INF: Understanding and resolving SQL Server blocking problems
The duration and transaction context of a query determine how long its locks are held and, thereby, their impact on other queries. If the query is not executed within a transaction (and no lock hints are used), the locks for SELECT statements will only be held on a resource at the time it is actually being read, not for the duration of the query. For INSERT, UPDATE, and DELETE statements, the locks are held for the duration of the query, both for data consistency and to allow the query to be rolled back if necessary.
For queries executed within a transaction, the duration for which the locks are held are determined by the type of query, the transaction isolation level, and whether or not lock hints are used in the query. For a description of locking, lock hints, and transaction isolation levels, see the following topics in SQL Server Books Online:
- Locking in the Database Engine
- Customizing Locking and Row Versioning
- Lock Modes
- Lock Compatibility
- Row Versioning-based Isolation Levels in the Database Engine
- Controlling Transactions (Database Engine)
- A SPID holds locks on a set of resources for an extended period of time before releasing them. This type of blocking resolves itself over time, but can cause performance degradation.
- A SPID holds locks on a set of resources and never releases them. This type of blocking does not resolve itself and prevents access to the affected resources indefinitely.
Gathering Blocking InformationTo counteract the difficulty of troubleshooting blocking problems, a database administrator can use SQL scripts that constantly monitor the state of locking and blocking on SQL Server. These scripts can provide snapshots of specific instances over time, leading to an overall picture of the problem. For a description of how to monitor blocking with SQL scripts, see the following articles in the Microsoft Knowledge Base:
- Identify the SPID (Session ID) at the head of the blocking chain and the SQL Statement.
In addition to using the scripts in the previously mentioned Knowledge Base article, you can identify the head of the blocking chain by using features that are provided through SQL Server Management Studio. To do this, use one of the following methods:
- Right-click the server object, expand Reports, expand Standard Reports, and then click Activity – All Blocking Transactions. This report shows the transactions at the head of blocking chain. If you expand the transaction, the report will show the transactions that are blocked by the head transaction. This report will also show the "Blocking SQL Statement" and the "Blocked SQL Statement."
- Use DBCC INPUTBUFFER(<spid>) to find the last statement that was submitted by a SPID.
- Find the transaction nesting level and process status of the blocking SPID.
The transaction nesting level of a SPID is available in the @@TRANCOUNT global variable. However, it can be determined from outside the SPID by querying the sysprocesses table as follows:The value returned is the @@TRANCOUNT value for the SPID. This shows the transaction nesting level for the blocking SPID, which in turn can explain why it is holding locks. For example, if the value is greater than zero, the SPID is in the midst of a transaction (in which case it is expected that it retains certain locks it has acquired, depending on the transaction isolation level).
SELECT open_tran FROM master.sys.sysprocesses WHERE SPID=<blocking SPID number>go
You can also check to see if any long-term open transaction exists in the database by using DBCC OPENTRAN database_name.
Gathering SQL Server Profiler Trace InformationIn addition to the above information, it is often necessary to capture a Profiler trace of the activities on the server to thoroughly investigate a blocking problem on SQL Server. If a SPID executes multiple statements within a transaction, only the last statementthat was submitted will show in the report, input buffer, or activity monitor output. However, one of the earlier commands may be the reason locks are still being held. A Profiler trace will enable you to see all of the commands executed by a SPID within the current transaction. The following steps help you to set up SQL Server Profiler to capture a trace.
- Open SQL Server Profiler.
- On the File menu, point to New, and then click Trace.
- On the General tab, specify a trace name and a file name to capture the data to.
Important The trace file should be written to a fast local or shared disk. Avoid tracing to a slow disk or network drive. Also make sure Server processes trace data is selected.
- On the Events Selection tab, click to select the Show all events and the Show all columns check boxes.
- On the Events Selection tab, add the Event types that are listed in Table 1 to your trace.
Additionally, you may include the additional Event types that are listed in Table 2 for further information. If you are running in a high-volume production environment, you may decide to use only the events in Table 1, as they are typically sufficient to troubleshoot most blocking problems. Including the additional events in Table 2 may make it easier to quickly determine the source of a problem (or these events may be necessary to identify the culprit statement in a multi-statement procedure). However, including events in Table 2 will also add to the load on the system and increase the trace output size.
|Errors and Warnings||Exception|
|Errors and Warnings||Attention|
|Security Audit||Audit Login|
|Security Audit||Audit Logout|
Table 2: Additional Event types
For more information about using the SQL Server Profiler, please see SQL Server Books Online.
