Hướng dẫn học Microsoft SQL Server 2008 part 144 pdf

Nielsen c66.tex V4 - 07/21/2009 4:13pm Page 1392 Part IX Performance Tuning and Optimization Level 2 – Read committed SQL Server’s default transaction isolation level, read committed, (described previously) is like a nice, polite white-picket fence between two good neighbors. It prevents dirty reads, but doesn’t bog the system down with excessive lock contention. For this reason, it’s SQL Server’s default isolation level and an ideal choice for most OTLP projects. Best Practice U nless there’s a specific reason to escalate the transaction isolation level, I strongly recommend that you keep your transactions at read committed. Level 3 – Repeatable read The third level of isolation, repeatable read, is like a 10-foot chain-link fence with barbed wire on top. There’s a significant difference between read committed’s white-picket fence and repeatable read. Read committed only has to lock the transaction that’s doing the writing. To ensure that reads are consistent, share locks on the reading transaction have to be extended as well. First, here’s a walk-through of a nonrepeatable read fault. Nonrepeatable read fault A nonrepeatable read is similar to a dirty read, b ut a nonrepeatable read occurs when a transaction can see the committed updates from another transaction (see Figure 66-8). True isolation means that one transaction never affects another transaction. If the isolation is complete, then no data changes from outside the transaction should be seen by the transaction. Reading a row inside a transaction should produce the same results every time. If reading a row twice results in different values, that’s a nonrepeatable read type of transaction fault. FIGURE 66-8 A nonrepeatable read transaction fault is when transaction 2 selects the same data twice and sees different values. Update Commit t1 Select Select t2 Isolation Retrieves Updated Value 1392 www.getcoolebook.com Nielsen c66.tex V4 - 07/21/2009 4:13pm Page 1393 Managing Transactions, Locking, and Blocking 66 To demonstrate a nonrepeatable read, the following sequence sets up two concurrent transactions. Transaction 2, on the right side, is in the default read committed transaction isolation level, which allows the nonrepeatable read fault. Assuming an unaltered copy of AdventureWorks2008, transaction 2 begins a logical transaction and then reads the department name as ‘‘Engineering’’: Transaction 2 USE AdventureWorks2008; SET TRANSACTION ISOLATION LEVEL READ COMMITTED; BEGIN TRANSACTION; SELECT Name FROM HumanResources.Department WHERE DepartmentID = 1; Result: Name Engineering Transaction 1 on the left side now updates the department name to ‘‘Non-Repeatable Read’’: Transaction 1 USE AdventureWorks2008; UPDATE HumanResources.Department SET Name = ‘Non-Repeatable Read’ WHERE DepartmentID = 1; Transaction 2, back on the right side, reads the row again. If it sees the value updated by transaction 1, that will be a nonrepeatable read transaction fault: SELECT Name FROM HumanResources.Department WHERE DepartmentID = 1; COMMIT TRANSACTION; Result: Name Non-Repeatable Read Sure enough, transaction 2’s read was not repeatable. 1393 www.getcoolebook.com Nielsen c66.tex V4 - 07/21/2009 4:13pm Page 1394 Part IX Performance Tuning and Optimization Preventing the fault Rerunning the same scripts with transaction 2’s transaction isolation level to Repeatable Read will result in a very different behavior (assuming an unaltered copy of AdventureWorks2008): Transaction 2 USE AdventureWorks2008; SET TRANSACTION ISOLATION LEVEL REPEATABLE READ; BEGIN TRANSACTION; SELECT Name FROM HumanResources.Department WHERE DepartmentID = 1; Result: Name Engineering Transaction 1 on the left side now updates the department name to ‘‘Non-Repeatable Read’’: Transaction 1 USE AdventureWorks2008; UPDATE HumanResources.Department SET Name = ‘Non-Repeatable Read’ WHERE DepartmentID = 1; Here’s the first major difference: There’s no ‘‘1 row(s) affected’’ message indicating that the update is paused, waiting for transaction 2’s share lock. Transaction 2, back on the right side, reads the row again. If it sees the value updated by transaction 1, then that’s a nonrepeatable read transaction fault: SELECT Name FROM HumanResources.Department