Performance Tuning with SQL Server Dynamic Management Views

Chapter 4: Transactions
Simply by un-commenting the calculation of the transaction duration in Listing 4.14, and
swapping the ORDER BY clause, we can investigate the activity of long-running transactions
that may be bloating the transaction log file, or preventing it from being truncated.
Snapshot Isolation and the tempdb Version Store
The snapshot isolation level was introduced in SQL Server 2005 and eliminates
blocking and deadlocking by using row versioning in the tempdb database to maintain
concurrency, rather than establishing locks on database objects. As such, the goal of
snapshot isolation is increased performance through greater concurrency. Snapshot
isolation is a huge topic and we can do little more than provide a brief overview here.
Please see Microsoft Books Online or MSDN for further details on isolation levels,
concurrency, and locking models.
Each time a row is modified in a database running under snapshot isolation, a version
of the row from prior to the modification is stored within tempdb in a version store. In
other words, this version store is populated with versions of data rows as they existed
before the initialization of an explicit transaction. The version store is shared by all
databases that are running under snapshot isolation on the SQL Server instance.
Read transactions targeting the affected rows will use the row version(s) from the
tempdb, while the writing transaction will modify the actual table data. Update locks
are issued for writes, and when a read transaction encounters such a lock, it is diverted
to the version store.
This row-versioning mechanism ensures that write transactions do not block reads. It
is no longer necessary to take exclusive locks on data being modified, to prevent dirty
reads, because the reads are made against versions of the rows that were in a committed
state from prior to the transaction or statement initialization. It also means that
readers do not block writers, since read transactions will no longer take shared read locks.
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