Storage and Space Management
Storage and Space Management
This domain tests logical and physical storage: tablespace and segment structures, space allocation and reclamation, undo and temporary space, and Automatic Storage Management disk groups.
Core rules
- Hierarchy: tablespace > segment > extent > Oracle block. Locally managed tablespaces track extents in a bitmap; dictionary-managed tablespaces are legacy and unsupported for new work.
- AUTOALLOCATE sizes extents adaptively (64K, then 1M, 8M, up to 64M). UNIFORM forces one extent size, which is the normal choice for temporary tablespaces with many small sorts.
- ASSM (SEGMENT SPACE MANAGEMENT AUTO) uses bitmaps for free space inside segments; PCTFREE and PCTUSED behave differently under ASSM than under manual segment space management.
- Bigfile tablespaces hold exactly one data file and can be very large; smallfile tablespaces hold many files and are required for undo and temporary tablespaces, which must be locally managed.
- AUTOEXTEND with MAXSIZE prevents a runaway file from filling a filesystem; space pressure surfaces as ORA-01653 (unable to extend table) and ORA-01654 (index).
- Space is reclaimed with ALTER TABLE ... SHRINK SPACE (ASSM and row movement where applicable), MOVE, or online redefinition; the Segment Advisor flags candidates.
- ASM: disk groups built from disks or files, with redundancy EXTERNAL, NORMAL (two-way) or HIGH (three-way) and failure groups aligned to hardware. Rebalancing is controlled by ASM_POWER_LIMIT; ASMCMD manages files and disks.
- RESUMABLE_TIMEOUT lets an allocation wait rather than fail immediately, raising ORA-01653 only if it cannot complete.
Worked example
A tablespace uses AUTOALLOCATE. A segment has 16 extents at 64K, 63 at 1M, and needs one more. The next extent allocation request is 8M because AUTOALLOCATE moves to the next size class once 64 extents of the current size exist. Reasoning: sizing calculations must use the current size class, not the initial 64K, or the space estimate will be far too small.
Traps that cost marks
- Assuming a data file with AUTOEXTEND on cannot fill the disk; always pair it with MAXSIZE.
- Expecting SHRINK to work on a segment without ASSM.
- Forgetting that NORMAL redundancy consumes roughly double raw capacity, so usable space from three 4 TB disks is about 8 TB, not 12 TB.