Global Allocation Map (GAM)
Shared Global Allocation Map (SGAM)
GAM pages record
what extents have been allocated. Each GAM covers 64,000 extents, or almost 4 GB
of data. The GAM has one bit for each extent in the interval it covers. If the
bit is 1, the extent is free; if the bit is 0, the extent is allocated.
Shared Global Allocation Map (SGAM)
SGAM pages record
which extents are currently being used as mixed extents and also have at least
one unused page. Each SGAM covers 64,000 extents, or almost 4 GB of data. The
SGAM has one bit for each extent in the interval it covers. If the bit is 1, the
extent is being used as a mixed extent and has a free page. If the bit is 0, the
extent is not used as a mixed extent, or it is a mixed extent and all its pages
are being used.
Each extent has the
following bit patterns set in the GAM and SGAM, based on its current use.
|
Current
use of extent
|
GAM bit
setting
|
SGAM bit
setting
|
|
Free, not being used
|
1
|
0
|
|
Uniform extent, or full mixed
extent
|
0
|
0
|
|
Mixed extent with free
pages
|
0
|
1
|
To allocate a
uniform extent, the Database Engine searches the GAM for a 1 bit and sets it to
0. To find a mixed extent with free pages, the Database Engine searches the SGAM
for a 1 bit. To allocate a mixed extent, the Database Engine searches the GAM
for a 1 bit, sets it to 0, and then also sets the corresponding bit in the SGAM
to 1. To de-allocate an extent, the Database Engine makes sure that the GAM bit
is set to 1 and the SGAM bit is set to 0. The algorithms that are actually used
internally by the Database Engine are more sophisticated than what is described
in this topic, because the Database Engine distributes data evenly in a
database. However, even the real algorithms are simplified by not having to
manage chains of extent allocation information.
Let me take a small example here to
explain you GAM and then you can work on SGAM. I will use the page database.
USe Page
GO
DBCC TRACEON(3604)
DBCC PAGE (Page, 1, 2, 1)
And the partial dump of the output is
Memory Dump @0x000000000E9DA0BE
0000000000000000: 0000381f 000080ff ffffffff ffffffff
†..8....ÿÿÿÿÿÿÿÿÿ
0000000000000010: ffffffff ffffffff ffffffff
ffffffff †ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
0000000000000020: ffffffff ffffffff ffffffff
ffffffff †ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
0000000000000030: ffffffff ffffffff ffffffff
ffffffff †ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
0000000000000040: ffffffff ffffffff ffffffff
ffffffff †ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
0000000000000050: ffffffff ffffffff ffffffff
ffffffff †ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
0000000000000060: ffffffff ffffffff ffffffff
ffffffff †ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
0000000000000070: ffffffff ffffffff
ffffffff ffffffff †ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
000080ff => these four bytes can be
expanded as below.
