Index Fragmentation Detect

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1，Fragmentation分为Internal Fragmentation and External Fragmentation。

Internal fragmentation 是指（index or data）page内部的fragmentation，在Page内部存在没有使用的space。

Internal fragmentation (often called physical fragmentation or page desnsity) means that index pages have wasted space, both at the leaf and non-leaf levels. Internal fragmentation means the index is taking more space than necessary, leading to increased disk space usage, more pages to read to process the data, and more memory used to hold the pages in the buffer pool.

External fragmentation 是指 pages 或 extents 的logical order 和 physical order 是不一致的，Logical order 是由 index key定义的，physical order 是page或extent 在disk space实际存储的order。如果在逻辑上连续的page或extent在物理上也是contiguous的，那么就不存在External fragmentation。

External fragmentation (often called logical fragmentation or extent fragmentation) means the pages or extents comprising the leaf level of a clustered or nonclustered index aren‘t in the most efficient order. The most efficient order is where the logical order of the pages and extents(as defined by the index keys, following the next-page pointers from the page headers) is the same as the physical order of the pages and extents with the data files. In other words, the index leaf-lelvel page that has the row with the next index key is also the next physical contiguous page int the data file.

SQL Server 提供DMF: sys.dm_db_index_physical_stats，用于查看External fragmentation，External Fragmentation 分为 Logical 和 Extent Fragmentation。如果扫描的table 有Clustered Index（BTree Structure），那么 DMF返回的Fragmentation是Pages 数量；如果扫描的table没有clustered index（Heap Structure），那么DMF返回的Fragmentation是Extents 数量，因为Heap的 Pages 是没有顺序的。在Heap的 Page header中，字段 next_page 和 Pre_page pointer是null。

Logical Fragmentation              

This is the percentage of out-of-order pages in the leaf pages of an index. An out-of-order page is a page for which the next physical page allocated to the index is not the page pointed to by the next-page pointer in the current leaf page.

Extent Fragmentation              

This is the percentage of out-of-order extents in the leaf pages of a heap. An out-of-order extent is one for which the extent that contains the current page for a heap is not physically the next extent after the extent that contains the previous page.

Fragmentation是由于对数据的更新（insert，update和update)，导致数据的存储的物理顺序和index key定义的逻辑顺序不一致而产生的，少量的Fragmentation是有利的，能够减少page split，提高更新性能；大量的fragmentation是有害的，增加IO的次数，降低查询性能。

Fragmentation occurs through the process of data modifications (INSERT, UPDATE, and DELETE statements) that are made against the table and, therefore, to the indexes defined on the table. Because these modifications are not ordinarily distributed equally among the rows of the table and indexes, the fullness of each page can vary over time. For queries that scan part or all of the indexes of a table, this kind of fragmentation can cause additional page reads. This hinders parallel scanning of data.

2，DMF：sys.dm_db_index_physical_stats 用法

2.1 Syntax

Returns size and fragmentation information for the data and indexes of the specified table or view.

For an index, one row is returned for each level of the B-tree in each partition.

For a heap, one row is returned for the IN_ROW_DATA allocation unit of each partition.

For large object (LOB) data, one row is returned for the LOB_DATA allocation unit of each partition.

If row-overflow data exists in the table, one row is returned for the ROW_OVERFLOW_DATA allocation unit in each partition.

Does not return information about xVelocity memory optimized columnstore indexes.

Syntax

sys.dm_db_index_physical_stats ( 
    { database_id | NULL | 0 | DEFAULT }
  , { object_id | NULL | 0 | DEFAULT }
  , { index_id | NULL | 0 | -1 | DEFAULT }
  , { partition_number | NULL | 0 | DEFAULT }
  , { mode | NULL | DEFAULT }
)

mode | NULL | DEFAULT              

Is the name of the mode. mode specifies the scan level that is used to obtain statistics. mode is sysname. Valid inputs are DEFAULT, NULL, LIMITED, SAMPLED, or DETAILED. The default (NULL) is LIMITED.

2.2 Scanning Modes

Mode 参数 indicate scan mode和返回信息的详细程度。在扫描的过程中，需要在扫描对象上申请IS table lock。

Limited Mode 不会扫描data pages，对于Index，扫描Leaf Level的Parent-level pages；对于Heap，扫描Object对应的IAM 和 PFS pages。

Detailed Mode 扫描all papges，耗时最久，返回的信息最详细。

2.3 Critical Columns

avg_fragmentation_in_percent

Logical fragmentation for indexes, or extent fragmentation for heaps in the IN_ROW_DATA allocation unit.

标识Fragmentation的百分比，MS推荐的值是0-10.

fragment_count

Number of fragments in the leaf level of an IN_ROW_DATA allocation unit.

avg_fragment_size_in_pages

Average number of pages in one fragment in the leaf level of an IN_ROW_DATA allocation unit.

avg_page_space_used_in_percent

Average percentage of available data storage space used in all pages.

For an index, average applies to the current level of the b-tree in the IN_ROW_DATA allocation unit.

For a heap, the average of all data pages in the IN_ROW_DATA allocation unit.

3，Detect Fragmentation

select ps.database_id,ps.object_id,ps.index_id,ps.partition_number,ps.index_type_desc,
    ps.alloc_unit_type_desc,
    ps.index_depth,
    ps.index_level,
    ps.avg_fragmentation_in_percent,
    ps.fragment_count,
    ps.avg_fragment_size_in_pages,
    ps.page_count,
    ps.avg_page_space_used_in_percent,
    ps.record_count,
    ps.ghost_record_count,
    ps.version_ghost_record_count,
    ps.min_record_size_in_bytes,
    ps.max_record_size_in_bytes,
    ps.avg_record_size_in_bytes,
    ps.forwarded_record_count,
    ps.compressed_page_count
from sys.dm_db_index_physical_stats(db_id(),object_id(‘dbo.FactProduct‘),1,1,‘detailed‘) as ps
order by ps.index_level

参考文档：

https://technet.microsoft.com/en-us/library/ms188917(v=sql.110).aspx

Index Fragmentation Detect

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原文地址：http://www.cnblogs.com/ljhdo/p/5129860.html