27. Fallback

Fallback

Fallback

protects your data by storing a second copy of each row of a

table on an alternate, Fallback AMP in the same cluster.

If an AMP fails,the system accesses the Fallback rows to meet requests.

Fallback provides AMP fault tolerance at the table level. With Fallback tables, if one

AMP fails, all table data is still available. Users may continue to use

Fallback tables without any loss of available data.

During table creation or after a table is created, you may specify whether

or not the system should keep a Fallback copy of the table. If Fallback is

specified, it is automatic and transparent.

Fallback guarantees that the two copies of a row will always be on

different AMPs. If either AMP fails, the alternate row copy is still available

on the other AMP.

There is a benefit to protecting your data, but there are costs associated

with that benefit. With Fallback use, you need twice the disk space for

storage and twice the I/O for INSERTs, UPDATEs, and DELETEs. (The

Fallback option does not require any extra I/O for SELECT operations and

the Fallback I/O will be performed in parallel with the primary I/O.)

The benefits of Fallback include:

·

Protects your data from hardware (disk) failure.

·

Protects your data from software (node) failure.

·

Automatically recovers with minimum recovery time, after repairs orfixes are complete.