DAT410 – SQL Always OnPartial Database AvailabilityDanny Tambs – Dannyt@microsoft.comSenior ConsultantMicrosoft

Session Objectives And TakeawaysSession Objective(s):

Overview of Partial Database AvailabilityDemonstrate Recovery Methods

Key TakeawaysPartial database availability can shorten downtime when a failure occurs.Partial DB should be designed in from project start.

DemosRecovery of a R/O Filegroup.Recovery of a R/W Filegroup.

What is it?Enables a database to be brought online fast after a disaster when one or more Filegroups are lost.

Enables a given database to be quickly restored and brought online while other missing data can be restored later.Eg:

Large OLTP database with small amount of transaction supporting data but huge volumes of history data.

Typical UsesVLDB systemsTypically over 1 TB since the time to restore the whole database means significant downtime.Solutions using multiple filegroups where the main application functionality resides in small No.# of filegroups.

Eg: An OLTP database with a relatively small amount of working data but a proportionaly huge amount of historical data.Not meant as a substitute for a sound backup strategy.

Steps in Recovery of a Database1. After the database fails examine error log to

determine the physical filename that failed to load.Error : 17207, Unable to open the physical file "C:\Data\MyFG_Database3_1_sec.ndf". Operating system error 2: "2(error not found)".

2. Determine the logical filename that the physical filename maps to by querying the system catalog.

3. Alter Database - set the logical filename to offline4. Alter Database - set Database to online5. DATABASE NOW ONLINE !

Continue operating with Partial Availability.6. Restore Missing filegroup(s) as time permits. 7. Restore Tran Log & recover DB if read/write

filegroup.8. Database now fully Online.

ALTER DATABASE MYDB SET ONLINEGO

Primary FG1 FG2

A Timeline

File1mdf

File2ndf

File3ndf

File4ndf File5

ldf

Arrays1 2 3

Database = ONLINE

X

Database = OFFLINE

ALTER DATABASE MYDB MODIFY FILE(NAME=‘LOGICAL FILE4’, OFFLINE)GO

BACKUP LOG MYDB TO DISK=‘…FILE’GORESTORE DATABASE MYDB FILE=‘LOGICAL FILE4’ FROM DISK WITH FILE=1, NORECOVERYGO

RESTORE LOG MYDB FROM DISK =‘…FILE’ WITH RECOVERYGO

FilegroupOFFLINE

Fix HW Issue

Database = ONLINE - PARTIAL

Database cannot be opened

Command:

Status:

SQL 2000 - ReviewPartial command under SQL2000 was less flexible. Nothing to do with setting a file offline and bringing the DB Online.

Used to restore a file group to a different location to recover lost data.Needed to restore the mdf file as well as required file group.Once the database was brought online any unrestored filegroups could not be restored.Could not really be used to bring a database online quickly in order to reduce downtime.

SQL 2005 ImprovementsMany Enhancements.

SQL 2005 – we can now set a file to OFFLINE and bring database ONLINE.Unaffected file groups are still available for read/write operations.Commands against online file groups will execute.Only Commands against offline file groups will fail. Typical Errors:

945, 8653, 17207, 5120, 17204Piecemeal restore - Unrestored filegroups can now be restored at a later time.

Syntax: SET FILE OFFLINEALTER DATABASE database_name     MODIFY FILE (    NAME = logical_file_name [ , OFFLINE ])

EG:

ALTER DATABASE MyFG_Database MODIFY FILE

(NAME = 'MyFG_Database3_sec' , OFFLINE)

*Set the unavailable file offline to be able to set the database online.

Syntax: SET DB ONLINE

ALTER DATABASE database_name     SET ONLINE

EG:

ALTER DATABASE MyFG_Database SET ONLINE

*Once unavailable files are to OFFLINE use this command to set the database to online state.

Recovery of a Read Only Filegroup

demo

Piecemeal RestoreMust specify PARTIAL in the 1st restore of the sequence.

RESTORE DATABASE DB_name FROM DISK = ... WITH PARTIAL

Involves a series of restore sequences, starting with the primary filegroup.Can also restore one or more secondary filegroups when restoring the primary.After the restore sequence is finished recovered files can be brought online directly.

Other Files can be restored while the database is online.SQL Server 2005 Enterprise Edition only.

Syntax: PARTIAL RESTORERESTORE DATABASE database_name FROM DISK = physical_filename[WITH , PARTIAL]

EG:

RESTORE DATABASE [MyFG_Database] FILEGROUP='Primary'

FROM DISK = N'C:\...\F1_backup.bak' WITH PARTIAL

Used for piecemeal restore of MDF and NDF files. When Completed, DB is online.

Piecemeal Restore 2Can be ONLINE or OFFLINE

Offline – All EditionsOnline – SQL Enterprise Edition Only

Offline piecemeal restoreThe database is online after the partial-restore Filegroups that have not yet been restored remain offline, but can be restored as needed after taking the database offline.

