一直習慣使用sys.master_files來統計數據庫的大小以及使用情況，但是發現sys.master_files不能准確統計tempdb的數據庫大小信息。如下所示：
SELECT database_id AS DataBaseId
,DB_NAME(database_id) AS DataBaseName
,Name AS LogicalName
,type_desc AS FileTypeDesc
,Physical_Name AS PhysicalName
,State_Desc AS StateDesc
,CASE WHEN max_size = 0 THEN N'不允許增長'
WHEN max_size = -1 THEN N'自動增長'
ELSE LTRIM(STR(max_size * 8.0 / 1024 / 1024, 14, 2))
+ 'G'
END AS MaxSize
,CASE WHEN is_percent_growth = 1
THEN RTRIM(CAST(Growth AS CHAR(10))) + '%'
ELSE RTRIM(CAST(Growth*8.0/1024 AS CHAR(10))) + 'M'
END AS Growth
,Is_Read_Only AS IsReadOnly
,Is_Percent_Growth AS IsPercentGrowth
,CAST(size * 8.0 / 1024 / 1024 AS DECIMAL(8, 4)) AS [Size(GB)]
FROM sys.master_files
WHERE database_id =2
ORDER BY 1
在Windows窗口里，你會看到這些文件實際大小為18G多，而不是1G大小，而使用sys.master_files統計的Size（GB)僅僅是tempdb文件的初始化大小，當然，你在SSMS里面使用UI去查看tempdb的屬性發現其大小值又是正確的，
如果你用Profile跟蹤看看具體SQL如下，你會發現，它統計的數據來源於視圖sys.database_files
USE tempdb;
GO
SELECT s.name AS [Name] ,
CAST(FILEPROPERTY(s.name, 'SpaceUsed') AS FLOAT) * CONVERT(FLOAT, 8) AS [UsedSpace] ,
CAST(CASE WHEN s.growth = 0 THEN ( CASE WHEN s.type = 2 THEN 0
ELSE 99
END )
ELSE s.is_percent_growth
END AS INT) AS [GrowthType] ,
s.physical_name AS [FileName] ,
s.size * CONVERT(FLOAT, 8) AS [Size] ,
CASE WHEN s.max_size = -1 THEN -1
ELSE s.max_size * CONVERT(FLOAT, 8)
END AS [MaxSize] ,
s.file_id AS [ID] ,
'Server[@Name='
+ QUOTENAME(CAST(SERVERPROPERTY(N'Servername') AS sysname), '''')
+ ']' + '/Database[@Name=' + QUOTENAME(DB_NAME(), '''') + ']'
+ '/LogFile[@Name=' + QUOTENAME(s.name, '''') + ']' AS [Urn] ,
CAST(CASE s.is_percent_growth
WHEN 1 THEN s.growth
ELSE s.growth * 8
END AS FLOAT) AS [Growth] ,
s.is_media_read_only AS [IsReadOnlyMedia] ,
s.is_read_only AS [IsReadOnly] ,
CAST(CASE s.state
WHEN 6 THEN 1
ELSE 0
END AS BIT) AS [IsOffline] ,
s.is_sparse AS [IsSparse]
FROM sys.database_files AS s
WHERE ( s.type = 1 )
ORDER BY [Name] ASC;
sys.database_files的具體定義如下
SET quoted_identifier ON
SET ansi_nulls ON
go
CREATE VIEW sys.database_files
AS
SELECT file_id = f.fileid,
file_guid = f.fileguid,
type = f.filetype,
type_desc = ft.NAME,
data_space_id = f.grpid,
NAME = f.lname,
physical_name = f.pname,
state = CONVERT(TINYINT, CASE f.filestate
-- Map enum EMDFileState to AvailablityStates
WHEN 0 THEN 0
WHEN 10 THEN 0 -- ONLINE
WHEN 4 THEN 7 -- DEFUNCT
WHEN 5 THEN 3
WHEN 9 THEN 3 -- RECOVERY_PENDING
WHEN 7 THEN 1
WHEN 8 THEN 1
WHEN 11 THEN 1 -- RESTORING
WHEN 12 THEN 4 -- SUSPECT
ELSE 6
END),-- OFFLINE
state_desc = st.NAME,
size = Isnull(Filepropertybyid(f.fileid, 'size'), size),
max_size = f.maxsize,
f.growth,
is_media_read_only = Sysconv(bit, f.status & 8),-- FIL_READONLY_MEDIA
is_read_only = Sysconv(bit, f.status & 16),-- FIL_READONLY
is_sparse = Sysconv(bit, f.status & 256),-- FIL_SPARSE_FILE
is_percent_growth = Sysconv(bit, f.status & 32),-- FIL_PERCENT_GROWTH
is_name_reserved = Sysconv(bit, CASE f.filestate
WHEN 3 THEN 1
ELSE 0
END),-- x_efs_DroppedReusePending
create_lsn = Getnumericlsn(f.createlsn),
drop_lsn = Getnumericlsn(f.droplsn),
read_only_lsn = Getnumericlsn(f.readonlylsn),
read_write_lsn = Getnumericlsn(f.readwritelsn),
differential_base_lsn = Getnumericlsn(f.diffbaselsn),
differential_base_guid = f.diffbaseguid,
differential_base_time = NULLIF(f.diffbasetime, 0),
redo_start_lsn = Getnumericlsn(f.redostartlsn),
redo_start_fork_guid = f.redostartforkguid,
redo_target_lsn = Getnumericlsn(f.redotargetlsn),