Identifying and Resolving Common Blocking ScenariosBy examining the above information, you can determine the cause of most blocking problems. The rest of this article is a discussion of how to use this information to identify and resolve some common blocking scenarios. This discussion assumes you have used the blocking scripts in article 271509 (referenced earlier) to capture information on the blocking SPIDs and have made a Profiler trace with the events described above.
Viewing the Blocking Script Output
Examine the sys.sysprocesses output to determine the heads of the blocking chainsIf you did not specify fast mode for the blocking scripts, there will be a section titled "SPIDs at the head of blocking chains" that lists the SPIDs that are blocking other SPIDs in the script output.
SPIDs at the head of blocking chains
Examine the sys.sysprocesses output for information on the SPIDs at the head of the blocking chain.It is important to evaluate the following sys.sysprocesses fields:
This column shows the status of a particular SPID. Typically, a sleeping status indicates that the SPID has completed execution and is waiting for the application to submit another query or batch. A runnable, running, or sos_scheduler_yield status indicates that the SPID is currently processing a query. The following table gives brief explanations of the various status values.
|Background||The SPID is running a background task, such as deadlock detection.|
|Sleeping||The SPID is not currently executing. This usually indicates that the SPID is awaiting a command from the application.|
|Running||The SPID is currently running on a scheduler.|
|Runnable||The SPID is in the runnable queue of a scheduler and waiting to get scheduler time.|
|Sos_scheduler_yield||The SPID was running, but it has voluntarily yielded its time slice on the scheduler to allow another SPID to acquire scheduler time.|
|Suspended||The SPID is waiting for an event, such as a lock or a latch.|
|Rollback||The SPID is in rollback of a transaction.|
|Defwakeup||Indicates that the SPID is waiting for a resource that is in the process of being freed. The waitresource field should indicate the resource in question.|
This field tells you the transaction nesting level of the SPID. If this value is greater than 0, the SPID is within an open transaction and may be holding locks acquired by any statement within the transaction.
Lastwaittype, waittype, and waittime
The lastwaittype field is a string representation of the waittype field, which is a reserved internal binary column. If the waittype is 0x0000, the SPID is not currently waiting for anything and the lastwaittype value indicates the last waittype that the SPID had. If the waittype is not zero, the lastwaittype value indicates the current waittype of the SPID.
For a brief description of the different lastwaittype and waittype values, see the following article in the Microsoft Knowledge base:
The waittime value can be used to determine if the SPID is making progress. When a query against the sys.sysprocesses table returns a value in the waittime column that is less than the waittime value from a previous query of sys.sysprocesses, this indicates that the prior lock was acquired and released and is now waiting on a new lock (assuming non-zero waittime). This can be verified by comparing the waitresource between sys.sysprocesses output.
This field indicates the resource that a SPID is waiting on. The following table lists common waitresource formats and their meaning:
In this case, database ID 5 is the pubs sample database and object ID 261575970 is the titles table and 1 is the clustered index.
In this case, database ID 5 is pubs, file ID 1 is the primary data file, and page 104 is a page belonging to the titles table.
To identify the object id that the page belongs to, use the DBCC PAGE (dbid, fileid, pageid, output_option) command, and look at the m_objId. For example:
|Key||DatabaseID:Hobt_id (Hash value for index key)||KEY: 5:72057594044284928 (3300a4f361aa)|
In this case, database ID 5 is Pubs, Hobt_ID 72057594044284928 corresponds to non clustered index_id 2 for object id 261575970 (titles table). Use the sys.partitions catalog view to associate the hobt_id to a particular index id and object id. There is no way to unhash the index key hash to a specific index key value.
In this case, database ID 5 is pubs , file ID 1 is the primary data file, page 104 is a page belonging to the titles table, and slot 3 indicates the row's position on the page.
|Compile||DatabaseID:ObjectID [[COMPILE]]||TAB: 5:834102012 [[COMPILE]] This is not a table lock, but rather a compile lock on a stored procedure. Database ID 5 is pubs, object ID 834102012 is stored procedure usp_myprocedure. See Knowledge Base Article 263889 for more information on blocking caused by compile locks.|
The remaining sys.sysprocesses columns can provide insight into the root of a problem as well. Their usefulness varies depending on the circumstances of the problem. For example, you can determine if the problem happens only from certain clients (hostname), on certain network libraries (net_library), when the last batch submitted by a SPID was (last_batch), and so on.
