SQL server Backup Restore Revealed

Antonios Chatzipavlis
DATA SOLUTIONS CONSULTANT & TRAINER
SQL Server Backup/Restore Revealed
SQL Night #43
Sep 26, 2020
20:30
SQL Nights

1988 2000 Founderv 6.0 60 +2010
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V.102010 - 2020
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Presentation Content
SQL Server
Backup/Restore
Revealed
• Database Architecture Basics
• Backup / Restore
• Backup / Restore Internals

Database Architecture
Data file(s)
8KB Pages
VLFs
T-log file
Database

Buffer Cache usage
Data file(s)
8KB Pages
VLFs
T-log file
Database
Buffer cache
Memory

Transaction log usage
Data file(s)
8KB Pages
VLFs
T-log file
Database
Buffer cache
Memory
Commit
returned
Pages marked as dirty

Checkpoint
Data file(s)
8KB Pages
VLFs
T-log file
Database
Buffer cache
Memory
Pages marked as dirty
checkpoint

Checkpoint frequency
Data file(s)
8KB Pages
VLFs
T-log file
Database
Buffer cache
Memory
DD D D D D

Automatic
Indirect
Manual
Internal
CHECKPOINT statement configures the target recovery duration
Checkpoint Options

Backup / Restore Strategy
Understanding RPO and RTO
(t)
DISASTER
RPO RTO
backup Restore end
Lost data DB Down
t1 t2 t3

Retention & Testing Policy Backup Integrity
•Insufficient Copies of Backups
•Insufficient Data on the Backups
•Unreadable Backups
•Unavailable Hardware
•Old Hardware
•Misaligned Hardware
•Mirrored Backups
•Backup Verification
•Backup storage size
•Backup security
Managing Backups

Backup Device(s)
Backup Device
Slot 1 – Backup A
Slot 2 – Backup B
Slot 3 – Backup C
Slot 4 – Backup D
Slot 5 – Backup E
Slot N – Backup …
Backup devices types
• Physical
• Logical
• Azure Blob storage

Media Set
Media Family
Backup Sets, Media Sets, Media Families
Backup Device A
Slot 1 – Backup Database A
Slot 2 – Backup Database B
Slot 3 – Backup Database C
Backup Set
Backup Set
Backup Set
Media Header

Media Set
Media Family 2/2Media Family 1/2
Backup Sets, Media Sets, Media Families
Backup Device A
Slot 1 – Backup Database A (50%)
Slot 2 – Backup Database B (50%)
Slot 3 – Backup Database C (50%)
Media Header
Backup Device B
Slot 1 – Backup Database A (50%)
Slot 2 – Backup Database B (50%)
Slot 3 – Backup Database C (50%)
Backup Set
Backup Set
Backup Set
Media Header

Recovery Models
Simple
No T-log backups
Auto truncate T-log
Full
T-log backups required
Avoids data loss due to a
damaged or missing data
file
Recovery at specific point
in time
Bulk logged
T-log backups required
Enhance the performance
of bulk copy operation
Reduce log space by using
minimal logging for many
bulk operations

Backup Types
Full Differential Transaction Log Tail Log
Copy Only Partial Filegroup Snapshot

BACKUP TYPES SIMPLE FULL BULK LOGGED
Full ● ● ●
Differential ● ● ●
Transaction Log ● ●
Tail Log ●
Copy Only ● ● ●
Partial ● ● ●
Filegroup ● ● ●
Snapshot ●
Backup Types / Recovery Model

Full Backup
(t)
New
DB FULL FULL FULL

Restore Full Backup
(t)
New
DB FULL FULL FULL
t4
DAMAGE
RESTORE

Differential Backup
(t)
New
DB FULL DIFF DIFF DIFF

Restore Differential Backup
(t)
New
DB FULL DIFF DIFF DIFF
t5
DAMAGE
RESTORE RESTORE

Transaction Log Backup
(t)
New
DB FULL LOG LOG DIFF LOG LOG

Restore Transaction Log Backup
(t)
New
t7
DAMAGE
RESTORE RESTORE RESTORE RESTORE

Tail Log Backup
(t)
New
DB FULL LOG LOG DIFF LOG LOG DAMAGE
TAIL
LOG
RPO
A tail-log backup captures any log records which has not
yet been backed up by the last transactional log backup.

