SQLServer Database Structures

SQL Server
Database
Structures

PRESENTERINFO
1982 I started working with computers
1988 I started my professional career in computers industry
1996 I started working with SQL Server 6.0
1998 I earned my first certification at Microsoft as
Microsoft Certified Solution Developer (3rd in Greece)
1999 I started my career as Microsoft Certified Trainer (MCT) with more than
30.000 hours of training until now!
2010 I became for first time Microsoft MVP on Data Platform
I created the SQL School Greece www.sqlschool.gr
2012 I became MCT Regional Lead by Microsoft Learning Program.
2013 I was certified as MCSE : Data Platform
I was certified as MCSE : Business Intelligence
2016 I was certified as MCSE: Data Management & Analytics
2017 Certified as MCSA : Machine Learning
Recertified as MCSE: Data Management & Analytics
Antonios
Chatzipavlis
Data Solutions
Consultant & Trainer
MVP on Data Platform
MCT, MCSE, MCITP, MCPD, MCSD, MCDBA,
MCSA, MCTS, MCAD, MCP, OCA, ITIL-F

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PRESENTATIONCONTENT
• Database Structure Internals
• Data File Internals
• tempdb Internals

• 32,767 Databases per instance
• 524,767 TB is the max size per
database
• 16 TB is the max size for each data file
• 2 TB is the max size for each log file
• 32,767 files per database
• 32,767 filegroups per database
• 64 KB is the size of each Extent
- 8 pages X 8 KB per page
• 8 KB is the size of each Page
DATABASE COMPONENTS
Database
File Group
(one or more)
Data File
(one or more)
Extent
(mixed or uniform)
Pages
(8 KB each)

File groups
• Logical grouping of data files
• Can bring administration and
performance benefits
• FILESTREAM has its own file group
FILEGROUPS AND DATABASE FILES
Database Files
• Data Files
- At least one per database
- Multiple files can:
- improve performance,
- aid maintainability
- circumvent operating system file size
limitations
• Transaction Log Files
- Usually only one per database
- No performance advantage from
multiple files

EXTENTS
Comprises eight physically continuous pages
Mixed
shared between
different
database objects
Uniform
owned by a
single
database object

PAGE STRUCTURE AND PAGE TYPES
Page Types
• Data (1)
• Index (2)
• Text (3 and 4)
• Boot (13)
• File Header (15)
• PFS (11)
• GAM (8)
• SGAM (9)
• IAM (10)
• DIFF_MAP (16)
• ML_MAP (17)

PAGE HEADER
pageID The file number and page number of this page in the database Metadata: ObjectId The ID of the object to which this page belongs
slotCnt The total number of slots (rows) used on this page Metadata: PartitionId The ID of the partition that this page is part of
Type Page Type Metadata: AllocUnitId The ID of the allocation unit that contains this page
LSN The LSN corresponding to the last log entry that changed this page freeData The byte offset of the first free space on this page
nextPage The file number and page number of the next page if this page is in
a page chain
Pminlen The number of bytes in fixed-length portion of rows
prevPage The file number and page number of the previous page if this page
is in a page chain
freeCnt The number of free bytes on the page
Level The level of this page in an index (always 0 for leaf pages) indexId The index ID of this page (always 0 for data pages)

• Data rows for in-row data
- The maximum size of a single data row is 8,060 bytes of in-row data.
- Rows can also have row-overflow and LOB data stored on separate pages.
- The number of rows stored on a given page varies depending on the table
structure and on the data being stored.
- A table with all fixed-length columns always can store the same number of
rows per page
- Variable-length rows can store as many rows as will fit based on the actual
length of the data entered.
- Keeping the row length shorter allows more rows to fit on a page, thus
reducing I/O and increasing the likelihood that needed data can stay in cache.
• Row offset array
- The row offset array is a block of 2-byte entries
- Each indicating the offset on the page at which the corresponding data row
begins.
- The row offset array indicates the logical order of rows on a page.
- For example, if a table has a clustered index, SQL Server stores the rows in the order of the
clustered index key.
- This doesn’t mean the rows are physically stored on the page in the order of
the clustered index key.
- Rather, slot 0 in the offset array refers to the first row in the clustered index key order, slot 1
refers to the second row, and so forth
DATA ROWS AND ROW OFFSET ARRAY

• Status Bits A
- Bit 0. This indicates special versioning information. In SQL Server 2012, this is always 0.
- Bits 1 through 3. Taken as a 3-bit value,
- 0(000) indicates a primary record,
- 1 (001) indicates a forwarded record,
- 2 (010) indicates a forwarding stub,
- 3 (011) indicates an index record,
- 4 (100) indicates a BLOB fragment or row-overflow data
- 5(101) indicates a ghost index record
- 6(110) indicates a ghost data record,
- 7(111) indicates a ghost version record.
- Bit 4. This indicates that a NULL bitmap exists. In SQL Server 2012, a NULL bitmap is always
present, even if no NULLs are allowed in any column.
- Bit 5. This indicates that variable-length columns exist in the row.
- Bit 6. This indicates that the row contains versioning information.
- Bit 7. This isn’t used in SQL Server 2012.
• Status Bits B
- The only bit used in the Status Bits B field indicates that the record is a ghost forwarded record.
RECORD STRUCTURE AND RECORD TYPES

