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Microsoft SQL Server 2008 Internals
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RCSI does not require any change in your applications. All that is needed is one change to the database options. Any of your applications written using the default Read Committed isolation level automatically uses RCSI after making the change at the database level.
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You can consider using SI in the following situations:
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The probability is low that any of your transactions have to be rolled back because of an update con ict. You have reports that need to be generated based on long-running, multistatement queries that must have point-in-time consistency. Snapshot isolation provides the bene t of repeatable reads without being blocked by concurrent modi cation operations.
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Optimistic concurrency does have bene ts, but you must also be aware of the costs. To summarize the bene ts:
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SELECT operations do not acquire shared locks, so readers and writers do not block each other. All SELECT operations retrieve a consistent snapshot of the data. The total number of locks needed is greatly reduced compared to pessimistic concurrency, so less system overhead is used. SQL Server needs to perform fewer lock escalations. Deadlocks are less likely to occur.
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Now let s summarize the other side. When weighing your concurrency options, you must consider the cost of the snapshot-based isolation levels:
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SELECT performance can be affected negatively when long-version chains must be scanned. The older the snapshot, the more time it takes to access the required row in an SI transaction. Row versioning requires additional resources in tempdb. Whenever either of the snapshot-based isolation levels are enabled for a database, UPDATE and DELETE operations must generate row versions. (Although I mentioned earlier that INSERT operations do not generate row versions, there are some cases where they might. In particular, if you insert a row into a table with a unique index, if there was an older version of the row with the same key value as the new row and that old row still exists as a ghost, your new row generates a version.) Row versioning information increases the size of every affected row by 14 bytes. UPDATE performance might be slower due to the work involved in maintaining the row versions.
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Transactions and Concurrency
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UPDATE operations using SI might have to be rolled back because of con ict detection. Your applications must be programmed to deal with any con icts that occur. The space in tempdb must be carefully managed. If there are very long-running transactions, all the versions generated by update transactions during the time must be kept in tempdb. If tempdb runs out of space, UPDATE operations won t fail, but SELECT operations that need to read versioned data might fail.
To maintain a production system using SI, you should allocate enough disk space for tempdb so that there is always at least 10 percent free space. If the free space falls below this threshold, system performance might suffer because SQL Server expends more resources trying to reclaim space in the version store. The following formula can give you a rough estimate of the size required by version store. For long-running transactions, it might be useful to monitor the generation and cleanup rate using Performance Monitor, to estimate the maximum size needed:
[size of common version store] = 2 * [version store data generated per minute] * [longest running time (minutes) of the transaction]
Controlling Locking
The SQL Server Query Optimizer usually chooses the correct type of lock and the lock mode. You should override this behavior only if thorough testing has shown that a different approach is preferable. Keep in mind that by setting an isolation level, you have an impact on the locks that held, the con icts that cause blocking, and the duration of your locks. Your isolation level is in effect for an entire session, and you should choose the one that provides the data consistency required by your application. Table-level locking hints can be used to change the default locking behavior only when necessary. Disallowing a locking level can adversely affect concurrency.
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