Examine the DBCC INPUTBUFFER output.For any SPID at the head of a blocking chain or with a non-zero waittype, the blocking script will execute DBCC INPUTBUFFER to determine the current query for that SPID.
In many cases, this is the query that is causing the locks that are blocking other users to be held. However, if the SPID is within a transaction, the locks may have been acquired by a previously executed query, not the current one. Therefore, you should also view the Profiler output for the SPID, not just the inputbuffer.
Note Because the blocking script consists of multiple steps, it is possible that a SPID may appear in the first section as the head of a blocking chain, but by the time the DBCC INPUTBUFFER query is executed, it is no longer blocking and the INPUTBUFFER is not captured. This indicates that the blocking is resolving itself for that SPID and it may or may not be a problem. At this point, you can either use the fast version of the blocking script to try to ensure you capture the inputbuffer before it clears (although there is still no guarantee), or view the Profiler data from that time frame to determine what queries the SPID was executing.
Viewing the Profiler DataViewing Profiler data efficiently is extremely valuable in resolving blocking issues. The most important thing to realize is that you do not have to look at everything you captured; be selective. Profiler provides capabilities to help you effectively view the captured data. In the Properties dialog box (on the File menu, click Properties), Profiler allows you to limit the data displayed by removing data columns or events, grouping (sorting) by data columns and applying filters. You can search the whole trace or only a specific column for specific values (on the Edit menu, click Find). You can also save the Profiler data to a SQL Server table (on the File menu, point to Save As and then click Table) and run SQL queries against it.
Be careful that you perform filtering only on a previously saved trace file. If you perform these steps on an active trace, you risk losing data that has been captured since the trace was started. Save an active trace to a file or table first (on the File menu, click Save As) and then reopen it (on the File menu, click Open) before proceeding. When working on a saved trace file, the filtering does not permanently remove the data being filtered out, it just does not display all the data. You can add and remove events and data columns as needed to help focus your searches.
What to look for:
- What commands has the SPID at the head of a blocking chain executed within the current transaction?
Filter the trace data for a particular SPID that is at the head of a blocking chain (on the File menu, click Properties; then on the Filters tab specify the SPID value). You can then examine the commands it has executed prior to the time it was blocking other SPIDs. If you include the Transaction events, they can easily identify when a transaction was started. Otherwise, you can search the Text column for BEGIN, SAVE, COMMIT, or ROLLBACK TRANSACTION operations. Use the open_tran value from the sysprocesses table to ensure that you catch all of the transaction events. Knowing the commands executed and the transaction context will allow you to determine why a SPID is holding locks.
Remember, you can remove events and data columns. Instead of looking at both starting and completed events, choose one. If the blocking SPIDs are not stored procedures, remove the SP:Starting or SP:Completed events; the SQLBatch and RPC events will show the procedure call. Only view the SP events when you need to see that level of detail.
- What is the duration of the queries for SPIDs at the head of blocking chains?
If you include the completed events above, the Duration column will show the query execution time. This can help you identify long-running queries that are causing blocking. To determine why the query is performing slowly, view the CPU, Read, and Writes columns, as well as the Execution Plan event.
Categorizing Common Blocking ScenariosThe table below maps common symptoms to their probable causes. The number indicated in the Scenario column corresponds to the number in the "Common Blocking Scenarios and Resolutions" section of this article below. The Waittype, Open_Tran, and Status columns refer to sysprocesses information. The Resolves? column indicates whether or not the blocking will resolve on its own.