• File Header Page (FH)
• Page Free Space (PFS)
• Global Allocation Map (GAM)
• Shared Global Allocation Map (SGAM)
• Differential Change Map (DCM)
• Bulk Change Map
• Boot Page
• Index Allocation Map (IAM)
ALLOCATION BITMAPS AND SPECIAL PAGES

• Page allocation
•The process of allocating new pages to an object
• Allocation units
•IN_ROW_DATA
•LOB_DATA
•ROW_OVERFLOW_DATA
• Page allocations can be analysed using DBCC page
PAGE ALLOCATION AND ALLOCATION UNITS

DBCC PAGE
DBCC PAGE ({dbid | dbname}, filenum, pagenum[, printopt])
DBCC TRACEON(3604);
GO
DBCC PAGE (AdventureWorks2012, 1, 266, 1);

VOLUME CONFIGURATION BEST PRACTICES
Volume
RAID 10 for
transaction log
files
RAID 10 or RAID
5 for database
data files
Store data and
transaction log
files on physically
separate volumes
Defragment disks
frequently
Size data and
transaction log
files to avoid
auto growth

NUMBER OF DATA FILES AND PL ACEMENT
For most databases
a single data file is
sufficient:
To improve
performance:

• Logical I/O
- pages read from cache
• Physical I/O
- pages read from disk
• You can view physical and logical
I/O stats using
- the DMV sys.dm_exec_query_stats
or
- by using the Transact-SQL
command SET STATISTICS IO ON
before issuing a query.
- SET STATISTICS IO ON will only return
statistics for queries in the current
session.
PHYSICAL I/O VS. LOGICAL I/O
• Buffer cache hit ratio
- percentage of pages read from cache
without having to be read from disk
- calculated by the formula ((logical
reads–physical reads)/logical reads) *
100.
- a SQL Server instance that is
performing well will have a buffer
cache hit ratio close to 100.
- The buffer cache hit ratio can be
improved by making more memory
available to SQL Server.

ALLOCATIONS WITHIN DATA FILES
Allocation is simple
in a file group that
has a single file
In a multifile group,
SQL Server uses:
• Round robin allocation
• Proportional fill

INSTANT FILE INITIALIZATION
Improves
performance by
skipping zeroing
of data pages on
file creation and
growth
Best practice is
to enable
following
assessment of
security risk
Disabled by
default: security
risk

Auto Grow
• Automatically expands a file
when space is low
• Manually manage file size for
better performance
• Auto grow best practices:
- Leave enabled to avoid
downtime in case file size limit is
reached
- Set to a fixed rather than a
percentage
AUTO GROW AND AUTO SHRINK
Auto Shrink
• Shrinks data file automatically
to release disk space
• Use with caution as:
- Auto shrink causes index
fragmentation
- It’s uncontrollable and affects
performance
- It can lead to growth-shrink cycle
severely affecting performance

DATA FILE SHRINKING
Shrink
• Causes index
fragmentation
• Resource intensive
• Degrades
performance
Best used for
• Emptying a file
before removing it
• Changing a file or
file group to read
only
• Reclaiming free
space following
large delete
operation
Alternative
• Move all indexes
to a new file group
• Move heaps with
shrink. Heaps are
not fragmented
with shrink
• Remove old file
group

Database file configuration
• SSMS database properties
• sp_helpfile and
sp_helpfilegroup
• sys.database_files and
sys.filegroups
• sys.master_files
• sys.dm_db_file_space_usage
MONITORING DATABASE FILES
Database file activity
•SSMS Activity Monitor – Data File
I/O
•Wait statistics –
PAGEIOLATCH_*, WRITELOG
•sys.dm_io_virtual_file_stats

TEMPDB USAGE
tempdb
tempdb Stores:
• Internal objects
• Version store
• Some user objects
Recreated at
startup
Must not be
allowed to run
out of space
Should be
actively

VERSION STORE INTERNALS
Stores all version
records from all
databases
New allocation unit
created every minute
Cleanup happens every
minute
Nonlogged operation
Version Store

Configuration
• Enable instant file initialization
• Size for expected workload
• Isolate tempdb data files
• Use multiple data files
• Enable auto growth as a
contingency
TEMPDB CONFIGURATION
Prohibited operations
• Backup and restore
• Add file groups
• Change collation
• Set offline or read only
tempdb size can be monitored using the sys.dm_db_file_space_usage DMV

EVALUATIONS
http://bit.ly/2xUr3XP
EVENT EVALUATION LINK SESSIONS EVALUATION LINK
http://bit.ly/2snEVoq

SQLServer Database Structures

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