Daily Archives: December 30, 2009

Reorganize data and index pages

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There are a few ways that this can be achieved:

1. Shrinking the database files (data file or transaction log file)

2. DBCC DBREINDEX (Similar to dropping and re-creating the indexes)

3. DBBC INDEXDEFRAG (De-fragmenting the indexed pages to remove logical scan fragmentation)

Note: It needs to be kept in mind that DBREINDEX is a fully-logged operation and can cause the transaction log to grow by large amounts when the database size is considerably large. So there have been situations when the transaction log has grown out of proportion and filled up the entire disk. If the T-log does grow out of proportion and you do choose to truncate/shrink the file, then please do so after verifying the points below:

1. Transactional replication is not configured for the database

2. Log shipping is not configured for the database

3. Database mirroring is not configured for the database (For SQL 2005 SP1 and above)

4. Any other operations are not being performed which need transactional log consistency in terms of LSN chains

Online re-indexing is a feature which is available in SQL Server 2005 Enterprise Edition only. Please note that rebuilding the indexes would cause the statistics associated with those indexes to also be updated. This however would not update the AUTO CREATED statistics.

The following articles could be useful:

SET OPTION considerations when running DBCC with indexes on computed columns

http://support.microsoft.com/kb/301292/

DBCC DBREINDEX

http://msdn2.microsoft.com/en-us/library/ms181671.aspx

The DBCC SHOWCONTIG output for SQL Server 2000 and 2005 will help you find out the current status of fragmentation for the database. A sample output would look like this:

DBCC SHOWCONTIG scanning ‘Customers’ table…

Table: ‘Customers’ (2073058421); index ID: 1, database ID: 6

TABLE level scan performed.

– Pages Scanned…………………………..: 3

– Extents Scanned…………………………: 2

– Extent Switches…………………………: 1

– Avg. Pages per Extent……………………: 1.5

– Scan Density [Best Count:Actual Count]…….: 50.00% [1:2]

– Logical Scan Fragmentation ………………: 0.00%

– Extent Scan Fragmentation ……………….: 50.00%

– Avg. Bytes Free per Page…………………: 246.7

– Avg. Page Density (full)…………………: 96.95%

The logical scan fragmentation should ideally be below 20% but anything above 40% would mean an INDEXDEFRAG or DBREINDEX is required. The script which is from the same MSDN article can help you run an INDEXDEFRAG or DBREINDEX based on the logical scan fragmentation count.

A sample script for the same can be found under the topic of "DBCC SHOWCONTIG" in SQL Server Books Online (2000 and 2005)

In SQL Server 2005, the following features may be helpful to you while using the ALTER INDEX command:

REBUILD [ WITH ( [ ,… n]) ]

Specifies the index will be rebuilt using the same columns, index type, uniqueness attribute, and sort order. This clause is equivalent to DBCC DBREINDEX. REBUILD enables a disabled index. Rebuilding a clustered index does not rebuild associated nonclustered indexes unless the keyword ALL is specified. If index options are not specified, the existing index option values stored in sys.indexes are applied. For any index option whose value is not stored in sys.indexes, the default indicated in the argument definition of the option applies.

The options ONLINE and IGNORE_DUP_KEY are not valid when you rebuild an XML index.

If ALL is specified and the underlying table is a heap, the rebuild operation has no affect on the table. Any nonclustered indexes associated with the table are rebuilt.

The rebuild operation can be minimally logged if the database recovery model is set to either bulk-logged or simple.

WITH ( LOB_COMPACTION = { ON OFF } )

Specifies that all pages that contain large object (LOB) data are compacted. The LOB data types are image, text, ntext, varchar(max), nvarchar(max), varbinary(max), and xml. Compacting this data can improve disk space use. The default is ON.

ON

All pages that contain large object data are compacted.Reorganizing a specified clustered index compacts all LOB columns that are contained in the clustered index. Reorganizing a nonclustered index compacts all LOB columns that are nonkey (included) columns in the index. For more information, see Creating Indexes with Included Columns. When ALL is specified, all indexes that are associated with the specified table or view are reorganized, and all LOB columns that are associated with the clustered index, underlying table, or nonclustered index with included columns are compacted. OFF Pages that contain large object data are not compacted.

OFF

has no affect on a heap.The LOB_COMPACTION clause is ignored if LOB columns are not present.

REORGANIZE

Specifies the index leaf level will be reorganized. This clause is equivalent to DBCC INDEXDEFRAG. ALTER INDEX REORGANIZE statement is always performed online. This means long-term blocking table locks are not held and queries or updates to the underlying table can continue during the ALTER INDEX REORGANIZE transaction. REORGANIZE cannot be specified for a disabled index or an index with ALLOW_PAGE_LOCKS set to OFF.

