MS SQL
Server Architecture
M A Srinivas
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SERVER
CLIENT
ARCHITECTURE
2
RELATIONAL ENGINE
• Query Processor
• It includes the components of SQL Server
that determine what your query exactly
needs to do and the best way to do it. It
manages the execution of queries as it
requests data from the storage engine and
processes the results returned
Different Tasks of Relational Engine:
• Query Processing
• Memory Management
• Thread and Task Management
• Buffer Management
• Distributed Query Processing
Query Processing
• SELECT SQL Statement (SELECT [CustomerID], [CompanyName], [City], [Region]
FROM [Northwind].[dbo].[Customers]
WHERE [Country] = 'Germany'
ORDER BY [CompanyName]) then press
Ctrl+K,it will display the following:
• Reading the Graphical Execution Plan
• the execution plan for the aforementioned query
Storage Engine
• Storage Engine is responsible for the following:
• Storage of data
• Retrieval of data
What is SQLOS ?
•
SQLOS is an engine that provides a service for multi-threaded worker pool based scheduling of
tasks, memory management, monitoring, hosting, tracing and debugging. SQLOS is powered by
the Windows API’s
• The key objective of SQLOS is to essentially have an operating system for SQL server and to
localize resource management and consumption
• SQLOS is not a service or a single dedicated thread. SQLOS is an API for developers and an
interface to the operating system of SQL Server. SQLOS is also NUMA aware as it was since its
inception in SQL 2005.
SQL OS – 1
• This lies between the host machine (Windows OS) and SQL Server
• All the activities performed on database engine are taken care of by
SQL OS
• It is a highly configurable operating system with powerful API
(application programming interface)
• It enables automatic locality and advanced parallelism
• SQL OS provides various operating system services :
SQLOS – 2
• To take care of massive workloads SQLOS is
introduced as a thin layer between windows and SQL
Server . SQLOS functions are
• Managing system resources (CPU,RAM,IO)
• Adapting to ever changing hardware platforms
• How SQL server integrates into Windows Operating
Systems
• Scalability
SQL OS - 3
• Memory Management
• Buffer Pool
• Log Buffer
• Deadlock detection using blocking and locking structure.
• Other Services. It includes following:
• exception handling,
• hosting for external components like Common Language Runtime, CLR etc.
What is NUMA?
• Non-Uniform Memory Architecture is a motherboard
design Improves scalability by removing memory
controller bottlenecks
• Physical memory becomes either local or remote to a
specific physical processor
• Accessing remote memory is slower than local
memory
• Each node (or CPU) has its own memory bank
• Memory access management is distributed
• Local Access Minimum latency (100-200 CPU cycles)
• Remote Access Increased latency (1-4x local access)
SQL Server’s NUMA objectives
• Align the motherboard and SQLOS
architectures
• Reduce remote memory access
• Create memory objects in a task’s NUMA
node
• E.g. Buffer pool pages, locks and
schedulers etc
• sys.dm_os_nodes, sys.dm_os_schedulers
SQL OS Hierarchy
Scheduling
• Pre-emptive scheduling : When a thread can be pre-empted by another
thread on the same CPU
• Non-Pre-emptive scheduling / Cooperative multitasking : A thread can
execute until it decides to allow another thread to execute or until it is
finished processing
• SQL Server uses Non-Pre-emptive scheduling
• A SQLOS scheduler is a logical “CPU” for SQLOS. It provides a method to
allow “threads” to be assigned to a CPU. A SQLOS scheduler allows nonpre-emptive scalable scheduling and also provides a mechanism for affinity
to NUMA and CPU’s.
Life Cycle of task
• User connects to SQL server Engine -- Session
• User executes SQL statement(s)
• Statement is divided into one or more tasks, if
parallelism is required -- Tasks
• Each task is assigned to a worker
• SQLOS Scheduler schedules worker
• Worker executes in RUNNING, RUNNABLE or
SUSPENDED state(s)
• Task is completed
Execution Model
• When an execution request is made within a session, SQL Server
divides the work into one or more tasks and then associates a
worker thread to each task for its duration. Each thread can be in
one of
• ➤ Running — A processor can only execute one thing at a time and
the thread currently executing on a processor will have a state of
running.
• ➤ Suspended — SQL Server has a co-operative scheduler so running
threads will yield the processor and become suspended while they
wait for a resource. This is called a wait in SQL Server.
• ➤ Runnable — When a thread has finished waiting, it becomes
runnable which means that it’s ready to execute again. This is known
as a signal wait
SQL Server Databases
• System databases
•
•
•
•
•
Master
Model
MSDB
Resource
Tempdb
• User databases
SQL Server Files
• Binaries
• Datafiles (.mdf, .ndf)
• Transaction log files (.ldf)
• Backup files and snapshot files
• File groups
SQL Server Recovery Model
19
Recovery
Model
Description
Work Loss Exposure
Recover to point in
time?
Simple
No log backups Changes since the most recent Can recover only to
backup are unprotected
the end of a backup
Full
Requires log
backups
Normally none
Can recover to a
specific point in time
Bulk logged
Requires log
backups
If the log is damaged or bulklogged operations occurred
since the most recent log
backup, changes since the last
backup must be redone
Point in time is not
supported
Database Files
• When we talk about any database in SQL server, there are 2 types of
files that are created at the disk level
1. Data file
2. Log file