WHERE DepartmentID = 1; Result: Name Engineering But the result is not the updated value from transaction 1. Instead, the original value is still in place. The read was repeatable and the nonrepeatable read fault has been prevented. When transaction 2 completes the transaction, it releases the share lock: COMMIT TRANSACTION; 1394 www.getcoolebook.com Nielsen c66.tex V4 - 07/21/2009 4:13pm Page 1395 Managing Transactions, Locking, and Blocking 66 Immediately, transaction 1, on the left side, is now free to complete its update and the ‘‘1 row(s)affected ’’ message appears in the Messages pane. Repeatable read protects against the selected rows being updated, but it doesn’t protect against new rows being added to or deleted from the selected range. Therefore, you could get a different value/set of results, if new rows are added or deleted. To avoid this, use the serializable transaction isolation level of protection. Best Practice R epeatable read has significant overhead, but it’s perfect for situations when a transaction must be able to read the data multiple times, perhaps performing calculations, and guarantee that no other transaction updates the data during these calculations. The key to using repeatable read is applying it conservatively — if an application requires repeatable read in some cases, be careful to only set repeatable read for those transactions that require it. Leave all the other transactions at the default read committed transaction isolation level. Level 4 – Serializable This most restrictive isolation level prevents all transactional faults and is like a high-security prison wall. Serializable protects against all three transactional faults: dirty reads, nonrepeatable reads, and phantom rows. Just as serialized inventory means that each item is uniquely identified and accounted for, the serial- ized transaction isolation level means that each row in every select’s result set is accounted for; and if that select is reissued, then the result will not include any row additions or deletions made by any other transaction. This mode is useful for databases for which absolute transactional integrity is more important than per- formance. Banking, accounting, and high-contention sales databases, such as the stock market, typically use serialized isolation. Best Practice U se the serialized transaction level when performing multiple aggregations on a ranged set of rows and there’s a risk that another connection might add or remove rows from that range during the transaction. An example might be when a transaction is reconciling multiple accounts and must ensure that no other transaction inserts within the same range as the adjustments during the reconciliation. Phantom rows The least severe transactional-integrity fault is a phantom row, which means that one transaction’s insert, update, or delete causes different rows to be returned in another transaction, as shown in Figure 66-9. 1395 www.getcoolebook.com Nielsen c66.tex V4 - 07/21/2009 4:13pm Page 1396 Part IX Performance Tuning and Optimization FIGURE 66-9 When the rows returned by a select are altered by another transaction, the phenomenon is called a phantom row . Insert Commit t1 Select Select t2 Isolation New additional rows are returned 4 Rows 6 Rows Beginning with a clean copy of AdventureWorks2008, transaction 2 selects all the rows in a specific range ( Name BETWEEN ‘A’ AND ‘G’): Transaction 2 USE AdventureWorks2008 SET TRANSACTION ISOLATION LEVEL REPEATABLEREAD BEGIN TRANSACTION SELECT DepartmentID as DeptID, Name FROM HumanResources.Department WHERE Name BETWEEN ‘A’ AND ‘G’ Result: DeptID Name 12 Document Control 1 Engineering 16 Executive 14 Facilities and Maintenance 10 Finance Transaction 1 now inserts a new row into the range selected by transaction 2: Transaction 1 Insert a row in the range INSERT HumanResources.Department (Name, GroupName) VALUES (’ABC Dept’, ‘Test Dept’) 1396 www.getcoolebook.com Nielsen c66.tex V4 - 07/21/2009 4:13pm Page 1397 Managing Transactions, Locking, and Blocking 66 When transaction 2 selects the same range again, if ‘ABC Dept’ is in the result list, then a phantom row transaction fault occurred: Transaction 2 re-selecting the same range SELECT DepartmentID as DeptID, Name FROM HumanResources.Department WHERE