to the block  After the block has been filled to the limit set by PCTFREE, Oracle considers the block to be unavailable for the insertion of new rows

31. Harvard University PCTUSED  New rows will not be added until the percentage free of that block falls below the parameter PCTUSED  Until that time the free space is used only for updates of existing rows  The parameter is set in the CREATE TABLE statement – PCTUSED 40

32. Harvard University Data Block Format  The data block format is the same, regardless of the type of data stored in that block

33. Harvard University Block Use Concurrency  The parameters used to control concurrency usage of the block are INITRANS and MAXTRANS  These set the initial and maximum number of transaction slots  Transaction slots are used to hold information about transactions that are making changes to the block at that point in time

34. Harvard University Block Use Concurrency  A transaction only uses one transaction slot, even if it is changing multiple rows in the block  INITRANS defaults to 1 in a data segment and to 2 in an index segment. So, if we set INITRANS=3, it ensures that at least 3 transactions can concurrently make changes to the block

35. Harvard University Block Use Concurrency  MAXTRANS, defaults to 255 and sets the limit for the number of concurrent transactions that can make changes to the data or index block  When set, this value restricts the space used for transaction slots  A transaction slot can take up to 20 bytes

36. Harvard University Locally Managed Tablespaces  The CREATE TABLESPACE command has the "extent_management_clause", that specifies how the extents of the tablespace will be managed. This clause uses one of the following parameters:  - DICTIONARY: Specifies that the tablespace is managed using dictionary tables. This is the default for 8i.  - LOCAL: Specifies that the tablespace is locally managed. Locally managed tablespaces have some part of the tablespace set aside for a bitmap. This is the default for 9i and 10g

37. Harvard University Locally Managed Tablespaces  AUTOALLOCATE specifies that the tablespace is system managed. Users cannot specify an extent size.  UNIFORM specifies that the tablespace is managed with uniform extents of SIZE bytes. The default SIZE is 1 megabyte.  If you do not specify either AUTOALLOCATE or UNIFORM with the LOCAL parameter, then AUTOALLOCATE is the default.

38. Harvard University Locally Managed Tablespaces  A tablespace that manages its own extents maintains a bitmap in each datafile, to keep track of the free or used status of blocks in that datafile.  Each bit in the bitmap corresponds to a block or a group of blocks.  When an extent is allocated or freed for reuse, Oracle changes the bitmap values to reflect the new status of the blocks.

39. Harvard University Locally Managed Tablespaces  Local management of extents automatically tracks adjacent free space, eliminating the need to coalesce free extents.  The sizes of extents are determined automatically by the system.  All extents can have the same size in a locally-managed tablespace, if UNIFORM is used.

40. Harvard University Locally Managed Tablespaces  For system-managed extents, you can specify the size of the initial extent and Oracle determines the optimal size of additional extents, with a minimum extent size of 64 KB. This is the default for permanent tablespaces.

41. Harvard University Locally Managed Tablespaces  For uniform extents, you can specify an extent size or use the default size, which is 1 MB.  Temporary tablespaces that manage their extents locally, can only use this type of allocation.

42. Harvard University Locally Managed Tablespaces  The storage parameters NEXT, PCTINCREASE, MINEXTENTS, MAXEXTENTS, and DEFAULT STORAGE are not valid for extents that are managed locally.

43. Harvard University Uniform Example  CREATE TABLESPACE USERS DATAFILE '/u04/oradata/ /test.dbf' SIZE 100M EXTENT MANAGEMENT LOCAL UNIFORM SIZE 128K;

44. Harvard University Autoallocate Example  CREATE TABLESPACE local_auto DATAFILE '/u04/11.1.0/oradata/ /test.dbf' SIZE 100M EXTENT MANAGEMENT LOCAL AUTOALLOCATE;

45. Harvard University Benefits  Locally managed tablespaces track all extent information in the tablespace itself, using bitmaps  This results in Improved concurrency and speed of space operations, as space allocations and deallocations predominantly modify locally managed resources (bitmaps stored in file header) rather than requiring centrally managed resources such as enqueues.

46. Harvard University Benefits  Improved performance, because recursive operations that are sometimes required during dictionary-managed space allocation are eliminated.  Simplified space allocation--when the AUTOALLOCATE clause is specified, appropriate extent sizes are automatically selected.

47. Harvard University Automatic Segment Space Management (ASSM)  CREATE TABLESPACE USERS DATAFILE '/u04/oradata/ /test.dbf' SIZE 1M EXTENT MANAGEMENT LOCAL UNIFORM SIZE 128K SEGMENT SPACE MANAGEMENT AUTO;

48. Harvard University ASSM  The tablespace must be permanent and locally managed.  If used, the segment ignores PCTUSED and FREELIST  This means that there is no advantage to having segments of various sizes. Keeping the sizes uniform is simpler to maintain

49. Harvard University Assignment 2  Name your database the same as your Unix Login  Your Unix Login is the first character of your first name and the next 7 of your last name  Source the Change file that is in your account