This post has been republished via RSS; it originally appeared at: Azure Database Support Blog articles.
Today, we addressed a service request from our customer who needed to identify both the session and the text of the query responsible for a significant increase in the transaction log of the tempdb among the databases within the Elastic DB Pool. This increment in the transaction log was leading to an error message: “Msg 9002, Level 17, State 4 - The transaction log for database 'tempdb' is full due to 'ACTIVE_TRANSACTION', with the holdup identified as (XXX:NNN:YYY)"
The TempDB database in Azure SQL plays a vital role in the performance and management of your SQL instances. Monitoring its transactions closely helps ensure smooth operation and efficient troubleshooting when issues arise. In this article, we’ll explore a PowerShell script designed to monitor transactions within TempDB in the context of Azure SQL Elastic Pools.
Objective
Our main aim is to retrieve and control information about TempDB's transaction log usage by different sessions and correlate this with the executing SQL commands. By doing so, we gain invaluable insights into which sessions and queries are consuming significant amounts of the transaction log space, a critical resource. Managing the space used by transactions is crucial for preventing potential issues associated with resource contention, and for ensuring the smooth performance of TempDB and the SQL instance as a whole. This monitoring approach is significant for administrators managing systems with high transaction rates or where resources are at a premium.
PowerShell Script Overview
1. Setting Up Variables:
The script begins by initializing variables with the user’s Azure SQL server name, user ID, password, elastic pool name, and the output file's path.
$serverName = "your_server_name.database.windows.net"
$userId = "your_user_id"
$password = "your_password"
$outputFilePath = "C:\path\to\your\directory\output.json"
2. Executing Queries with Retry Policy:
Two functions, ExecuteDbNamesQuery and ExecuteTransactionQuery, are defined. Both are designed to execute SQL queries with a retry policy in case of transient failures, a common scenario in cloud environments.
3. Retrieving Database Names:
The script retrieves the names of all databases within a specified elastic pool. This step is crucial since transactions are tracked per database.
$databasesQuery = "SELECT name FROM sys.databases WHERE database_id IN (SELECT database_id FROM [sys].[database_service_objectives] WHERE elastic_pool_name = '$elasticPoolName')"
4. Transaction Monitoring Query:
For each database, a detailed query is executed. This query fetches essential transaction information, including session ID, transaction ID, database ID, database name, transaction log used (in KB), transaction begin time, transaction type, transaction state, and the executing command text.
5. Data Compilation & Export:
Results from each database are compiled and exported as a JSON file. JSON format is chosen for its readability and ease of use in subsequent data manipulation or analysis tasks.
$allResults | ConvertTo-Json | Out-File $outputFilePath
Why Monitoring Space is Essential
- Preventing Overuse: Transactions that consume excessive log space can lead to performance degradation and can cause the system to run out of space.
- Optimizing Performance: Understand which sessions and queries are heavy users of the transaction log, and optimize or redesign them for efficient log usage.
- Resource Management: Managing the use of transaction log space is crucial for cost control and efficient resource allocation in environments where resources are finite.
- Troubleshooting: Having detailed information on transaction log usage aids in quickly identifying and resolving transaction processing issues.
Understanding the Results
- Session ID & Transaction ID: Identifies sessions and transactions within those sessions.
- Database ID & Name: These fields provide context for understanding the transaction's role and impact.
- Log Used (in KB) & Begin Time: Helps identify long-running or resource-intensive transactions.
- Transaction Type & State: Describes the nature and current state of the transaction.
- Executing Command Text: Retrieves the SQL command being executed in the session, offering insight into the operations being performed.
Conclusion
Monitoring and controlling TempDB transactions, especially regarding transaction log space, is indispensable in Azure SQL Elastic Pools. The provided PowerShell script offers a robust solution for retrieving, understanding, and analyzing transaction data, aiding database administrators in their ongoing management and optimization efforts. Always ensure to run the script in a secure and test environment before deploying it in a production setting.
$serverName = "your_server_name.database.windows.net"
$userId = "your_user_id"
$password = "your_password"
$outputFilePath = "C:\path\to\your\directory\output.json"
$elasticPoolName = "elasticdbpoolname"
function ExecuteDbNamesQuery($connectionString, $query) {
$maxRetries = 3
$retryInterval = 2 # seconds
$results = @()
for ($i = 0; $i -lt $maxRetries; $i++) {
try {
$conn = New-Object System.Data.SqlClient.SqlConnection
$conn.ConnectionString = $connectionString
$conn.Open()
$command = $conn.CreateCommand()
$command.CommandText = $query
$reader = $command.ExecuteReader()
while ($reader.Read()) {
$results += "$($reader["name"])"
}
$conn.Close()
return $results
} catch {
Write-Output "Error: $_"
Start-Sleep -Seconds $retryInterval
}
}
Write-Output "The query failed after $maxRetries."