Online piecemeal restoreAfter the partial-restore sequence, the database is online, and the primary filegroup and any recovered secondary filegroups are available (ONLINE). Other Filegroups to be restored are offline, but can be restored as needed while the database remains online.

Piecemeal Restore 3Works with all recovery models.Recovery Model NotesSimple Recovery -Any as yet unrestored read/write

filegroups cannot be restored. (No Tran Log)

- Only Read only Filegroups can be restored

Full and Bulk Logged

-Any as yet unrestored read/write filegroups can be restored.

-Transaction Log must be restored to make the DB consistent.

Mapping Logical to Physical filesUse sys.master_files system catalogue

view.Uses data in Master DB.

EG.SELECT name FROM sys.master_files WHERE physical_name = ‘Physical Name‘

Logical files – known within SQLPhysical names – The file on the disk

Get From Error log

Get the File Info per DBGet information about the DB files, sizes and if

Read only or / Read Write. Use to easily find file to set offline.

SELECT s.data_space_id as Id, g.name as Filegroup_name, s.name as Logical_name, physical_name, g.is_default AS [IsDefault], g.is_read_only AS [ReadOnly],

CAST(ISNULL((select sum(gs.size)*convert(float,8)

FROM sys.database_files gs WHERE gs.data_space_id = g.data_space_id), 0) AS

float) AS [Size] FROM sys.filegroups AS g INNER JOIN sys.master_files AS s ON (s.data_space_id = g.data_space_id) WHERE s.type = 0 AND s.database_id = db_id() AND (s.drop_lsn IS NULL)

Get Logical File Information Use sys.database_files Catalog View.Determine if a file is online

EG:

SELECT File_id, is_read_only, -- Is this Readonly?state_desc , -- Either Online or Offline.name -- Logical Filename

FROM sys.database_files

Recovery of a Read / Write Filegroup

demo

Designing for Partial AvailabilityQueries against an offline filegroup will fail.

REASONSJoining to tables are that not available.Tables with DRI against tables on offline filegroups.Triggers referring to unavailable tables.Partitioned Tables - where a required partition is offline.

- Group related schema Functionality in related filegroups.Note :Functions and procedures are still in PRIMARY

filegroup

Design for Partial Availability ..Detect if a table is unavailable

We know the table we want but what if the file is unavailable?Only use in filegroup to filegroup query cases. Ie boundary conditions.

Use function. IsAvailable(@Schema, @Objname)

Specify schema name, object_name to determine if the table is available.This is sample code and does not ship with SQL server 2005

Get the code from the white paperhttp://www.microsoft.com/technet/prodtechnol/sql/bestpractice/pdbavail.mspx

Tables to Filegroups mapping

SELECT sysob.object_id AS [ObjectId] ,schm.[name] AS SchemaName,sysob.[name] AS ObjectName,sysdbf.name AS [LogicalName] -- = LOGICAL FILE NAME,sysdbf.physical_name AS [PhysicalName] -- = PHYSICAL FILE NAME,sysdbf.state_desc AS [FileGroupState] -- = FILE GROUP ONLINE/OFFLINE ETC.,sysdbf.STATE

FROM sys.indexes AS iINNER JOIN sys.data_spaces AS ds ON i.data_space_id = ds.data_space_idINNER JOIN sys.objects AS sysob ON i.object_id = sysob.object_idINNER JOIN sys.database_files AS sysdbf ON (ds.data_space_id = sysdbf.data_space_id) INNER JOIN sys.schemas AS schm ON sysob.schema_id = schm.schema_id

WHERE sysob.type = 'U'AND is_ms_shipped = 0 AND sysdbf.type = 0 AND sysdbf.drop_lsn IS NULL

Understand what tables are affected if a file is offline.

Considerations for UseSetting a file to OFFLINE is a one way operation. Must restore to make online.When restoring a missing filegroup by using offline restore the database is unavailable for user interaction.Only Secondary files (*.ndf) can be set to OFFLINE state.Queries against OFFLINE filegroups will fail.Setting a file in a filegroup to OFFLINE affects the entire filegroup.

Best PracticesKeep your *.mdf files as small as possible.

Makes the first mdf restore faster.Keep your minimal functionality to be online in the MDF for fast initial restore. (critical tables, procs, Functions, reference tables.) Restore other FG later.Backup the DB transaction logs often.Record the mapping of logical to physical file names.Have a clear and well rehearsed partial DB recovery plan.

Related ContentWhitepaper – Partial Database Availability

http://www.microsoft.com/technet/prodtechnol/sql/bestpractice/pdbavail.mspx

Customer Advisory Team Blog Sitehttp://blogs.msdn.com/sqlcat/archive/2005/10/27/how-can-sql-server-2005-online-piecemeal-restore-improve-availability.aspx

MSDNhttp://msdn2.microsoft.com/en-us/library/ms177425.aspx

Questions?

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