内容简介：SQL Server使用sys.master_files计算tempdb大小不正确
一直习惯使用 sys . master_files 来统计数据库的大小以及使用情况，但是发现 sys . master_files 不能准确统计 tempdb 的数据库大小信息。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'不允许增长' WHEN max_size = -1 THEN N'自动增长' 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，你会看到这些文件实际大小为 18G 多，而不是 1G 大小，而使用 sys.master_files 统计的 Size （ GB) 仅仅是 tempdb 文件的初始化大小，当然，你在 SSMS 里面使用 UI 去查看 tempdb 的属性发现其大小值又是正确的，
如果你用 Profile 跟踪看看具体 SQL 如下，你会发现，它统计的数据来源于视图 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 的具体定义如下
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 的具体定义如下：
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
从上面 SQL 脚本可以看到，统计数据库的大小分别来自于 sys.sysprufiles 和 master.sys.sysbrickfiles 这两个表，然后我们就很难再深入了解具体的原因了。在 https://connect.microsoft.com/SQLServer/feedback/details/377223/sys-master-files-does-not-show-accurate-size-information 这个链接里面，对 tempdb 相关的问题有一些描述：
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.
大体意思， sys.master_files 中的数据是异步更新的，而不是同步更新的。它不会立即更新。当你重启你的 SQL Server 时， SQL Server 启动时候都会根据 sys.master_files 中值重新创建、初始化 tempdb 文件大小。而 sys.database_files 显示当前使用的 tempdb 文件。 很可能并非所有的 tempdb 数据文件都在线。 所以，如果要查询 tempdb 的准确大小，就要使用 sys.database_files 来查询！
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