Row Versioning Resource Usage
The row versioning framework supports the following features available in SQL Server:
Multiple Active Results Sets (MARS)
The row versioning framework also supports the following row versioning-based transaction isolation levels, which by default are not enabled:
When the READ_COMMITTED_SNAPSHOT database option is ON, READ_COMMITTED transactions provide statement-level read consistency using row versioning.
When the ALLOW_SNAPSHOT_ISOLATION database option is ON, SNAPSHOT transactions provide transaction-level read consistency using row versioning.
Row versioning-based isolation levels reduce the number of locks acquired by transaction by eliminating the use of shared locks on read operations. This increases system performance by reducing the resources used to manage locks. Performance is also increased by reducing the number of times a transaction is blocked by locks acquired by other transactions.
Row versioning-based isolation levels increase the resources needed by data modifications. Enabling these options causes all data modifications for the database to be versioned. A copy of the data before modification is stored in tempdb even when there are no active transactions using row versioning-based isolation. The data after modification includes a pointer to the versioned data stored in tempdb. For large objects, only part of the object that changed is copied to tempdb.
Space Used in tempdb
For each instance of the Database Engine, tempdb must have enough space to hold the row versions generated for every database in the instance. The database administrator must ensure that tempdb has ample space to support the version store. There are two version stores in tempdb:
The online index build version store is used for online index builds in all databases.
The common version store is used for all other data modification operations in all databases.
Row versions must be stored for as long as an active transaction needs to access it. Once every minute, a background thread removes row versions that are no longer needed and frees up the version space in tempdb. A long-running transaction prevents space in the version store from being released if it meets any of the following conditions:
It uses row versioning-based isolation.
It uses triggers, MARS, or online index build operations.
It generates row versions.
When a trigger is invoked inside a transaction, the row versions created by the trigger are maintained until the end of the transaction, even though the row versions are no longer needed after the trigger completes. This also applies to read-committed transactions that use row versioning. With this type of transaction, a transactionally consistent view of the database is needed only for each statement in the transaction. This means that the row versions created for a statement in the transaction are no longer needed after the statement completes. However, row versions created by each statement in the transaction are maintained until the transaction completes.
When tempdb runs out of space, the Database Engine forces the version stores to shrink. During the shrink process, the longest running transactions that have not yet generated row versions are marked as victims. A message 3967 is generated in the error log for each victim transaction. If a transaction is marked as a victim, it can no longer read the row versions in the version store. When it attempts to read row versions, message 3966 is generated and the transaction is rolled back. If the shrinking process succeeds, space becomes available in tempdb. Otherwise, tempdb runs out of space and the following occurs:
Write operations continue to execute but do not generate versions. An information message (3959) appears in the error log, but the transaction that writes data is not affected.
Transactions that attempt to access row versions that were not generated because of a tempdb full rollback terminate with an error 3958.
Space Used in Data Rows
Each database row may use up to 14 bytes at the end of the row for row versioning information. The row versioning information contains the transaction sequence number of the transaction that committed the version and the pointer to the versioned row. These 14 bytes are added the first time the row is modified, or when a new row is inserted, under any of these conditions:
READ_COMMITTED_SNAPSHOT or ALLOW_SNAPSHOT_ISOLATION options are ON.
The table has a trigger.
Multiple Active Results Sets (MARS) is being used.
Online index build operations are currently running on the table.
These 14 bytes are removed from the database row the first time the row is modified under all of these conditions:
READ_COMMITTED_SNAPSHOT and ALLOW_SNAPSHOT_ISOLATION options are OFF.
The trigger no longer exists on the table.
MARS is not being used.
Online index build operations are not currently running.
If you use any of the row versioning features, you might need to allocate additional disk space for the database to accommodate the 14 bytes per database row. Adding the row versioning information can cause index page splits or the allocation of a new data page if there is not enough space available on the current page. For example, if the average row length is 100 bytes, the additional 14 bytes cause an existing table to grow up to 14 percent.
Space Used in Large Objects
The SQL Server Database Engine supports six data types that can hold large strings up to 2 gigabytes (GB) in length: nvarchar(max), varchar(max), varbinary(max), ntext, text, and image. Large strings stored using these data types are stored in a series of data fragments that are linked to the data row. Row versioning information is stored in each fragment used to store these large strings. Data fragments are a collection of pages dedicated to large objects in a table.
As new large values are added to a database, they are allocated using a maximum of 8040 bytes of data per fragment. Earlier versions of the Database Engine stored up to 8080 bytes of ntext, text, or image data per fragment.
Existing ntext, text, and image large object (LOB) data is not updated to make space for the row versioning information when a database is upgraded to SQL Server from an earlier version of SQL Server. However, the first time the LOB data is modified, it is dynamically upgraded to enable storage of versioning information. This will happen even if row versions are not generated. After the LOB data is upgraded, the maximum number of bytes stored per fragment is reduced from 8080 bytes to 8040 bytes. The upgrade process is equivalent to deleting the LOB value and reinserting the same value. The LOB data is upgraded even if only one byte is modified. This is a one-time operation for each ntext, text, or image column, but each operation may generate a large amount of page allocations and I/O activity depending upon the size of the LOB data. It may also generate a large amount of logging activity if the modification is fully logged. WRITETEXT and UPDATETEXT operations are minimally logged if database recovery mode is not set to FULL.
