Commands that allocate many data pages perform extent allocation to allocate eight pages at a time, rather than allocating just one page at a time. Extent allocation reduces logging, since it writes one log record instead of eight.
Commands that perform extent allocation are: select into, create index, reorg rebuild, bcp, alter table...lock, and the alter table...unique and primary key constraint options, since these constraints create indexes. alter table commands that add, drop, or modify columns sometimes require a table-copy operation also. All of these commands allocate an extent, and, unless a reserve page gap value is in effect, fill all eight pages.
You specify the reservepagegap in pages, indicating a ratio of empty pages to filled pages. For example, if you specify a reservepagegap of 8, an operation doing extent allocation fills seven pages and leaves the eighth page empty.
These empty pages can be used to store forwarded rows and for maintaining the clustering of data rows in index key order, for data-only-locked tables with clustered indexes.
Since extent allocation operations must allocate entire extents, they do not use the first page on each allocation unit, because it stores the allocation page. For example, if you create a clustered index on a large table and do not specify a reserve page gap, each allocation unit has 7 empty, unallocated pages, 248 used pages, and the allocation page. These 7 pages can be used for row forwarding and inserts to the table, which helps keep forwarded rows and inserts with clustered indexes on the same allocation unit. Using reservepagegap leaves additional empty pages on each allocation unit.
Figure 14-1shows how an allocation unit might look after a clustered index is created with a reservepagegap value of 16 on the table. The pages that share the first extent with the allocation unit are not used and are not allocated to the table. Pages 279, 295, and 311 are the unused pages on extents that are allocated to the table.
Figure 14-1: Reserved pages after creating a clustered index