PFS pages
Uniform extent, or full mixed extent
Uniform extent, or full mixed extent
0
0
Mixed extent with free pages
0
1
To manage extents, the Database Engine uses the following conceptual algorithms:
To allocate a uniform extent, the Database Engine searches the GAM page for a
bit and
sets it to.
To find a mixed extent with free pages, the Database Engine searches the SGAM page for
a
bit.
To allocate a mixed extent, the Database Engine searches the GAM page for a
bit, sets
it to
, and then also sets the corresponding bit on the SGAM page to.
To deallocate an extent, the Database Engine makes sure that the bit on the GAM page is
set to
, and the bit on the SGAM page is set to.
The Database Engine allocates extents from those available in the filegroup using a
proportional fill allocation
algorithm. For example, in a filegroup with two files, if one file has
double the free space of the other, two pages are allocated from that file for every one page
allocated from the other file. This means that if allocations continue, all files in a filegroup end
up with similar percentage of space used.
For more information, see
File and filegroup fill strategy.
pages record the allocation status of each page and the amount of free
space on each page. A PFS page has 1 byte for each page it tracks. The byte records whether
the page is allocated, and if so, whether it’s empty, 1 to 50 percent full, 51 to 80 percent full, 81
to 95 percent full, or 96 to 100 percent full.
After an extent has been allocated to an object, the Database Engine uses PFS pages to track
which pages in the extent have data or are free. This information is used when the Database
Engine allocates a new page. The amount of free space in a page is only maintained for heap
and text/LOB pages. This information is used when the Database Engine has to find a page
with enough free space available to hold a newly inserted row.
BTree indexes don’t require page free space tracking because the point at which to insert a new
row is always determined by the index key values. If a page in a BTree index doesn’t have
Index Allocation Map (IAM)
varchar(max)
nvarchar(max)
varbinary(max)
json
10110110