|1||Non-zero||>= 0||runnable||Yes, when query finishes.||Physical_IO, CPU and/or Memusage columns will increase over time. Duration for the query will be high when completed.|
|2||0x0000||>0||sleeping||No, but SPID can be killed.||An attention signal may be seen in the Profiler trace for this SPID, indicating a query timeout or cancel has occurred.|
|3||0x0000||>= 0||runnable||No. Will not resolve until client fetches all rows or closes connection. SPID can be killed, but it may take up to 30 seconds.||If open_tran = 0, and the SPID holds locks while the transaction isolation level is default (READ COMMMITTED), this is a likely cause.|
|4||Varies||>= 0||runnable||No. Will not resolve until client cancels queries or closes connections. SPIDs can be killed, but may take up to 30 seconds.||The hostname column in sysprocesses for the SPID at the head of a blocking chain will be the same as one of the SPID it is blocking.|
|5||0x0000||>0||rollback||Yes.||An attention signal may be seen in the Profiler trace for this SPID, indicating a query timeout or cancel has occurred, or simply a rollback statement has been issued.|
|6||0x0000||>0||sleeping||Eventually. When Windows NT determines the session is no longer active, the SQL Server connection will be broken.||The last_batch value in sysprocesses is much earlier than the current time.|
Common Blocking Scenarios and ResolutionsThe scenarios listed below will have the characteristics listed in the table above. This section provides additional details when applicable, as well as paths to resolution.
- Blocking Caused by a Normally Running Query with a Long Execution Time
The solution to this type of blocking problem is to look for ways to optimize the query. Actually, this class of blocking problem may just be a performance problem, and require you to pursue it as such. For information on troubleshooting a specific slow-running query, see the following Microsoft Knowledge Base article:243589 How to troubleshoot slow-running queries on SQL Server 7.0 or on later versionsFor overall application performance troubleshooting, see the following Knowledge Base article:224587 HOW TO: Troubleshoot Application Performance with SQL ServerFor more information, see the Performance Monitoring and Tuning How-to Topics SQL Server 2008 Books Online topic on the following MSDN Web site:
- Blocking Caused by a Sleeping SPID That Has Lost Track of the Transaction Nesting Level
This type of blocking can often be identified by a SPID that is sleeping or awaiting a command, yet whose transaction nesting level (@@TRANCOUNT, open_tran from sysprocesses) is greater than zero. This can occur if the application experiences a query timeout, or issues a cancel without also issuing the required number of ROLLBACK and/or COMMIT statements. When a SPID receives a query timeout or cancel, it will terminate the current query and batch, but does not automatically roll back or commit the transaction. The application is responsible for this, as SQL Server cannot assume that an entire transaction must be rolled back simply due to a single query being canceled. The query timeout or cancel will appear as an ATTENTION signal event for the SPID in the Profiler trace.
To demonstrate this, issue the following simple query from Query Analyzer:While the query is executing, click the red Cancel button. After the query is canceled, SELECT @@TRANCOUNT indicates that the transaction nesting level is one. Had this been a DELETE or an UPDATE query, or had HOLDLOCK been used on the SELECT, all the locks acquired would still be held. Even with the query above, if another query had acquired and held locks earlier in the transaction, they would still be held when the above SELECT was canceled.
BEGIN TRAN SELECT * FROM SYSOBJECTS S1, SYSOBJECTS S2-- Issue this after canceling querySELECT @@TRANCOUNTROLLBACK TRAN
- Applications must properly manage transaction nesting levels, or they may cause a blocking problem following the cancellation of the query in this manner. This can be accomplished in one of several ways:
- In the error handler of the client application, submit an IF @@TRANCOUNT > 0 ROLLBACK TRAN following any error, even if the client application does not believe a transaction is open. This is required, because a stored procedure called during the batch could have started a transaction without the client application's knowledge. Note that certain conditions, such as canceling the query, prevent the procedure from executing past the current statement, so even if the procedure has logic to check IF @@ERROR <> 0 and abort the transaction, this rollback code will not be executed in such cases.
- Use SET XACT_ABORT ON for the connection, or in any stored procedures which begin transactions and are not cleaning up following an error. In the event of a run-time error, this setting will abort any open transactions and return control to the client. Note that T-SQL statements following the statement which caused the error will not be executed.
- If connection pooling is being used in an application that opens the connection and runs a small number of queries before releasing the connection back to the pool, such as a Web-based application, temporarily disabling connection pooling may help alleviate the problem until the client application is modified to handle the errors appropriately. By disabling connection pooling, releasing the connection will cause a physical logout of the SQL Server connection, resulting in the server rolling back any open transactions.