First byte = 00 = 00000000 = each bit
covers 8 pages (1 extent) = 0 to 63 pages
Second byte = 00 = 00000000 = 64 to 127
pages
Third byte = 80 = 10000000 = 7 * 8 = 128
to 183 pages
Let’s check PFS page
USe Page
GO
DBCC TRACEON(3604)
DBCC PAGE (Page, 1, 1, 3)
PFS: Page Alloc Status @0x000000000F92C000
(1:0) - (1:3) = ALLOCATED
100_PCT_FULL
(1:4) - (1:5) = NOT ALLOCATED
0_PCT_FULL
(1:6) - (1:7) = ALLOCATED
100_PCT_FULL
(1:8) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:9) - = ALLOCATED
100_PCT_FULL Mixed Ext
(1:10) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:11) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:12) - = ALLOCATED
100_PCT_FULL IAM Page Mixed Ext
(1:13) - (1:16) = ALLOCATED
0_PCT_FULL Mixed Ext
(1:17) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:18) - (1:19) = ALLOCATED
0_PCT_FULL Mixed Ext
(1:20) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:21) - (1:25) = ALLOCATED
0_PCT_FULL Mixed Ext
(1:26) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:27) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:28) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:29) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:30) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:31) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:32) - = ALLOCATED
50_PCT_FULL Mixed Ext
(1:33) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:34) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:35) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:36) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:37) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:38) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:39) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:40) - (1:41) = ALLOCATED
0_PCT_FULL
(1:42) - = NOT ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:43) - = NOT ALLOCATED
0_PCT_FULL Mixed Ext
(1:44) - = NOT ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:45) - (1:46) = NOT ALLOCATED
0_PCT_FULL Mixed Ext
(1:47) - = NOT ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:48) - (1:53) = ALLOCATED
0_PCT_FULL Mixed Ext
(1:54) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:55) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:56) - (1:63) = ALLOCATED
0_PCT_FULL
(1:64) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:65) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:66) - (1:71) = ALLOCATED
0_PCT_FULL Mixed Ext
(1:72) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:73) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:74) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:75) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:76) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:77) - (1:78) = ALLOCATED
0_PCT_FULL Mixed Ext
(1:79) - = NOT ALLOCATED
0_PCT_FULL Mixed Ext
(1:80) - = NOT ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:81) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:82) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:83) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:84) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:85) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:86) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:87) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:88) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:89) - = NOT ALLOCATED
0_PCT_FULL Mixed Ext
(1:90) - = NOT ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:91) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:92) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:93) - = NOT ALLOCATED
0_PCT_FULL Mixed Ext
(1:94) - = NOT ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:95) - = ALLOCATED
0_PCT_FULL Has Ghost Mixed Ext
(1:96) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:97) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:98) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:99) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:100) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:101) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:102) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:103) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:104) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:105) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:106) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:107) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:108) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:109) - = NOT ALLOCATED
0_PCT_FULL Mixed Ext
(1:110) - = NOT ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:111) - (1:113) = ALLOCATED
0_PCT_FULL Mixed Ext
(1:114) - (1:115) = NOT ALLOCATED
0_PCT_FULL Mixed Ext
(1:116) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:117) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:118) - = NOT ALLOCATED
0_PCT_FULL Mixed Ext
(1:119) - = NOT ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:120) - = NOT ALLOCATED
0_PCT_FULL Mixed Ext
(1:121) - = NOT ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:122) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:123) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:124) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:125) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:126) - = NOT ALLOCATED
0_PCT_FULL Has Ghost Mixed Ext
(1:127) - = NOT ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:128) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:129) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:130) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:131) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:132) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:133) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:134) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:135) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:136) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:137) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:138) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:139) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:140) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:141) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:142) - (1:144) = ALLOCATED
0_PCT_FULL Mixed Ext
(1:145) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:146) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:147) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:148) - = NOT ALLOCATED
50_PCT_FULL Mixed Ext
(1:149) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:150) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:151) - = NOT ALLOCATED
100_PCT_FULL Mixed Ext
(1:152) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:153) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:154) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:155) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:156) - (1:158) = ALLOCATED
0_PCT_FULL Mixed Ext
(1:159) - = ALLOCATED
100_PCT_FULL Mixed Ext
(1:160) - = ALLOCATED
50_PCT_FULL Mixed Ext
(1:161) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:162) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:163) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:164) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:165) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:166) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:167) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:168) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:169) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:170) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:171) - = ALLOCATED
0_PCT_FULL Mixed Ext
(1:172) - = ALLOCATED
0_PCT_FULL IAM Page Mixed Ext
(1:173) - (1:175) = NOT ALLOCATED
0_PCT_FULL
(1:176) - (1:181) = ALLOCATED
0_PCT_FULL
(1:182) - (1:383) = NOT
ALLOCATED 0_PCT_FULL
As you can see all extents after page 183
are not allocated. Similarly you check for page 3 (SGAM).