Name BETWEEN ‘A’ AND ‘G’ COMMIT TRANSACTION Result: DeptID Name 17 ABC Dept 12 Document Control 1 Engineering 16 Executive 14 Facilities and Maintenance 10 Finance Sure enough, ‘ABC Dept’ is in the result list, and that’s the phantom row. Serialized transaction isolation level The highest transaction isolation level can defend the transaction against the phantom row. Transaction 2 will first insert a sample row, ‘ Amazing FX Dept,’ so transaction 1 will have a row that can be deleted without worrying about referential integrity issues. It then sets the transaction isolation level, begins a transaction, and reads a range of data: Transaction 2 USE AdventureWorks2008 insert test row for deletion INSERT HumanResources.Department (Name, GroupName) VALUES (’Amazing FX Dept’, ‘Test Dept’) SET TRANSACTION ISOLATION LEVEL SERIALIZABLE BEGIN TRANSACTION SELECT DepartmentID as DeptID, Name FROM HumanResources.Department WHERE Name BETWEEN ‘A’ AND ‘G’ 1397 www.getcoolebook.com Nielsen c66.tex V4 - 07/21/2009 4:13pm Page 1398 Part IX Performance Tuning and Optimization Result: DeptID Name 17 Amazing FX Dept 12 Document Control 1 Engineering 16 Executive 14 Facilities and Maintenance 10 Finance Transaction 2’s SELECT returned six rows. With transaction 2 in a transaction and serialized transaction isolation level protecting the range of names from ‘ A’to‘G’, transaction 1 will attempt to insert, update, and delete into and from that range: Transaction 1 Insert a row in the range INSERT HumanResources.Department (Name, GroupName) VALUES (’ABC Dept’, ‘Test Dept’) Update Dept into the range UPDATE HumanResources.Department SET Name = ‘ABC Test’ WHERE DepartmentID = 1 Engineering Delete Dept from range DELETE HumanResources.Department WHERE DepartmentID = 17 Amazing FX Dept The significant point here is that none of transaction 1’s DML commands produced a ‘‘ 1 row(s)affected’’ message. Transaction 2 now reselects the same range: Transaction 2 SELECT DepartmentID as DeptID, Name FROM HumanResources.Department WHERE Name BETWEEN ‘A’ AND ‘G’ Result: DeptID Name 17 Amazing FX Dept 12 Document Control 1 Engineering 16 Executive 14 Facilities and Maintenance 10 Finance 1398 www.getcoolebook.com Nielsen c66.tex V4 - 07/21/2009 4:13pm Page 1399 Managing Transactions, Locking, and Blocking 66 The SELECT returns the same six rows with the same values. Transactional integrity is intact, and the phantom row fault has been thwarted. Transaction 1 is still on hold, waiting for transaction 2 to complete its transaction: COMMIT TRANSACTION As soon as transaction 2 issues a commit transaction and releases its locks, transaction 1 is free to make its changes, and three ‘ ‘ 1 row(s)affected’’ messages appear in transaction 1’s Messages pane: SELECT * FROM HumanResources.Department Result: DeptID Name 18 ABC Dept 1 ABC Test 12 Document Control 1 Engineering 16 Executive 14 Facilities and Maintenance 10 Finance Selecting the range after transaction 2 is committed and transaction 1 has made its updates reveals the inserted and updated rows added to the range. In addition, the Amazing FX department has disappeared. Concurrency and the serialized isolation level are not friends because in order to get the protection needed for the serialized isolation level, more locks are required; worse, those locks have to be on key ranges to prevent someone else from inserting rows. In the event that you don’t have the correct indexes, the only way SQL can prevent phantoms is to lock the table. Locking the table is obviously not good for concurrency. For this reason, if you need to use serialized transactions, you must ensure that you have the correct indexes in order to avoid table locks. Snapshot isolations Traditionally, writers block readers, and readers block writers, but version-based isolations are a completely different twist. When version-based isolations are enabled, if a transaction modifies (irrespective of the isolation level) data, a pre-modification version of the data is stored. This allows other transactions to read the original version of thedataevenwhiletheoriginaltransactionisinan uncommitted state. Therefore, snapshot isolation eliminates writer versus reader contention. Nevertheless, contention isn’t completely gone — you still have writers