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This chapter is from the book

Managing Very Large Databases

Because scalability is composed of many things, designing for scale is difficult, especially for applications that come packaged from software providers, such as SAP and Siebel. In SQL Server 2005, a number of features provide mechanisms for increasing scalability for Very Large Database (VLDB) systems. The fact is, it’s significantly more complex to scale a 500GB database than a 500MB database. The processing overhead for backup and recovery functions can’t impede system availability. SQL Server 2005 enables new scalability features that include a horizontal partitioning technology, new methods for backing up and restoring large data sets, and providing higher levels of data reading concurrency. This section covers the most significant and complex VLDB technology: the table partitioning functionality.

Table and Index Partitioning

In previous releases of SQL Server, partitions were created through Distributed Partition Views (DPVs). DPVs were neither easy to set up nor easy to maintain. In SQL Server 2005, DPVs are still available, but they are deprecated. How can you implement partitioning?

Table partitioning is broken into several steps. First, you determine if a table should be partitioned. This is the most important step. Not every table benefits from partitioning. Determine which tables are performing poorly. Take normal corrective action, such as optimizing the indexes. At this point, it would be instrumental to run the DTA against the table to see if it recommends a partition. Also, look at the data. Does the table contain a mix of older data and new data? Find out if there are regulatory reasons for keeping this data live in the system.

After choosing a table, you need to define a partitioning key and decide on the number of partitions. This is trickier; the partitioning key is used to generate the partitioning function. The column used for partitioning should be able to be broken into ranges. Additionally, the range of values ultimately determines how many partitions your table will support. The maximum number of potential partitions is 1,000. Simply stated, the partitioning function maps each row to its appropriate partition. When determining your partition column, consider the total plan. The number of subsets provides the realm of possibilities for long-term partition maintenance. One of the most common partitioning columns is a data column. It provides the most natural means for dividing table data. Once you decide on a partition column, you must design a partitioning scheme.

The partition scheme maps each partition specified by the partition function to a filegroup. Essentially, the partitioning scheme maps the partition to a physical location. Planning the partition scheme essentially involves deciding which filegroup(s) you want to place your partitions on. The primary reason you may want to place your partitions on separate filegroups is to ensure that you can perform backup operations on partitions independently, because you can perform backups on individual filegroups. Additionally, you want to align your data by placing indexes on the same filegroups as the partitioned data. When you align your indexes with your partitioned data, maintenance and query performance are improved. Also, remember that partition schemes are logically separate from the partition function; you can have multiple schemes. Your scheme and function should have the same number of partitions.

Creating a filegroup is the next step, and it requires you to think about hardware. For performance and easier maintenance, filegroups should make it easier to separate the data. The number of filegroups may be limited by hardware resources. Generally, it’s best to have filegroups on different spindles so that disk I/O issues are avoided. Separating the data also has a performance benefit, because parallelism is increased across partitions. It’s also worth considering whether your partitions allow different quality and quantity of disks. For example, if your system uses a RAID 10 disk array, you might consider keeping the hottest data on those disks. Doing so has many benefits. You might also consider using less-expensive disks for partitions and filegroups of older data that doesn’t have significant workload pressure. With the portioning scheme and column planning completed and the filegroups decided on and created, you can focus on the task of creating the partitioning function and scheme and partitioning the data. When you create the partitions, remember that you must consider two boundaries: the left and the right. The partitioning function must include all data and should be restricted through a check constraint.

When you actually create and set up partitioning, you must create new tables. This may be a problem in systems where partitioning is needed but you can’t rebuild the table structure. Partitioning has some other limitations, such as data type limits. You cannot use SQL CLR, timestamp, image, or ntext types as the partitioning column. The columns must be deterministic and persisted in the column, so you can’t use a derived column. This affects mostly data warehouses, so for non-data-warehouse usages, these barriers should not be a problem. Finally, the partitions must be on the same node.

