Sunday, 21 December 2014

Introduction to DMV's and DMF's

Introduction to DMV's and DMF's in SQL Server 2005 

The DMVs; newly introduced in SQL Server 2005 gives the database administrator information about the current state of the SQL Server machine. These values will help the administrator to diagnose problems and tune the server for optimal performance. DMVs are designed to be used instead of system tables and various other functions provided in SQL Server 2000. In this article, I will be explaining only about the frequently used DMVs.
Two types of dynamic management views:
  1. Server-scoped DMV: Stored in Master Database
  2. Database-scoped DMV: Specific to each database

Permission to Execute DMV [Security]
To query a server scoped DMV, the database user must have SELECT privilege on VIEW SERVER STATE and for database scoped DMV, the user must have SELECT privilege on VIEW DATABASE STATE.
  • GRANT VIEW SERVER STATE to <Login>
  • GRANT VIEW DATABASE STATE to <User>
If you want to deny a user permission to query certain DMVs, you can use the DENY command to restrict access to a specific DMV.
Getting Started
All the DMVs exits in SYS schema and their names start with DM_. So when you need to query a DMV, you should prefix the view name with SYS. As an example, if you need to see the total physical memory of the SQL Server machine; then execute the below TSQL command:
SELECT
(Physical_memory_in_bytes/1024.0)/1024.0 AS Physical_memory_in_Mb
FROM
sys.dm_os_sys_info
In this article, I will be explaining some of the DMVs which can be used frequently to understand the current behavior of SQL Server:
  1. SQL Server related [Hardware Resources] DMV
  2. Database related DMV
  3. Index related DMV
  4. Execution related DMV
1. SQL Server Related DMV
This section details the DMVs associated with SQL Server system. SQL DMV is responsible to manage server level resources specific to a SQL Server instance.
This section covers DMVs related to OS, Disk and Memory.
a. sys.dm_os_sys_info
This view returns the information about the SQL Server machine, available resources and the resource consumption.
This view returns information like the following:
  1. CPU Count: Number of logical CPUs in the server
  2. Hyperthread-ratio: Ratio of logical and physical CPUs
  3. Physical_memory_in_bytes: Amount of physical memory available
  4. Virtual_memory_in_bytes: Amount of virtual memory available
  5. Bpool_commited: Committed physical memory in buffer pool
  6. OS_Priority_class: Priority class for SQL Server process
  7. Max_workers_thread: Maximum number of workers which can be created
b. sys.dm_os_hosts
This view returns all the hosts registered with SQL Server 2005. This view also provides the resources used by each host.
  1. Name: Name of the host registered
  2. Type: Type of hosted component [SQL Native Interface/OLE DB/MSDART]
  3. Active_tasks_count: Number active tasks host placed
  4. Active_ios_count: I/O requests from host waiting
c. sys.dm_os_schedulers
Sys.dm_os_schedulers view will help you identify if there is any CPU bottleneck in the SQL Server machine. The number of runnable tasks is generally a nonzero value; a nonzero value indicates that tasks have to wait for their time slice to run. If the runnable task counts show high values, then there is a symptom of CPU bottleneck.
SELECT
scheduler_id,current_tasks_count,runnable_tasks_count
FROM sys.dm_os_schedulers
WHERE scheduler_id < 255
The above query will list all the available schedulers in the SQL Server machine and the number of runnable tasks for each scheduler.
d. sys.dm_io_pending_io_requests
This dynamic view will return the I/O requests pending in SQL Server side. It gives you information like:
  1. Io_type: Type of pending I/O request
  2. Io_pending: Indicates whether the I/O request is pending or has been completed by Windows
  3. Scheduler_address: Scheduler on which this I/O request was issued
e. sys.dm_io_virtual_file_stats
This view returns I/O statistics for data and log files [MDF and LDF file]. This view is one of the commonly used views and will help you to identify I/O file level. This will return information like:
  1. Sample_ms: Number of milliseconds since the instance of SQL Server has started
  2. Num_of_reads: Number of reads issued on the file
  3. Num_of_bytes_read: Total number of bytes read on this file
  4. Io_stall_read_ms: Total time, in milliseconds, that the users waited for reads issued on the file
  5. Num_of_writes: Number of writes made on this file
  6. Num_of_bytes_written: Total number of bytes written to the file
  7. Io_stall_write_ms: Total time, in milliseconds, that users waited for writes to be completed on the file
  8. Io_stall: Total time, in milliseconds, that users waited for I/O to be completed
  9. Size_on_disk_bytes: Number of bytes used on the disk for this file
f. sys.dm_os_memory_clerks
This DMV will help how much memory SQL Server has allocated through AWE.
SELECT
SUM(awe_allocated_kb) / 1024 as [AWE allocated, Mb]
FROM sys.dm_os_memory_clerks
The same DMV can be used to get the memory consumption by internal components of SQL Server 2005.