Furthermore, the dynamic management view sys.dm_db_index_physical_stats would help you get statistical information pertaining to indexes on SQL Server 2005. The columns mentioned below in this DMV output could be helpful in analyzing the need for index reorganization.

avg_fragmentation_in_percent: Logical fragmentation for indexes, or extent fragmentation for heaps in the IN_ROW_DATA allocation unit. The value is measured as a percentage and takes into account multiple files. For definitions of logical and extent fragmentation, see Remarks. 0 for LOB_DATA and ROW_OVERFLOW_DATA allocation units.NULL for heaps when mode = SAMPLED.

fragment_count: Number of fragments in the leaf level of an IN_ROW_DATA allocation unit. For more information about fragments, see Remarks.NULL for nonleaf levels of an index, and LOB_DATA or ROW_OVERFLOW_DATA allocation units. NULL for heaps when mode = SAMPLED.

avg_fragment_size_in_pages: Average number of pages in one fragment in the leaf level of an IN_ROW_DATA allocation unit. NULL for nonleaf levels of an index, and LOB_DATA or ROW_OVERFLOW_DATA allocation units. NULL for heaps when mode = SAMPLED.

The following article could be helpful:Microsoft SQL Server 2000 Index Defragmentation Best Practices

http://www.microsoft.com/technet/prodtechnol/sql/2000/maintain/ss2kidbp.mspx

Database Maintenance Plans

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Database Maintenance Plans for SQL Server 2000
Database Maintenance Plans offer the following features in SQL Server 2000:
1. Backing up databases
2. Reorganization of indexes and data pages
3. Statistics Update
4. Shrinking the database
5. Database integrity checks
The above tasks of the maintenance plans would have associated with respective SQL Agent jobs and also these could be scheduled to suit your needs. Furthermore, these jobs could be created manually also without the help of maintenance plans.

Database Maintenance Plans for SQL Server 2005
In addition to the above mentioned features, SQL Server 2005 Enterprise Edition provides the option of rebuilding the indexes as an online operation. This makes it possible for the indexes to remain online while a rebuild index task is being performed on the database.
ONLINE = { ON OFF }
Specifies whether underlying tables and associated indexes are available for queries and data modification during the index operation. The default is OFF.
Note: Online index operations are available only in SQL Server 2005 Enterprise Edition.
ON
Long-term table locks are not held for the duration of the index operation. During the main phase of the index operation, only an Intent Share (IS) lock is held on the source table. This enables queries or updates to the underlying table and indexes to continue. At the start of the operation, a Shared (S) lock is held on the source object for a very short amount of time. At the end of the operation, for a short time, an S (Shared) lock is acquired on the source if a nonclustered index is being created; or an SCH-M (Schema Modification) lock is acquired when a clustered index is created or dropped online and when a clustered or nonclustered index is being rebuilt. ONLINE cannot be set to ON when an index is being created on a local temporary table.
OFF
Table locks are applied for the duration of the index operation. An offline index operation that creates, rebuilds, or drops a clustered index, or rebuilds or drops a nonclustered index, acquires a Schema modification (Sch-M) lock on the table. This prevents all user access to the underlying table for the duration of the operation. An offline index operation that creates a nonclustered index acquires a Shared (S) lock on the table. This prevents updates to the underlying table but allows read operations, such as SELECT statements.

The frequency of these tasks and which tasks are needed need to be performed need to be decided on the following factors:
1. Importance of the database in question
2. Size of the database
3. Maintenance window frequency
4. Load on the server
5. Whether it is an OLTP or OLAP database or both
6. The kind of transactions that take place on the database (Point 5 and 6 would decide how often index reorganization/index rebuild/statistics updates are required and whether a shrink database/file is required)

Stuck Stalled IO Messages

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Sometimes, we see the following messages in the SQL Server 2000 (Service Pack 4 and above) and SQL Server 2005 ERRORLOGs:

2004-11-11 00:21:25.26 spid1 SQL Server has encountered 192 occurrence(s) of IO requests taking longer than 15 seconds to complete on file [E:\db_data2.ndf] in database [user_db] (7). The OS file handle is 0x00000000000074D4. The offset of the latest long IO is:
0x00000000022000″.

A given I/O request that becomes stalled or stuck is only reported one time. If the message reports 10 I/Os as stalled, these 10 reports will not occur again. If the next message reports 15 I/Os stalled, it indicates that 15 new I/O requests have become stalled.
Most of these messages if occur frequently in a SQL Server, then it could be due to one of the following:

1. There is an underlying hardware issue where in your firmware and disk drivers need to be upgraded. This is mostly done by the Internal Storage Team or preferably by the Hardware Vendor
2. Or the disks are overwhelmed with the IO Requests being posted on the risks. In this case, you would want to go ahead and make sure that if possible some of the data files and log files would need to be moved to a different drive
3. Or it could be due to bad plans SQL Server ends up performing extra number reads than required

Also, monitoring the disks using the following counters under performance monitor under the would be helpful (under the performance objects – Logical Disk and Physical Disk):
1. Average Disk Queue Length
2. Average Disk Secs\Transfer
3. Average Disk Secs\ Write
4. Average Disk Secs\ Reads
The last three counters should never show a prolonged peak period which shows values of 30 milli-seconds and above. Please refer my blog article Setting up Perfmon Logs on how to setup a perfmon trace.