- If connection pooling is enabled and the destination server is SQL Server 2000, upgrading the client computer to MDAC 2.6 or later may be beneficial. This version of the MDAC components adds code to the ODBC driver and OLE DB provider so that the connection would be "reset" before it is reused. This call to sp_reset_connection aborts any server-initiated transactions (DTC transactions initiated by the client app are not affected), resets the default database, SET options, and so forth. Note that the connection is not reset until it is reused from the connection pool, so it is possible that a user could open a transaction and then release the connection to the connection pool, but it might not be reused for several seconds, during which time the transaction would remain open. If the connection is not reused, the transaction will be aborted when the connection times out and is removed from the connection pool. Thus, it is optimal for the client application to abort transactions in their error handler or use SET XACT_ABORT ON to avoid this potential delay.
- Actually, this class of blocking problem may also be a performance problem, and require you to pursue it as such. If the query execution time can be diminished, the query timeout or cancel would not occur. It is important that the application be able to handle the timeout or cancel scenarios should they arise, but you may also benefit from examining the performance of the query.
- Applications must properly manage transaction nesting levels, or they may cause a blocking problem following the cancellation of the query in this manner. This can be accomplished in one of several ways:
- Blocking Caused by a SPID Whose Corresponding Client Application Did Not Fetch All Result Rows to Completion
After sending a query to the server, all applications must immediately fetch all result rows to completion. If an application does not fetch all result rows, locks can be left on the tables, blocking other users. If you are using an application that transparently submits SQL statements to the server, the application must fetch all result rows. If it does not (and if it cannot be configured to do so), you may be unable to resolve the blocking problem. To avoid the problem, you can restrict poorly-behaved applications to a reporting or a decision-support database.
The application must be re-written to fetch all rows of the result to completion.
- Blocking Caused by a Distributed Client/Server Deadlock
Unlike a conventional deadlock, a distributed deadlock is not detectable using the RDBMS lock manager. This is due to the fact that only one of the resources involved in the deadlock is a SQL Server lock. The other side of the deadlock is at the client application level, over which SQL Server has no control. The following are two examples of how this can happen, and possible ways the application can avoid it.
- Client/Server Distributed Deadlock with a Single Client Thread
If the client has multiple open connections, and a single thread of execution, the following distributed deadlock may occur. For brevity, the term "dbproc" used here refers to the client connection structure.
SPID1------blocked on lock------->SPID2 /\ (waiting to write results | back to client) | | | | Server side | ================================|================================== | <-- single thread --> | Client side | \/ dbproc1 <------------------- dbproc2 (waiting to fetch (effectively blocked on dbproc1, awaiting next row) single thread of execution to run)In the case shown above, a single client application thread has two open connections. It asynchronously submits a SQL operation on dbproc1. This means it does not wait on the call to return before proceeding. The application then submits another SQL operation on dbproc2, and awaits the results to start processing the returned data. When data starts coming back (whichever dbproc first responds -- assume this is dbproc1), it processes to completion all the data returned on that dbproc. It fetches results from dbproc1 until SPID1 gets blocked on a lock held by SPID2 (because the two queries are running asynchronously on the server). At this point, dbproc1 will wait indefinitely for more data. SPID2 is not blocked on a lock, but tries to send data to its client, dbproc2. However, dbproc2 is effectively blocked on dbproc1 at the application layer as the single thread of execution for the application is in use by dbproc1. This results in a deadlock that SQL Server cannot detect or resolve because only one of the resources involved is a SQL Server resource.
- Client/Server Distributed Deadlock with a Thread per Connection
Even if a separate thread exists for each connection on the client, a variation of this distributed deadlock may still occur as shown by the following.
SPID1------blocked on lock-------->SPID2 /\ (waiting on net write) Server side | | | | | INSERT |SELECT | ================================|================================== | <-- thread per dbproc --> | Client side | \/ dbproc1 <-----data row------- dbproc2 (waiting on (blocked on dbproc1, waiting for it insert) to read the row from its buffer)This case is similar to Example A, except dbproc2 and SPID2 are running a SELECT statement with the intention of performing row-at-a-time processing and handing each row through a buffer to dbproc1 for an INSERT, UPDATE, or DELETE statement on the same table. Eventually, SPID1 (performing the INSERT, UPDATE, or DELETE) becomes blocked on a lock held by SPID2 (performing the SELECT). SPID2 writes a result row to the client dbproc2. Dbproc2 then tries to pass the row in a buffer to dbproc1, but finds dbproc1 is busy (it is blocked waiting on SPID1 to finish the current INSERT, which is blocked on SPID2). At this point, dbproc2 is blocked at the application layer by dbproc1 whose SPID (SPID1) is blocked at the database level by SPID2. Again, this results in a deadlock that SQL Server cannot detect or resolve because only one of the resources involved is a SQL Server resource.
