SQL Server Architecture - SQL Server 2012 Internals and Troubleshooting - page 18

Query Tree Language Event

Cmd Parser TDS

TDS

Query Plan SQL Server

Network Interface

Optimizer Query Executor SNI

Protocol
Layer

Relational Engine

Plan Cache

OLE DB

Transaction Log

D

Access Methods

Transaction
Manager

Data Cache

Data file

Bufer
Manager

Storage Engine Bufer Pool

FIGURE 1-7

Recovery
In the previous section you read about the life cycle of an UPDATE query, which introduced
write-ahead logging as the method by which SQL Server maintains the durability of any changes.

Modii cations are written to the transaction log i rst and are then actioned in memory only. This
is done for performance reasons and enables you to recover the changes from the transaction log if
necessary. This process introduces some new concepts and terminology that are explored further in
this section on “recovery.”

Dirty Pages
When a page is read from disk into memory it is regarded as a clean page because it's exactly the
same as its counterpart on the disk. However, once the page has been modii ed in memory it is
marked as a dirty page .

Clean pages can be l ushed from cache using dbcc dropcleanbuffers , which can be handy when
you're troubleshooting development and test environments because it forces subsequent reads to be
fuli lled from disk, rather than cache, but doesn't touch any dirty pages.