One important point that needs to be kept is  the amount of I/O being performed by SQL Server. You can find out the amount of I/O being driven by SQL Server by using the Process performance object with the SQL Server instance (sqlservr#) that you are concerned with and looking at the values for I/O Data Bytes value. It might be a case where your queries have sub-optimal plans due to which they are performing an excessive amount of reads/writes which is causing the disk to be throttled.

If you find out that this is actually an issue with the disk sub-system, then it is always a good idea to involve the disk vendor/or your internal storage team to find out if all the drivers/firmware are updated with the latest patches.

Addition: May 26, 2011:
If you want to get your hands dirty and debug a Stuck-Stalled IO request, then Bob Dorr, Principal Escalation Engineer at Microsoft, has a blog post on this:
http://blogs.msdn.com/b/psssql/archive/2008/03/03/how-it-works-debugging-sql-server-stalled-or-stuck-i-o-problems-root-cause.aspx

Helpful Articles:
SQL IO Basics

SQL Stuck Stalled IO KB Article

Stuck Stalled IO Messages

SQL Server 2005 IO Basics

Working with TempDB in SQL Server 2005

Troubleshooting Performance Problems (section on I/O)

Physical Database Storage

Storage Top 10 Best Practices

Ruling Out Disk-Bound Problems

Troubleshooting SQL Setup Failures

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  1. The Setup Logs for SQL Server 2005 are created in the following location
    “(system drive):\Program Files\Microsoft SQL Server\90\Setup Bootstrap\LOG” 
    There are two folders underneath this location:
    a. Files
    b. Hotfix
  2. The files under the Files folder are created during the RTM installation of SQL Server 2005
  3. The files under the Hotfix folder are created during Service Pack/Hotfix installation
  4. For every failed setup of SQL Server 2005, there will be a .CAB (Cabinet file) created under the Files or Hotifx depending on what installation you are performing: RTM/Hotfix/Service Pack installation
  5. Also, the LOG folder would have a summary.txt file which would give a brief overview of which component(s) failed to install
  6. SQL Server 2005 uses MSI to install the components. For RTM setup, we use .MSI files and for patches, we use MSP files. The error # that you see in the setup summary.txt file would be present in the associated MSP or MSI log along with extended information of which function was being called. Eg. If setup fails during the configuration phase, then you might get the connectivity errors related to the attempts that setup made to connect to SQL Server. This would further provide some more hints to troubleshoot the issue
  7. Using the Error Number and the Component Name, find out if you come back with any helpful links from http://support.microsoft.com/
  8. If you are getting nowhere, then please post a question on the MSDN/TechNet Forums (Setup and Upgrade) or open up a support incident with Microsoft Product Support Services

Please refer the following for more details:
http://technet.microsoft.com/en-us/library/ms143702.aspx
http://msdn2.microsoft.com/en-us/library/ms144287.aspx

Another comprehensive blog post which talks about SQL Server 2005 Setup Logs:

http://blogs.msdn.com/raja_sivagaminathan/archive/2010/01/07/sql-server-2005-setup-log-files.aspx

SQL Server 2000 Setup Failures
The setup logs for RTM installations for SQL Server 2000 are installed with the following log sqlstp.log getting created in the %windows% folder.
The Service Pack setup logs are created having the following filename sqlsp.log under the same location specified above.
However, the hotfix logs are created with the following prefix “KB***.log”

SQL Server 2000 setup doesn’t create multiple setup logs. You can use sqlstp.log (RTM setup) or sqlsp.log (Service Pack setups) or KB*****.log (Patch setup) and look for “return value 3“. If an action specified by a setup function errors out, then it prints the Return Value as 3 in the setup logs.
The drawback with the SQL setup logs for SQL Server 2000 is that the setup logs get overwritten every time setup is run. But in SQL Server 2005, this is not the case. All the logs are retained in the BOOTSTRAP folder and a new set of log files are created with incremental numbers in the same location.