The nvarchar(max), varchar(max), and varbinary(max) data types are not available in earlier versions of SQL Server. Therefore, they have no upgrade issues.
Enough disk space should be allocated to accommodate this requirement.
Monitoring Row Versioning and the Version Store
For monitoring row versioning, version store, and snapshot isolation processes for performance and problems, SQL Server provides tools in the form of Dynamic Management Views (DMVs) and performance counters in Windows System Monitor.
The following DMVs provide information about the current system state of tempdb and the version store, as well as transactions using row versioning.
sys.dm_db_file_space_usage. Returns space usage information for each file in the database. For more information, see sys.dm_db_file_space_usage (Transact-SQL).
sys.dm_db_session_space_usage. Returns page allocation and deallocation activity by session for the database. For more information, see sys.dm_db_session_space_usage (Transact-SQL).
sys.dm_db_task_space_usage. Returns page allocation and deallocation activity by task for the database. For more information, see sys.dm_db_task_space_usage (Transact-SQL).
sys.dm_tran_top_version_generators. Returns a virtual table for the objects producing the most versions in the version store. It groups the top 256 aggregated record lengths by database_id and rowset_id. Use this function to find the largest consumers of the version store. For more information, see sys.dm_tran_top_version_generators (Transact-SQL).
sys.dm_tran_version_store. Returns a virtual table that displays all version records in the common version store. For more information, see sys.dm_tran_version_store (Transact-SQL).
sys.dm_tran_top_version_generators and sys.dm_tran_version_store are potentially very expensive functions to run, since both query the entire version store, which could be very large.
sys.dm_tran_active_snapshot_database_transactions. Returns a virtual table for all active transactions in all databases within the SQL Server instance that use row versioning. System transactions do not appear in this DMV. For more information, see sys.dm_tran_active_snapshot_database_transactions (Transact-SQL).
sys.dm_tran_transactions_snapshot. Returns a virtual table that displays snapshots taken by each transaction. The snapshot contains the sequence number of the active transactions that use row versioning. For more information, see sys.dm_tran_transactions_snapshot (Transact-SQL).
sys.dm_tran_current_transaction. Returns a single row that displays row versioning-related state information of the transaction in the current session. For more information, see sys.dm_tran_current_transaction (Transact-SQL).
sys.dm_tran_current_snapshot. Returns a virtual table that displays all active transactions at the time the current snapshot isolation transaction starts. If the current transaction is using snapshot isolation, this function returns no rows. sys.dm_tran_current_snapshot is similar to sys.dm_tran_transactions_snapshot, except that it returns only the active transactions for the current snapshot. For more information, see sys.dm_tran_current_snapshot (Transact-SQL).
SQL Server performance counters provide information about the system performance impacted by SQL Server processes. The following performance counters monitor tempdb and the version store, as well as transactions using row versioning. The performance counters are contained in the SQLServer:Transactions performance object.
Free Space in tempdb (KB). Monitors the amount, in kilobytes (KB), of free space in the tempdb database. There must be enough free space in tempdb to handle the version store that supports snapshot isolation.
The following formula provides a rough estimate of the size of the version store. For long-running transactions, it may be useful to monitor the generation and cleanup rate to estimate the maximum size of the version store.
[size of common version store] = 2 * [version store data generated per minute] * [longest running time (minutes) of the transaction]
The longest running time of transactions should not include online index builds. Because these operations may take a long time on very large tables, online index builds use a separate version store. The approximate size of the online index build version store equals the amount of data modified in the table, including all indexes, while the online index build is active.
Version Store Size (KB). Monitors the size in KB of all version stores. This information helps determine the amount of space needed in the tempdb database for the version store. Monitoring this counter over a period of time provides a useful estimate of additional space needed for tempdb.
Version Generation rate (KB/s). Monitors the version generation rate in KB per second in all version stores.
Version Cleanup rate (KB/s). Monitors the version cleanup rate in KB per second in all version stores.
Information from Version Generation rate (KB/s) and Version Cleanup rate (KB/s) can be used to predict tempdb space requirements.
Version Store unit count. Monitors the count of version store units.
Version Store unit creation. Monitors the total number of version store units created to store row versions since the instance was started.
Version Store unit truncation. Monitors the total number of version store units truncated since the instance was started. A version store unit is truncated when SQL Server determines that none of the version rows stored in the version store unit are needed to run active transactions.
Update conflict ratio. Monitors the ratio of update snapshot transaction that have update conflicts to the total number of update snapshot transactions.
Longest Transaction Running Time. Monitors the longest running time in seconds of any transaction using row versioning. This can be used to determine if any transaction is running for an unreasonable amount of time.
Transactions. Monitors the total number of active transactions. This does not include system transactions.
Snapshot Transactions. Monitors the total number of active snapshot transactions.
Update Snapshot Transactions. Monitors the total number of active snapshot transactions that perform update operations.
NonSnapshot Version Transactions. Monitors the total number of active non-snapshot transactions that generate version records.
The sum of Update Snapshot Transactions and NonSnapshot Version Transactions represents the total number of transactions that participate in version generation. The difference of Snapshot Transactions and Update Snapshot Transactions reports the number of read-only snapshot transactions.