conflicting with writers. If a second writer attempts to update a resource that’s already being updated, the second resource is blocked. There are two version-based isolations — snapshot isolation and read committed snapshot isolation: 1399 www.getcoolebook.com Nielsen c66.tex V4 - 07/21/2009 4:13pm Page 1400 Part IX Performance Tuning and Optimization ■ Snapshot isolation: Operates like serializable isolation without the locking and blocking issues. The same select within a transaction will see before image of the data. ■ Read committed snapshot isolation: Sees any committed data, similar to SQL Server’s default isolation level of read committed. However, importantly, it doesn’t place any shared locks on the data being read. Best Practice O racle’s default transaction behavior is just like snapshot isolation, which is why some DBAs moving up to SQL Server love snapshot isolation, and why some assume snapshot isolation must be better somehow than traditional transaction isolation levels. It’s true that snapshot isolation can eliminate some locking and blocking issues and therefore improves performance given the right hardware. However, the best practice is as follows: If you choose snapshot isolation, it should be an architecture issue, not a performance issue. If another transaction is updating the data, should the user wait for the new data, or should the user see the before image of the data? For many applications, returning the before image would paint a false picture. Enabling row versioning Snapshot actually leverages SQL Server’s row-versioning technology, which copies any row being updated into TempDB. Configuring snapshot isolation, therefore, requires first enabling row versioning for the database. Besides the TempDB load, row versioning also adds a 14-byte row identifier to each row. This extra data is added to the row when the row is modified if it hasn’t been done previously. It is used to store the pointer to the versioned row. Because snapshot isolation uses row versioning, which writes copies of the rows to TempDB, this can put an incredible load on TempDB. If you enable the row-version-based isolations, be prepared to watch TempDB and perhaps locate TempDB’s data and transaction logs on their own disk subsystems. Row versioning alters the row structure so that a copy of the row can be sent to TempDB. The following code enables snapshot isolation. To alter the database and turn on snapshot isolation, there can no other connections to the database: USE Aesop; ALTER DATABASE Aesop SET ALLOW_SNAPSHOT_ISOLATION ON check snapshot isolation select name, snapshot_isolation_state, snapshot_isolation_state_desc, 1400 www.getcoolebook.com Nielsen c66.tex V4 - 07/21/2009 4:13pm Page 1401 Managing Transactions, Locking, and Blocking 66 is_read_committed_snapshot_on from sys.databases where database_id = DB_ID() Transaction 1 now begins a reading transaction, leaving the transaction open (uncommitted): USE Aesop SET TRANSACTION ISOLATION LEVEL Snapshot; BEGIN TRAN SELECT Title FROM FABLE WHERE FableID = 2 Result: Title The Bald Knight A second transaction begins an update to the same row that the first transaction has open: USE Aesop; SET TRANSACTION ISOLATION LEVEL Snapshot; BEGIN TRAN UPDATE Fable SET Title = ‘Rocking with Snapshots’ WHERE FableID = 2; SELECT * FROM FABLE WHERE FableID = 2 Result: Title Rocking with Snapshots This is pretty amazing. The second transaction is able to update the row even though the first transac- tion is still open. Going back to the first transaction, it will still see the original data: SELECT Title FROM FABLE WHERE FableID = 2 Result: Title The Bald Knight 1401 www.getcoolebook.com . HumanResources.Department WHERE DepartmentID = 1; Result: Name Engineering Transaction 1 on the left side now updates the department name to ‘‘Non-Repeatable Read’’: Transaction 1 USE AdventureWorks2008; UPDATE. of AdventureWorks2008): Transaction 2 USE AdventureWorks2008; SET TRANSACTION ISOLATION LEVEL REPEATABLE READ; BEGIN TRANSACTION; SELECT Name FROM HumanResources.Department WHERE DepartmentID =. now updates the department name to ‘‘Non-Repeatable Read’’: Transaction 1 USE AdventureWorks2008; UPDATE HumanResources.Department SET Name = ‘Non-Repeatable Read’ WHERE DepartmentID = 1; Here’s