Two reliable solutions are to use either a query timeout or bound connections.
- Query Timeout
When a query timeout has been provided, if the distributed deadlock occurs, it will be broken when then timeout happens. See the DB-Library or ODBC documentation for more information on using a query timeout.
- Bound Connections
This feature allows a client having multiple connections to bind them into a single transaction space, so the connections do not block each other. For more information, see the "Using Bound Connections" topic in SQL Server 7.0 Books Online.
- Client/Server Distributed Deadlock with a Single Client Thread
- Blocking Caused by a SPID That Is in a "Golden," or Rollback, State
A data modification query that is KILLed, or canceled outside of a user-defined transaction, will be rolled back. This can also occur as a side effect of the client computer restarting and its network session disconnecting. Likewise, a query selected as the deadlock victim will be rolled back. A data modification query often cannot be rolled back any faster than the changes were initially applied. For example, if a DELETE, INSERT, or UPDATE statement had been running for an hour, it could take at least an hour to roll back. This is expected behavior, because the changes made must be completely rolled back, or transactional and physical integrity in the database would be compromised. Because this must happen, SQL Server marks the SPID in a "golden" or rollback state (which means it cannot be KILLed or selected as a deadlock victim). This can often be identified by observing the output of sp_who, which may indicate the ROLLBACK command. The Status column of sys.sysprocesses will indicate a ROLLBACK status, which will also appear in sp_who output or in SQL Server Management Studio Activity Monitor.
You must wait for the SPID to finish rolling back the changes that were made.
If the server is shut down in the midst of this operation, the database will be in recovery mode upon restarting, and it will be inaccessible until all open transactions are processed. Startup recovery takes essentially the same amount of time per transaction as run-time recovery, and the database is inaccessible during this period. Thus, forcing the server down to fix a SPID in a rollback state will often be counterproductive.
To avoid this situation, do not perform large batch INSERT, UPDATE, or DELETE operations during busy hours on OLTP systems. If possible, perform such operations during periods of low activity.
- Blocking Caused by an Orphaned Connection
If the client application traps or the client workstation is restarted, the network session to the server may not be immediately canceled under some conditions. From the server's perspective, the client still appears to be present, and any locks acquired may still be retained. For more information, click the following article number to view the article in the Microsoft Knowledge Base:137983 How to troubleshoot orphaned connections in SQL Server
If the client application has disconnected without appropriately cleaning up its resources, you can terminate the SPID by using the KILL command. The KILL command takes the SPID value as input. For example, to kill SPID 9, simply issue the following command:
Note The KILL command may take up to 30 seconds to complete, due to the interval between checks for the KILL command.
Application Involvement in Blocking ProblemsThere may be a tendency to focus on server-side tuning and platform issues when facing a blocking problem. However, this does not usually lead to a resolution, and can absorb time and energy better directed at examining the client application and the queries it submits. No matter what level of visibility the application exposes regarding the database calls being made, a blocking problem nonetheless frequently requires both the inspection of the exact SQL statements submitted by the application and the application's exact behavior regarding query cancellation, connection management, fetching all result rows, and so on. If the development tool does not allow explicit control over connection management, query cancellation, query timeout, result fetching, and so on, blocking problems may not be resolvable. This potential should be closely examined before selecting an application development tool for SQL Server, especially for business-critical OLTP environments.
It is vital that great care be exercised during the design and construction phase of the database and application. In particular, the resource consumption, isolation level, and transaction path length should be evaluated for each query. Each query and transaction should be as lightweight as possible. Good connection management discipline must be exercised. If this is not done, it is possible that the application may appear to have acceptable performance at low numbers of users, but the performance may degrade significantly as the number of users scales upward.
With proper application and query design, Microsoft SQL Server is capable of supporting many thousands of simultaneous users on a single server, with little blocking.
Article ID: 224453 - Last Review: 06/22/2014 20:04:00 - Revision: 7.0
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