Copy Only Backup
(t)
New
t2-3
FULL
COPY ONLY

Partial Backup
(t)
New
DB FULL
Primary
filegroup
Read only
Filegroup
Read/Write
Filegroup
FULL
READ/WRITE
PARTIAL
FULL
READ ONLY
PARTIAL
DIFFERENTIAL
READ/WRITE
PARTIAL
DIFFERENTIAL
READ/WRITE
PARTIAL
All SQL Server recovery
models support partial
backups
Partial backups are designed for
use under the
simple recovery model
to improve flexibility for backing
up very large databases that
contain one or more read-only
filegroups.

Restore Partial Backup
(t)
New
DB FULL
Primary
filegroup
Read only
Filegroup
Read/Write
Filegroup
FULL
READ/WRITE
PARTIAL
FULL
READ ONLY
PARTIAL
DIFFERENTIAL
READ/WRITE
PARTIAL
DIFFERENTIAL
READ/WRITE
PARTIAL
t6
DAMAGE
RESTORE RESTORERESTORE

Filegroup Backup
(t)
New
DB FULL
Primary
filegroup
Other
Filegroups
FILEGROUP
A
FILEGROUP
B
FILEGROUP
C
FILEGROUP
A
FILEGROUP
B
FILEGROUP
C
t0A
LOG
t1A
LOG
t1B
LOG
t2A
LOG
t2B
LOG
t3A
LOG
t3B
LOG
t4A
LOG
t4B
LOG
t5A
LOG
t5B
LOG
t6A
LOG
t6B
LOG
t7A
LOG
t7B
LOG

Restore Filegroup Backup
(t)
New
DB FULL
Primary
filegroup
Other
Filegroups
FILEGROUP
A
FILEGROUP
B
FILEGROUP
C
FILEGROUP
A
FILEGROUP
B
FILEGROUP
C
t0A
LOG
t1A
LOG
t1B
LOG
t2A
LOG
t2B
LOG
t3A
LOG
t3B
LOG
t4A
LOG
t4B
LOG
t5A
LOG
t5B
LOG
t6A
LOG
t6B
LOG
t7A
LOG
t7B
LOG
t8
DAMAGE
FILEGROUP B
t8’
TAIL LOG

8KB Pages
Database Snapshot
Data file(s)
Database
Snapshot
SELECT on
Snapshot
We can restore this snapshot, but it should not be considered as a backup

File-snapshot backup
SQL Server instance on Azure Virtual Machine ( SQL Server 2016 + )
SQL Server Instance
On Azure VM
DB
Azure Blob Storage
(t)
FULL LOG LOG LOG LOG LOG LOG LOG
t9
DAMAGETAIL LOG
RESTORE ALL (SEQUENCIAL)
Azure Blob Storage
TO URL https://<storagename>.blob.core.windows.net/<container>/xxx.bak
FILE_SNAPSHOT
RESTORE

When already running FULL DIFFERENTIAL LOG
Full Backup Blocked Blocked
Allowed
( >SQL 2005)
Differential Backup Blocked Blocked
Allowed
( >SQL 2005)
Log Backup
Allowed
( >SQL 2005)
Allowed
( >SQL 2005)
Blocked
Backup Concurrency in the same database

Data Copy
Creates files and copies data to the files
Redo Phase
Applies committed transactions from restored log entries
Undo Phase
Rolls back transactions that were uncommitted at the recovery point
Phases of the Restore Process

RESTORE LABELONLY
Returns information about the backup media on a specified backup device.
RESTORE HEADERONLY
Returns all the backup header information for all backup sets on a particular backup device.
RESTORE FILELISTONLY
Returns a list of data and log files contained in a backup set.
RESTORE VERIFYONLY
Verifies the backup but does not restore it and checks to see that the backup set is complete, and the entire
backup is readable.
Retrieving Backup Metadata

Page(s) Restore
Online Restore – Enterprise Edition
(t)
DB
Full Recovery
Model
FULL
LOG LOG DIFF LOG LOG
t7
Page corruption
823, 824
PAGE(s)
TAIL LOG
RESTORE
msdb dbo suspect_pages;

Page(s) Restore
Offline Restore – Enterprise and Standard Editions
(t)
DB
Full Recovery
Model
FULL
t7
Page corruption
823, 824
PAGE(s)
TAIL LOG
RESTORE
msdb dbo suspect_pages;

