What is Fallback in Teradata?
FALLBACK ensures that a second copy of each row inserted into a table is stored on another AMP in the same cluster. This is done in case the AMP goes down or the disks fail. There are usually four AMPs grouped together in a cluster. FALLBACK allows for processing to continue in the event that an AMP is lost. As a matter of fact, FALLBACK allows for the loss of one AMP in each cluster. If 2 AMPS are lost, then the system comes to a halt.
The cost for FALLBACK is that twice as much disk storage space is used for row storage as compared to not using FALLBACK. The cost is also twice the I/O on inserts, updates, and deletes because there are always two copies to write. One copy goes to the primary AMP where it belongs and the other copy goes to different AMP in the same cluster. Since Teradata AMPs operate in parallel; so, it does not take twice as long to store a duplicate row. Both rows are placed on their respective AMPs at nearly the same time.
To overcome the Storage disadvantage of Fallback, TD also allows Virtual Processor migration. Since AMP and PE are both software modules, they can be migrated to a different node.
The VProc migration provides fault-tolerance without using much extra space.
The below image explains a PE + AMP migration.
FALLBACK is an optional feature. Only important tables are fallback protected.
FALLBACK ensures that a second copy of each row inserted into a table is stored on another AMP in the same cluster. This is done in case the AMP goes down or the disks fail. There are usually four AMPs grouped together in a cluster. FALLBACK allows for processing to continue in the event that an AMP is lost. As a matter of fact, FALLBACK allows for the loss of one AMP in each cluster. If 2 AMPS are lost, then the system comes to a halt.
The cost for FALLBACK is that twice as much disk storage space is used for row storage as compared to not using FALLBACK. The cost is also twice the I/O on inserts, updates, and deletes because there are always two copies to write. One copy goes to the primary AMP where it belongs and the other copy goes to different AMP in the same cluster. Since Teradata AMPs operate in parallel; so, it does not take twice as long to store a duplicate row. Both rows are placed on their respective AMPs at nearly the same time.
To overcome the Storage disadvantage of Fallback, TD also allows Virtual Processor migration. Since AMP and PE are both software modules, they can be migrated to a different node.
The VProc migration provides fault-tolerance without using much extra space.
The below image explains a PE + AMP migration.
FALLBACK is an optional feature. Only important tables are fallback protected.
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