SELECT TOP 10 type,
SUM(single_pages_kb) as [SPA Mem, Kb]
FROM sys.dm_os_memory_clerks
GROUP BY type
ORDER BY SUM(single_pages_kb) DESC
g. sys.dm_os_ring_buffers
This DMV uses RING_BUFFER_RESOURCE_MONITOR and gives information from resource monitor notifications to identify memory state changes. Internally, SQL Server has a framework that monitors different memory pressures. When the memory state changes, the resource monitor task generates a notification. This notification is used internally by the components to adjust their memory usage according to the memory state.

SELECT
Record FROM sys.dm_os_ring_buffers
WHERE ring_buffer_type = 'RING_BUFFER_RESOURCE_MONITOR'
The output of the above query will be in XML format. The output will help you in detecting any low memory notification.
RING_BUFFER_OOM: Ring buffer oom contains records indicating server out-of-memory conditions.
SELECT
record FROM sys.dm_os_ring_buffers
WHERE ring_buffer_type = 'RING_BUFFER_OOM'
2. Database Related DMV
This section details the DMVs associated with SQL Server Databases. These DMVs will help to identify database space usages, partition usages, session information usages, etc...
a. sys.dm_db_file_space_usage
This DMV provides the space usage information of TEMPDB database.
b. sys.dm_db_session_space_usage
This DMV provides the number of pages allocated and de-allocated by each session for the database
c. sys.dm_db_partition_stats
This DMV provides page and row-count information for every partition in the current database.
The below query shows all counts for all partitions of all indexes and heaps in the MSDB database:
USE MSDB;
GO
SELECT * FROM sys.dm_db_partition_stats;
The following query shows all counts for all partitions of Backup set table and its indexes
USE MSDB
GO
SELECT * FROM sys.dm_db_partition_stats
WHERE object_id = OBJECT_ID('backupset');
d. sys.dm_os_performance_counters
Returns the SQL Server / Database related counters maintained by the server.
The below sample query uses the dm_os_performance_counters DMV to get the Log file usage for all databases in KB.
SELECT instance_name
,cntr_value 'Log File(s) Used Size (KB)'
FROM sys.dm_os_performance_counters
WHERE counter_name = 'Log File(s) Used Size (KB)'
3. INDEX Related DMV
This section details the DMVs associated with SQL Server Databases. These DMVs will help to identify database space usages, Partition usages, Session information usages, etc.
a. sys.dm_db_index_usage_stats
This DMV is used to get useful information about the index usage for all objects in all databases. This also shows the amount of seeks and scan for each index.
SELECT object_id, index_id, user_seeks, user_scans, user_lookups
FROM sys.dm_db_index_usage_stats
ORDER BY object_id, index_id
All indexes which have not been used so far in as database can be identified using the below Query:

