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DB_FILE_MULTIBLOCK_READ_COUNT Parameter

DB_FILE_MULTIBLOCK_READ_COUNT specifies the maximum number of Oracle blocks that can be read in a single I/O call during a sequential scan (for example, a full-table scan). Conceptually, it controls the “batch size” of reads when Oracle is reading table blocks in order.

In modern Oracle (including 23ai), the parameter still exists, but it is typically best treated as a costing and scan-throughput influence rather than a day-to-day tuning knob. If the parameter is not explicitly set, Oracle derives an efficient value for the platform and storage stack.

What the parameter does

  • Scan efficiency: Larger values can reduce the number of I/O calls required to scan a segment (fewer, larger reads).
  • Optimizer costing: The cost-based optimizer estimates the cost of full scans partly from the assumed multiblock read size. Increasing the value can make full-table scans look cheaper relative to index access.
  • Scope: It applies to sequential scan-style reads. It does not “speed up” random single-row index lookups.

The total number of I/Os for a full scan depends on the table size, the multiblock read count, and whether the operation uses parallel execution or direct-path read strategies.

Modern guidance: do not anchor this to a fixed 64KB rule

Older guidance often claimed that Oracle “always reads a minimum of 64KB” on UNIX and that DB_BLOCK_SIZE × DB_FILE_MULTIBLOCK_READ_COUNT should equal 64KB. That is not a reliable rule in modern environments. Practical I/O sizes are influenced by the OS, filesystem, ASM, storage array capabilities, and Oracle’s internal scan strategy.

A better way to reason about it is:

  • Effective I/O size (bytes) ≈ DB_BLOCK_SIZE × DB_FILE_MULTIBLOCK_READ_COUNT
  • Oracle will cap or adjust the effective I/O to what is efficient and supported on the platform.

Examples: converting blocks to an approximate I/O size

-- Approximate effective read size (bytes) = DB_BLOCK_SIZE * DB_FILE_MULTIBLOCK_READ_COUNT

-- Example A: 8K blocks, read 128 blocks per I/O
-- 8192 * 128 = 1,048,576 bytes  (~1 MB)

-- Example B: 16K blocks, read 64 blocks per I/O
-- 16384 * 64 = 1,048,576 bytes  (~1 MB)

If you set an invalid or excessively large value, Oracle will use a safe effective maximum for the platform. In modern releases, the default behavior is typically already aligned to the maximum efficient scan I/O size for the storage subsystem.

Reference summary

Property Description
Parameter type Integer
Default value Platform-dependent; derived from the maximum efficient I/O size
Modifiable ALTER SESSION, ALTER SYSTEM
Range of values Operating system-dependent
Basic No

When to change DB_FILE_MULTIBLOCK_READ_COUNT

In most Oracle 23ai environments, the recommended posture is: leave it unset unless you have a controlled reason to override defaults (for example, a targeted benchmark showing a consistent benefit for a scan-heavy workload).

  • OLTP systems: typically do not benefit from forcing this parameter; performance is usually driven by indexed access, concurrency, and cache efficiency.
  • Scan-heavy analytics / batch: may benefit in specific cases, but validate with execution plans and measured I/O throughput rather than relying on generic “set it high” guidance.
  • Risk to avoid: forcing an aggressive value can distort optimizer costing (favoring scans where index access is actually better).

If you need to experiment, do it in a controlled window and validate outcomes using execution plans (DBMS_XPLAN) and workload metrics, then revert to defaults if the change does not produce a measurable improvement.


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