ALTER TABLE, FALLBACK, and Read From Fallback
Fallback is very important when a system needs to reconstruct data from fallback copies if a hardware read error occurs when it attempts to read the primary copy of the data. When a read error occurs in this case, the file system reads the fallback copy of the rows and reconstructs a memory‑resident image of them on their home AMP. This is referred to as Read From Fallback. See Database Design.
Without this feature, the file system fault isolation logic would abort the transaction and, depending on the error, possibly mark the table as being down. See “SET DOWN and RESET DOWN Options” on page 138. Support for Read From Fallback is limited to the following cases:
In some cases, Active Fallback can repair the damage to the primary data dynamically. In situations where the bad data block cannot be repaired, Read From Fallback substitutes an error-free fallback copy of the corrupt rows each time the read error occurs.
To avoid the overhead of this substitution, you must rebuild the primary copy of the data manually from the fallback copy using the Table Rebuild utility. For information about Table Rebuild, see Utilities: Volume 2 (L-Z).