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On certain file systems (e.g. ext4) pg_control may appear torn if there is a concurrent write while reading the file. To prevent an invalid read, retry until the checksum matches the control data. Special handling is required for the pg-version-force feature since the offset of the checksum is not known. In this case, scan from the default position to the end of the data looking for a checksum match. This is a bit imprecise, but better than nothing, and the chance of a random collision in the control data seems very remote considering the ratio of data size (< 512 bytes) to checksum size (4 bytes). This was discovered and a possible solution proposed for PostgreSQL in [1]. The proposed solution may work for backup, but pgBackRest needs to be able to read pg_control reliably outside of backup. So no matter what fix is adopted for PostgreSQL, pgBackRest need retries. Further adjustment may be required as the PostgreSQL fix evolves. [1] https://www.postgresql.org/message-id/20221123014224.xisi44byq3cf5psi%40awork3.anarazel.de |
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