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分类: Oracle

2006-12-14 17:19:35

28.In addition to all the regular Oracle features, Real Application Clusters exploits the
redundancy provided by clustering to deliver availability with n-1 node failures in
an n-node cluster. In other words, all users have access to all data as long as there is
one available node in the cluster.
29.Data Blocks, Extents, and Segments
A segment and all its extents are stored in one tablespace.Within a tablespace, a
segment can include extents from more than one file; that is, the segment can span
datafiles. However, each extent can contain data from only one datafile.
Although you can allocate additional extents, the blocks themselves are allocated
separately. If you allocate an extent to a specific instance, the blocks are immediately
allocated to the free list. However, if the extent is not allocated to a specific instance,
then the blocks themselves are allocated only when the high water mark moves.
The high water mark is the boundary between used and unused space in a segment
Data Block Format
1)Header (Common and Variable)
The header contains general block information, such as the block address and the
type of segment (for example, data or index).
2)Table Directory
This portion of the data block contains information about the table having rows in
this block.
3)Row Directory
This portion of the data block contains information about the actual rows in the
block (including addresses for each row piece in the row data area).
After the space has been allocated in the row directory of a data block’s overhead,
this space is not reclaimed when the row is deleted. Therefore, a block that is
currently empty but had up to 50 rows at one time continues to’ have 100 bytes
allocated in the header for the row directory. Oracle reuses this space only when
new rows are inserted in the block.
这个空间,即使row被删除了也不会释放。 只有新插入的row 的偏移量信息可以重用这个空间,
The data block header, table directory, and row directory are referred to collectively
as overhead. Some block overhead is fixed in size; the total block overhead size is
variable. On average, the fixed and variable portions of data block overhead total 84
to 107 bytes.
(The data block header, table directory, and row directory三者“结合”就是overhead,
5)free space
In data blocks allocated for the data segment of a table or cluster, or for the index
segment of an index, free space can also hold transaction entries. A transaction
entry is required in a block for each INSERT, UPDATE, DELETE, and SELECT...FOR
UPDATE statement accessing one or more rows in the block. The space required for
transaction entries is operating system dependent; however, transaction entries in
most operating systems require approximately 23 bytes.
(free space也能够支持transaction entries。在数据块中每个insert,update,delete,select for update
语句访问一条或多条记录时transaction entries都有记录,分配的free space依赖于操作系统,在大多数操作系统上,
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