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A cluster is a group of AMPs that provide for fallback capability. When a table is protected with the FALLBACK option, a copy of each permanent data row is stored on a separate AMP in the cluster. A large system usually consists of many of these AMP clusters. (For details on the FALLBACK option, see of row distribution within a cluster, see "AMP Clustering and Fallback" on page 7-32.)
A smaller cluster size reduces the chances of a down AMP causing a non-operational configuration, while a large cluster size causes less performance degradation while an AMP is down.
If you activate RAID 1 or RAID 5, you may not want to use fallback protection for all of your data. It might be more cost-effective in terms of disk space to activate fallback protection for only those tables where an added measure of protection is needed—in case of software failure or the loss of two failed disks in a rank—which RAID 5 and cannot protect you from.
The following SQL statements demonstrate how to define the default when tables are created. The fallback option is actually activated when you use the CREATE TABLE or ALTER TABLE statements:
CREATE USER maxim ,AS PERMANENT =1000000, ,PASSWORD = mxm, ,FALLBACK;
MODIFY USER maxim as FALLBACK ;
AMPs are virtual processes, so they themselves cannot experience a hardware failure. If an AMP loses two disks in a rank, it will be unable to access its data and is the only situation where an AMP will stay down.
7 - 2 Teradata RDBMS Database Administration
Chapter 7: Protecting Data
Automatic Data Protection Mechanisms
Two down AMPs in the same cluster causes the Teradata database to halt. A software problem can cause an AMP to go down and the database to restart, but as long as the AMP can access its disk, it should come back up during the restart. (For details, see "Restarts of the Teradata RDBMS" on page 9-3.)
Down AMP Recovery Journal
The DownAMP Recovery Journal provides automatic data recovery on fallback-protected data tables when a clustered AMP is out of service. This journal consists of two system files stored in user DBC:
When a clustered AMP is out of service, the Down AMP Recovery Journal automatically captures changes to fallback-protected tables from the other AMPs in the cluster (see "AMP Clustering and Fallback" on page 7-32).
Each time a change is made to a fallback protected row whose copy resides on a down AMP, the Down AMP Recovery Journal stores the table ID and row ID of the committed changes. When the AMP comes back online, Teradata opens the Down AMP Recovery Journal to update, or roll forward, any changes made while the AMP was down.
The recovery operation uses fallback rows to replace primary rows and primary rows to replace fallback rows. The journal ensures that the information on the fallback AMP and on the primary AMP is identical. Once the transfer of information is complete and verified, the Down AMP Recovery Journal is discarded automatically.
Note: Space for the Down AMP Recovery Journal is allocated from user DBC.
An AMP is placed out of service if two physical disks fail in a single rank. The AMP remains out of service until you replace the disks and run the Table Rebuild utility to reconstruct the table headers of fallback tables on the failed AMP. (For instructions on using Table Rebuild, see Teradata RDBMS Utilities.)
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Teradata RDBMS Database AdministrationChapter 7: Protecting Data
Disk Arrays and RAID Technology
Disk Arrays and RAID Technology
A disk array is a configuration that consists of a number of drives that utilize specialized disk controllers to manage and distribute data and parity across disks, while providing fast access and data integrity. (Also see "Hardware Data Protection" on page 7-23.)
A disk array is a parallel collection of disk drives connected through an array controller board (by a SCSI interface) to an SMP or MPP node. The processors do not directly access the disks, but instead issue requests for data on logical units maintained by the array controller.
Various recording techniques offer different degrees of data protection. Teradata RDBMS works with Redundant Array of Independent Disk (RAID) technology at levels RAID 1, RAID 5, and RAID S, the EMC implementation of RAID 4. All levels provide data protection in the event of a single disk failure.
RAID 1 technology uses pairs of disk drives that contain mirrored data. The two disks are identical representations of the same data. If one drive fails, the alternate, or mirrored drive, continues operation.
RAID 1 mirrors data by striping segments across data drives and mirrored data drives. Any time the system writes data to a drive, it writes it to its mirrored drive. Once a failed drive is replaced, the array controller automatically restores the contents using data recorded on the mirrored drive. (Although you can continue to operate a RAID 1 array with a failed data drive, you should replace the drive as soon as possible.)