Database Integrity Checks

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Database integrity checks are recommended to ensure that the database consistency is intact and if there is a problem with consistency, then it is reported to the appropriate team(s) so that necessary action can be taken to rectify it. This can be done with the help of Database Maintenance Plans. In the event that a CHECKDB on a database fails, then it needs to be reported by Email(using xp_sendmail or SQL Agent Mail or Database Mail for SQL Server 2005) or events fired in the Operating System Event Logs (if these are monitored regularly) with the help of Operators which can be configured for SQL Server Agent. The xp_sendmail feature is not available for SQL 2005 64-bit versions.
The frequency of these checks largely depends on the following factors:
1. Importance of the database
2. How often data changes in the database (If a database integrity check fails for a database where data is not modified, then it would be advisable to restore the last known good backup rather than trying to repair the consistency database)
3. The size of the database
4. In the event of consistency checks failing, it needs to be determined which is the most feasible option:
a. Restore the last known good backups in accordance with the recovery model for that database to allow for a minimum amount of data loss
b. Or try and repair the database and falling back on Option (a) only if this fails In case a repair option is suggested in the CHECKDB output, it is important to note that a REPAIR_ALLOW_DATA_LOSS be never performed on the database without understanding its full consequences and consulting Microsoft PSS. In the event, that this route needs to be taken, it is always recommended to fall back on the last known good backups if possible. The REPAIR_FAST and REPAIR_REBUILD repair options can be performed without having any data loss. Please note that these are time consuming operations and in the event of database inconsistency it is not possible for us to predict how long these tasks would run for. Also, the time taken for CHECKDB on a database cannot be predicted. An educated guess can be made to how long it would take by referring to the last durations of the CHECKDB operations on that particular database. For the above mentioned repair options, please refer the following article: http://msdn2.microsoft.com/en-us/library/aa258278(SQL.80).aspx

REPAIR_ALLOW_DATA_LOSS
Performs all repairs done by REPAIR_REBUILD and includes allocation and deallocation of rows and pages for correcting allocation errors, structural row or page errors, and deletion of corrupted text objects. These repairs can result in some data loss. The repair may be done under a user transaction to allow the user to roll back the changes made. If repairs are rolled back, the database will still contain errors and should be restored from a backup. If a repair for an error has been skipped due to the provided repair level, any repairs that depend on the repair are also skipped. After repairs are completed, back up the database. This option shouldn’t be used without the assistance of a SQL Support Engineer.
REPAIR_FAST
Performs minor, nontime-consuming repair actions such as repairing extra keys in nonclustered indexes. These repairs can be done quickly and without risk of data loss.
REPAIR_REBUILD
Performs all repairs done by REPAIR_FAST and includes time-consuming repairs such as rebuilding indexes. These repairs can be done without risk of data loss. In case of SQL Server 2005, you have the option of checking the suspect_pages in the MSDB database to find out the affected pages.

Please refer the following articles for more detailed information:
Suspect_pages table (SQL 2005)
http://msdn2.microsoft.com/en-us/library/ms174425.aspx
Understanding and managing the suspect_pages table
http://msdn2.microsoft.com/en-us/library/ms191301.aspx
Designing a Backup and Restore Strategy
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/adminsql/ad_bkprst_63eh.asp

In the event, that only a few pages have a problem, then for SQL Server 2005 a page level restore can be performed. Online page restore operation is a feature available in SQL Server 2005 Enterprise Edition but all other Editions of SQL Server 2005 support offline page level restores. The page level restores are done using the NORECOVERY option for that database. Then a backup of the current transaction log is taken and applied to the database with the RECOVERY option. This feature is applicable to databases in FULL or BULK-LOGGED recovery models.
For performing page level restores, please refer the following article: http://msdn2.microsoft.com/en-us/library/ms175168.aspx
It is highly important that a disaster recovery plan is in place to ensure the following:

  • · A plan to acquire hardware in the event of hardware failure
  • · A communication plan.
  • · A list of people to be contacted in the event of a disaster.
  • · Instructions for contacting the people involved in the response to the disaster.
  • · Information on who owns the administration of the plan.
  • · A checklist of required tasks for each recovery scenario. To help you review how disaster recovery progressed, initial each task as it is completed, and indicate the time of completion on the checklist.

In conclusion, if the database is of a considerably large size, then an integrity check needs to be scheduled during a window when the load on the server is at a minimum. The definition of minimum here refers to a load which is lesser than the normal workload on the server. If the database sizes are quite small, then daily integrity checks on the database would be the order of the day.
It is recommended that DBCC CHECKDB be run during hours when the load is light on the server. If DBCC CHECKDB is run during heavy peak usage time, expect a performance hit on the transaction throughput as well as DBCC CHECKDB completion time.

Recommendations for Good DBCC Performance
· Run CHECKDB when the system usage is low.
· Be sure that you are not performing other disk I/O operations, such as disk backups.
· Place tempdb on a separate disk system or a fast disk subsystem.
· Allow enough room for tempdb to expand on the drive. Use DBCC with ESTIMATE ONLY to estimate how much space will be needed for tempdb.
· Avoid running CPU-intensive queries or batch jobs.
· Reduce active transactions while a DBCC command is running.
· Use the NO_INFOMSGS option to reduce processing and tempdb usage significantly.