return $results
}
function ExecuteTransactionQuery($connectionString, $query) {
$maxRetries = 3
$retryInterval = 2 # seconds
$results = @()
for ($i = 0; $i -lt $maxRetries; $i++) {
try {
$conn = New-Object System.Data.SqlClient.SqlConnection
$conn.ConnectionString = $connectionString
$conn.Open()
$command = $conn.CreateCommand()
$command.CommandText = $query
$reader = $command.ExecuteReader()
while ($reader.Read()) {
$row = @{
SessionId = $reader["session_id"]
TransactionId = $reader["transaction_id"]
DatabaseId = $reader["database_id"]
DatabaseName = $reader["database_name"]
LogUsedKb = $reader["database_transaction_log_used_Kb"]
BeginTime = $reader["database_transaction_begin_time"]
TypeDesc = $reader["transaction_type_desc"]
StateDesc = $reader["transaction_state_desc"]
CommandText = $reader["executing_command_text"]
}
$results += ,$row
}
$conn.Close()
return $results
} catch {
Write-Output "Error: $_"
Start-Sleep -Seconds $retryInterval
}
}
Write-Output "The query failed after $maxRetries ."
return $results
}
$masterConnString = "Server=$serverName;Database=master;User Id=$userId;Password=$password;"
$databasesQuery = "SELECT name FROM sys.databases WHERE database_id IN (SELECT database_id FROM [sys].[database_service_objectives] WHERE elastic_pool_name = '$elasticPoolName')"
$databases = ExecuteDbNamesQuery $masterConnString $databasesQuery
$allResults = @()
foreach ($db in $databases) {
$dbConnString = "Server=$serverName;Database=$db;User Id=$userId;Password=$password;"
$transactionQuery = @"
SELECT
ST.session_id,
ST.transaction_id,
DT.database_id,
CASE WHEN D.name IS NULL AND DT.database_id = 2 THEN 'TEMPDB' ELSE D.name END as [database_name],
CONVERT(numeric(18,2), DT.database_transaction_log_bytes_used / 1024.0 ) as [database_transaction_log_used_Kb],
DT.database_transaction_begin_time,
CASE database_transaction_type
WHEN 1 THEN 'Read/write transaction'
WHEN 2 THEN 'Read-only transaction'
WHEN 3 THEN 'System transaction'
WHEN 4 THEN 'Distributed transaction'
END as [transaction_type_desc],
CASE database_transaction_state
WHEN 0 THEN 'The transaction has not been completely initialized yet'
WHEN 1 THEN 'The transaction has been initialized but has not started'
WHEN 2 THEN 'The transaction is active'
WHEN 3 THEN 'The transaction has ended. This is used for read-only transactions'
WHEN 4 THEN 'The commit process has been initiated on the distributed transaction. This is for distributed transactions only. The distributed transaction is still active but further processing cannot take place'
WHEN 5 THEN 'The transaction is in a prepared state and waiting resolution.'
WHEN 6 THEN 'The transaction has been committed'
WHEN 7 THEN 'The transaction is being rolled back'
WHEN 8 THEN 'The transaction has been rolled back'
END as [transaction_state_desc],
substring
(REPLACE
(REPLACE
(SUBSTRING
(T.text
, (req.statement_start_offset/2) + 1
, (
(CASE statement_end_offset
WHEN -1
THEN DATALENGTH(T.text)
ELSE req.statement_end_offset
END
- req.statement_start_offset)/2) + 1)
, CHAR(10), ' '), CHAR(13), ' '), 1, 4000) as [executing_command_text]
FROM
sys.dm_tran_database_transactions DT
INNER JOIN sys.dm_tran_session_transactions ST
ON DT.transaction_id = ST.transaction_id
LEFT JOIN sys.databases D
ON DT.database_id = D.database_id
INNER JOIN sys.dm_exec_requests req
ON ST.session_id = Req.session_id
OUTER APPLY sys.dm_exec_sql_text(req.sql_handle) as T
ORDER BY
ST.session_id
"@
$results = ExecuteTransactionQuery $dbConnString $transactionQuery
$allResults += $results
}
# Convierte todos los resultados a JSON y escribe en el archivo
$allResults | ConvertTo-Json | Out-File $outputFilePath
Additional Information: Azure SQL DB and TEMPDB usage tracking - Microsoft Community Hub
This script contains an example how to fill up the transaction log of TEMPDB:
-- Create a temporal table
CREATE TABLE #LargeTempTable (
ID INT PRIMARY KEY IDENTITY(1,1),
Name NVARCHAR(50),
Value INT
);
-- Variables
DECLARE @counter INT = 0;
DECLARE @name NVARCHAR(50);
DECLARE @value INT;
-- Start the transaction
BEGIN TRANSACTION;
-- Insert loop
WHILE @counter < 1000000 --
BEGIN
SET @name = 'Name' + CAST(@counter AS NVARCHAR(50));
SET @value = CAST(RAND() * 1000 AS INT);
INSERT INTO #LargeTempTable (Name, Value)
VALUES (@name, @value);
SET @counter = @counter + 1;
END
-- ROLLBACK TRANSACTION;
DROP TABLE #LargeTempTable;
