The Relational and Storage
•The relational engine comprises all the components necessary
to parse and optimize any query
•It also manages the execution of queries as it requests data
from the storage engine in terms of OLE DB row sets and then
processes the row sets returned. (Row set is the OLE DB term
for a result set.)
•The storage engine comprises the components needed to
actually access and modify data on disk.
The SQL manager is responsible for everything having to
do with managing stored procedures and their plans. It
determines when a stored procedure needs recompilation
based on changes in the underlying objects' schemas, and
it manages the caching of procedure plans so that other
processes can reuse them.
•Checkpoints are run on a database-by-database basis.
•They flush dirty pages from the current database out to disk
so that those changes don't have to be redone during database
•A dirty page is one that has been modified since it was
brought from disk into the buffer pool.
•When a checkpoint occurs, SQL Server writes a checkpoint
record to the transaction log, which lists all the transactions
that are active
•This allows the recovery process to build a table containing a
list of all the potentially dirty pages.
Transaction Logging and
•Recovery performs both redo (rollforward) and undo (rollback)
•If the change doesn't appear in the database, it is again performed
from the information in the log
•Undo requires the removal of partial changes made by a
transaction if the transaction did not entirely complete
Three phases to the recovery
•Phase 1: Analysis, Phase 2: Redo, Phase 3: Undo
Phase 1: Analysis
•The first phase is a forward pass starting at the last
checkpoint record in the transaction log.
•This pass determines and constructs a dirty page table (DPT)
consisting of pages that might have been dirty at the time of
the crash (or when SQL Server stopped)
•An active transaction table is built that consists of
uncommitted transactions at the time of the crash
Phase 2: Redo
•This phase repeats history by returning the database to the state it
was in at the time of the crash
•The starting point for this forward pass is the minimum of all the
LSNs (Log sequence number) in the DPT (dirty page table)
•. The DPT is used to avoid reading pages that don't need
recovering and to avoid overwriting nonlogged changes
Phase 3: Undo
•This phase moves backward from the end of the log
•Any transaction that was not committed at the time of the
crash is undone so that none of its changes are actually
reflected in the database.
Locking and Transaction
•A transaction can be rolled back only if all affected data was
locked exclusively so that no other process could have made
changes to resources used by the transaction that would prevent its
being rolled back.
•Only one active transaction can modify a row at a time
•This is why exclusive locks must be held until a transaction is
either committed or aborted.
Interaction with the Operating
•The SQL Server kernel is responsible for interacting with the
•All requests to operating system services are made via the
Win32 API and C run-time libraries.
•SQL Server supports multiprocessing at the thread level rather
than at the process level, which allows for preemptive operation
and dynamic load balancing across multiple CPUs. Using
multiple threads is significantly more efficient than using