In addition to partitioning table data, indexes can be partitioned. Secondary indexes can be set up completely separately from primary indexes. The syntax for creation is the same. When the indexes and partitions are within the same filegroup, the indexes are aligned. Alignment provides several advantages; most importantly, it provides a means for simplifying data backup. Query performance is better in aligned index systems, because the I/O aspects of query processing are increased.

Backup and Restore Enhancements

For database administrators, the most gut-wrenching experience is being called in to back up from a media set for a database and having the media fail. Moreover, in previous versions of SQL Server, you couldn’t mirror the backups, so you had only one set of backups to work with. If there was a disaster and the backup was lost, the data was gone forever. SQL Server 2005 has new check features for ensuring the quality of the backed-up data.

Checksum Integrity Checks

SQL Server 2005 introduces a dbcc_checksum statement that enables extra data verification. The checksum is enabled using the SET page_verify recovery option of the alter database command. The page_verify command provides three options to discover incomplete I/O transactions caused by disk I/O errors:

  • Torn page detection. If this option is specified, a bit is reversed for each 512-byte sector in the 8KB database page when the page is written to disk. If a bit is in the wrong state when the page is later read, the page was written incorrectly, and a torn page is detected. This is the default option.
  • Checksum. If this option is specified, a checksum is taken over the contents of the entire page and is stored in the page header when a page is written to disk. When a page is read from disk, the checksum is recomputed and compared to the checksum value stored in the page header. If the values don’t match, an error message is reported to both the SQL Server error log and the NT Event Viewer.
  • None. If this option is specified, the page_verify_option is set to OFF. Future data page writes will not contain a checksum, and checksums will not be verified at read time even if a checksum is present.

Disk I/O errors can cause database corruption that is often the result of a power failure or a disk hardware error that occurs when data is being written to disk. The CHECKSUM option provides the most comprehensive level of integrity checking, offering an extra level of protection for detecting disk I/O errors that may not be detected by the disk hardware itself.

Fast Recovery

SQL Server 2005 improves the availability of SQL Server databases with a new, faster recovery option. Users can reconnect to a recovering database after the transaction log has been rolled forward. Earlier versions of SQL Server required users to wait until incomplete transactions had rolled back, even if they did not need to access affected parts of the database. A new database option, ALLOW ACTIVITY DURING UNDO, is turned on by default.

Online Restore

SQL Server 2005 introduces the ability to perform a restore operation while an instance of SQL Server is running. Online restore improves SQL Server’s availability, because only the data being restored is unavailable. The rest of the database remains online and available. Earlier versions of SQL Server required that you take a database offline before performing a restoration.

You can choose between two options when using online restore:

  • An online file-level restoration of an entire database file
  • An online page restoration of a single page of data

SQL Server 2005 also supports the online restoration of a filegroup, because a filegroup is nothing more than a collection of files.

Mirrored Backups

SQL Server 2005 introduces support for mirrored backup sets, which increases the reliability of SQL Server backups. Earlier versions of SQL Server supported only a single copy of a given backup. If backup media were damaged, roll-forward would take longer or fail. In SQL Server 2005, backup media can now be mirrored. For example, an administrator can set up four tape devices to back up two media families, with a mirror for each media family. The corresponding volumes in each mirror have identical content, making them interchangeable at restoration time. Administrators can implement up to four mirrored backup sets.

Full-Text Catalog Inclusion

SQL Server 2005 provides integrated backup and restore facilities for full-text catalogs. Earlier versions of SQL Server did not provide an integrated and reliable mechanism by which full-text catalogs could be backed up and restored. In SQL Server 2005, full-text catalogs can be backed up and restored along with, or separate from, database data. This functionality reduces the time needed to recover from a disaster and simplifies the task of moving data, including catalogs, from one computer to another without the need to fully repopulate the catalog.

This backup and restore feature provides the following capabilities:

  • You can back up and restore one or more full-text catalogs to and from media in the same manner as other data.
  • It eliminates the need to fully repopulate data after a restoration.
  • It updates full-text data to reflect changes by rolling logs forward after a restoration. Change tracking must be enabled for this capability to work.
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