Oracle Space Management Experiment (III) the influence of district Management area on I/O performance

The advantages and disadvantages of a large community, more than one m area meaningful? Table Space management technology Management is the district, the Local management table space LMT in each data file header added bitmap area management is extent usage.

When extent is used and released, Oracle updates the record in the bitmap area of the header of the data file.

For large residential areas, in fact, even in the system of automatic allocation of the size of the management mode, 8M area is also very common, as follows:

The system Admin area size is automatically allocated by the system to the extents of the expansion.

In the front 1M space of the segment: The district size is 8 blocks =64k, the first 16 districts are such.

Between segment 1M---64M: District size 1m,128 Block

After segment 64M, the area size is 8M.

Large cell advantages and disadvantages Summary:

Community: The advantages of the province space

: The number of disadvantages can be allocated to many, even resulting in the data file header bitmap area contention.

Area: Disadvantages waste space, and consume a little more CPU when allocating and recycling

: Benefits: Reduce the number of I/O during full scan--explain the next question

Reduce the number of allocations.

Scanning segments also reduces the number of segment headers--The segment headers are cached

Large area, large IO, full table scan faster without concurrency

Cell, small Io, with concurrent full table scan speed faster

What is the impact of a large community on concurrency?

Large-area community does not have much impact on concurrency, because the concurrent final operation is the size of the block-the operation block will be locked-buffer pin, may produce a buffer busy waits. such as block large, row more, such as 500 lines, competition, and relatively small concurrency. While the block is small, lines less, such as 100 lines, less competition, relatively large concurrency support.

But the large community will also have a little impact on concurrency, that is, the advance of high water level, generally in a district as a unit to advance. See:

Will the large area increase OLTP I/O performance? Why?

Has no effect on OLTP or has little impact.

For OLAP, you can reduce the number of I/o times, or let random I/O become sequential I/O.

But consider:

OS, hardware driver: Max IO size 1M.

The space within the zone is continuous. --table fragments???

Read a 8M zone that contains 8 1M I/O, and most likely, 8 I/O, is sequential io.

Read 8 1M zones, at least 8 random I/O.

However, when the system is automatically allocated, it is automatically allocated 8M after 64M, using the system allocation area size.

You can consider setting the table space to the uniform size, unless you have a lot of table space and many smaller tables are larger than 8M and are less than hundred m full table scans.

This is to consider the underlying storage of AU. such as 8M AU, 8M zone, so that each au 8M in the same disk, such as storage maximum support 4M, then can be area size 4M or no matter, the ASM layer AU is also 4 m. --It doesn't seem very practical.

This section is to be considered when OLAP.

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