SELECT object_name(i.object_id),
i.name,
s.user_updates,
s.user_seeks,
s.user_scans,
s.user_lookups
from sys.indexes i
left join sys.dm_db_index_usage_stats s
on s.object_id = i.object_id and
i.index_id = s.index_id and s.database_id = 5
where objectproperty(i.object_id, 'IsIndexable') = 1 and
s.index_id is null or
(s.user_updates > 0 and s.user_seeks = 0
and s.user_scans = 0 and s.user_lookups = 0)
order by object_name(i.object_id)
Replace the Database_id with the database you are looking at.
4. Execution Related DMV
Execution related DMVs will provide information regarding sessions, connections, and various requests which are coming into the SQL Server.
a. sys.dm_exec_sessions
This DMV will give information on each session connected to SQL Server. This DMV is similar to running sp_who2 or querying Master..sysprocesses table.
SELECT
session_id,login_name,
last_request_end_time,cpu_time
FROM sys.dm_exec_sessions
WHERE session_id >= 51 – All user Sessions
b. sys.dm_exec_connections
This DMV shows all the connection to SQL Server. The below query uses sys.dm_exec_connections DMV to get connection information. This view returns one row for each user connection (Sessionid > =51).
SELECT
connection_id,
session_id,client_net_address,
auth_scheme
FROM sys.dm_exec_connections

c. sys.dm_exec_requests
This DMV will give details on what each connection is actually performing in SQL Server.
SELECT
session_id,status,
command,sql_handle,database_id
FROM sys.dm_exec_requests
WHERE session_id >= 51
d. sys.dm_exec_sql_text
This dynamic management function returns the text of a SQL statement given a SQL handle.
SELECT
st.text
FROM
sys.dm_exec_requests r
CROSS APPLY
sys.dm_exec_sql_text(sql_handle) AS st
WHERE r.session_id = 51
Conclusion
Dynamic Management views (DMV) and Dynamic Management Functions (DMF) in SQL Server 2005 give a transparent view of what is going on inside various areas of SQL Server. By using them, we will be able to query the system for information about its current state in a much more effective manner and provide solutions much faster. DMVs can be used to performance tune and for troubleshooting server and queries. This article has shown an overview of what they are and how we can use them.

 

There are multiple categories in which these views and functions have been organized. The below table shows the split:  
Categories
Count
dm_broker*
4
dm_clr*
4
dm_db*
12
dm_exec*
14
dm_fts*
5
dm_io*
4
dm_os*
27
dm_qn*
1
dm_repl*
4
dm_tran*
10
So we have 85 of these views and function. To give a further split, 76 of these are views and 9 of them are functions. So these information can be queried from the system_objects system catalog table. A typical query I used was:
select * from sys.system_objects Where name like 'dm_%' Order by 1

Each of these views and functions have different parameters or output columns and in the next couple of queries we will try to find out how to get these values.
-- Getting the column details of the DMV's
Select o.name, c.name, t.name, c.column_id, c.max_length, c.precision, c.scale
FROM sys.system_columns c
INNER JOIN sys.system_objects o
ON c.object_id = o.object_id
INNER JOIN sys.types t
ON c.user_type_id = t.user_type_id
Where o.name = 'dm_os_loaded_modules'
order by 1

In the above query we query we get the output columns for the DMV (dm_os_loaded_modules) using the system objects. In the above query we get details like name of the output column, datatype and other length specific values. Even though this will not get us the values for the table valued functions. We will have to tweak the above query for DMF's.
-- Getting the column details of the DMF's
Select o.name, t.name, p.*
FROM sys.system_parameters p
INNER JOIN sys.system_objects o
ON p.object_id = o.object_id
INNER JOIN sys.types t
ON p.user_type_id = t.user_type_id
Where o.name = 'dm_exec_sql_text'
order by 1
In the above query we try to get the parameters for the DMF (dm_exec_sql_text) using the systtem_parameters system catalog. So the output would show the above DMF has a parameter @handle. So if we queried this function for the sql text for a given query in the cache. The handle can be got from dm_exec_query_stats or other related views.


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