Restore to a point in time
(t)
DB
Full Recovery
Model
FULL
t6’
Some crucial
data deleted
LOG
RESTORE RESTORE RESTORE RESTORE RESTORE
t7’
Discover
RESTORE LOG ‘<dbname>’
FROM backupdevice
WITH STOPAT = ‘YYYY/MM/DD HH:MM:SS’
STOPAT = { 'datetime'| @datetime_var }
STOPATMARK = { 'mark_name' | 'lsn:lsn_number’ }
STOPBEFOREMARK = { 'mark_name' | 'lsn:lsn_number' } [ AFTER 'datetime']
DATA
LOSS

master
• Start the server instance in single-user mode by
using the –m startup option.
• Use a RESTORE DATABASE statement to restore a
full database backup of the master database by
using the sqlcmd utility.
• Terminate your sqlcmd connection.
• Remove the single-user startup parameter.
• Restart SQL Server.
model
• Start the server instance with TF3608.
• Use a RESTORE DATABASE statement to restore a
full database backup of the master database by
using the sqlcmd utility.
• Terminate your sqlcmd connection.
• Remove the single-user startup parameter.
• Restart SQL Server.
msdb
• You can restore it by using the RESTORE
DATABASE statement as you would a user db
Restore System databases

Transaction Log Truncation
T-logfile
Start End End
File growth

Transaction Log Truncation
T-logfile
Start
End
Start
End

Differential Backup
Commonly used case
Restore chain too long impacting
RTO

Smart differential backup
New approach ( SQL Server 2016 + )
modified_extent_page_count -> sys.dm_db_file_space_usage
Total number of pages modified in allocated extents of the file since last full database backup. The modified page count
can be used to track differential changes in the database since last full backup to decide if differential backup is beneficial
SELECT file_id,
total_page_count,
modified_extent_page_count,
cast (( modified_extent_page_count * 1.0 /
total_page_count * 1.0 )
* 100.0 as decimal(5,2)) as pctchanges
FROM sys.dm_db_file_space_usage

Transaction Log Backup
Commonly used case
Not Adaptive
• Frequent Auto grows
• VLF Fragmentation
• Inconsistent backup size

Smart transaction log backup
New approach ( SQL Server 2016 + )
select from sys.dm_db_log_stats DB_ID
Adaptive
• Minimal Auto grows
• No VLF Fragmentation
• Consistent backup size

Memory out of buffer cache
How SQL Server Backup Works
DB
1 Backup command executed
2 Checkpoints (few times)
Checkpoints
3 Workers creation
Controller
Read thread
(one per volume)
Write thread
(one per backup device)
4 Start buffering data (async I/O)
backup
device
BUFFER
BUFFER
BUFFER
BUFFER
BUFFER
BUFFER
BUFFER
BUFFER
5 Writing buffered data
6 Exchange the buffers

BufferCount
• Specifies the total number of I/O buffers to be used
for the backup operation.
• You can specify any positive integer
• Large numbers of buffers might cause “out of
memory” errors because of inadequate virtual
address space in the Sqlservr.exe process.
MaxTransferSize
• Specifies the largest unit of transfer in bytes to be
used between SQL Server and the backup media.
• The possible values are multiples of 65536 bytes (64
KB) ranging up to 4194304 bytes (4 MB).
Important Backup Options
Calculate the Buffer Count Formula

Backup related wait types
BACKUPBUFFER
Occurs when a backup
task is waiting for data
or is waiting for a
buffer in which to store
data.
BACKUPIO
Occurs when a backup
task is waiting for data
or is waiting for a
buffer in which to store
data.
BACKUPTHREAD
Occurs when a task is
waiting for a backup
task to finish.

SQL Server tracks all backup activity
in the following tables in the msdb
database:
• msdb.dbo.backupfile
• msdb.dbo.backupfilegroup
• msdb.dbo.backupmediafamily
• msdb.dbo.backupmediaset
• msdb.dbo.backupset
Viewing Backup History
https://bit.ly/365LosB

Deleting Backup History
sp_delete_backuphistory @oldest_date
sp_delete_database_backuphistory @database_name

Backup/Restore operation execution info
How to know the execution percentage and completion time for a backup / restore operation
SELECT SUBSTRING
case
when then
else
end
DATEADD
FROM sys.dm_exec_requests
CROSS APPLY sys.dm_exec_sql_text
INNER JOIN sys.dm_exec_sessions
INNER JOIN sys.dm_exec_connections
WHERE
https://www.sqlschool.gr/blog/how-to-know-the-execution-percentage-and-completion-time-for-a-task-1054.aspx

Thank you!
@antoniosch - @sqlschool
./sqlschoolgr - ./groups/sqlschool
./c/SqlschoolGr
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SQL server Backup Restore Revealed

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