redo_target_fork_guid = f.forkguid,
backup_lsn = Getnumericlsn(f.backuplsn)
FROM sys.sysprufiles f
LEFT JOIN sys.syspalvalues st
ON st.class = 'DBFS'
AND st.value = f.filestate
LEFT JOIN sys.syspalvalues ft
ON ft.class = 'DBFT'
AND ft.value = f.filetype
WHERE filestate NOT IN ( 1, 2 ) -- x_efs_Dummy, x_efs_Dropped
go
sys.master_files的具體定義如下：
SET quoted_identifier ON
SET ansi_nulls ON
go
CREATE VIEW sys.master_files
AS
SELECT database_id = f.dbid,
file_id = f.fileid,
file_guid = f.fileguid,
type = f.filetype,
type_desc = ft.NAME,
data_space_id = f.grpid,
NAME = f.lname,
physical_name = f.pname,
state = CONVERT(TINYINT, CASE f.filestate
-- Map enum EMDFileState to AvailablityStates
WHEN 0 THEN 0
WHEN 10 THEN 0 -- ONLINE
WHEN 4 THEN 7 -- DEFUNCT
WHEN 5 THEN 3
WHEN 9 THEN 3 -- RECOVERY_PENDING
WHEN 7 THEN 1
WHEN 8 THEN 1
WHEN 11 THEN 1 -- RESTORING
WHEN 12 THEN 4 -- SUSPECT
ELSE 6
END),-- OFFLINE
state_desc = st.NAME,
f.size,
max_size = f.maxsize,
f.growth,
is_media_read_only = Sysconv(bit, f.status & 8),-- FIL_READONLY_MEDIA
is_read_only = Sysconv(bit, f.status & 16),-- FIL_READONLY
is_sparse = Sysconv(bit, f.status & 256),-- FIL_SPARSE_FILE
is_percent_growth = Sysconv(bit, f.status & 32),-- FIL_PERCENT_GROWTH
is_name_reserved = Sysconv(bit, CASE f.filestate
WHEN 3 THEN 1
ELSE 0
END),-- x_efs_DroppedReusePending
create_lsn = Getnumericlsn(f.createlsn),
drop_lsn = Getnumericlsn(f.droplsn),
read_only_lsn = Getnumericlsn(f.readonlylsn),
read_write_lsn = Getnumericlsn(f.readwritelsn),
differential_base_lsn = Getnumericlsn(f.diffbaselsn),
differential_base_guid = f.diffbaseguid,
differential_base_time = NULLIF(f.diffbasetime, 0),
redo_start_lsn = Getnumericlsn(f.redostartlsn),
redo_start_fork_guid = f.redostartforkguid,
redo_target_lsn = Getnumericlsn(f.redotargetlsn),
redo_target_fork_guid = f.forkguid,
backup_lsn = Getnumericlsn(f.backuplsn)
FROM master.sys.sysbrickfiles f
LEFT JOIN sys.syspalvalues st
ON st.class = 'DBFS'
AND st.value = f.filestate
LEFT JOIN sys.syspalvalues ft
ON ft.class = 'DBFT'
AND ft.value = f.filetype
WHERE f.dbid < 0x7fff -- consistent with sys.databases
AND f.pruid = 0
AND f.filestate NOT IN ( 1, 2 ) -- x_efs_Dummy, x_efs_Dropped
AND Has_access('MF', 1) = 1
go
從上面SQL腳本可以看到，統計數據庫的大小分別來自於sys.sysprufiles 和master.sys.sysbrickfiles這兩個表，然后我們就很難再深入了解具體的原因了。在https://connect.microsoft.com/SQLServer/feedback/details/377223/sys-master-files-does-not-show-accurate-size-information 這個鏈接里面，對tempdb相關的問題有一些描述：
1. The view sys.master_files is something new and is updated asynchronously. It doesn't updates immediately.
2. When you re-start your SQL Server, SQL Server will re-create tempdb based on sys.master_files.
3. The sys.master_files tell you about any tempdb data file which was there on your system (the number of tempdb files) with which your server have started.
4. While sys.database_files shows currently used tempdb files. Its quite possible that not all tempdb data files came online.
5. You can read the errorlog look for any error meesage did any of the files did not come online.
6. After you started sql server somebody might have executed SQL commands to remove or add tempdb files.
大體意思，sys.master_files中的數據是異步更新的，而不是同步更新的。它不會立即更新。當你重啟你的SQL Server時，SQL Server啟動時候都會根據sys.master_files中值重新創建、初始化tempdb文件大小。而sys.database_files顯示當前使用的tempdb文件。 很可能並非所有的tempdb數據文件都在線。 所以，如果要查詢tempdb的准確大小，就要使用sys.database_files來查詢！