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Encrypt doublewrite buffers

A summary of Doublewrite buffer and Doublewrite buffer encryption changes:

Percona Server for MySQL Versions Doublewrite Buffer and Doublewrite Buffer Encryption Implementation
Percona Server from Percona-Server-8.0.23-14 MySQL 8.0.23 implemented its own version of parallel doublewrite encryption. Pages that belong to encrypted tablespaces are also written into the doublewrite buffer in an encrypted form. The Percona implementation was reverted and theinnodb_parallel_dblwr_encrypt is deprecated and may be removed in later releases.
Percona Server from Percona-Server-8.0.20-11 to Percona-Server-8.0.22-13 inclusive MySQL 8.0.20 implemented its own parallel doublewrite buffer, which is stored in external files (#ib_16384_xxx.dblwr) and not stored in the system tablespace. Percona’s implementation was reverted. As a result, innodb_parallel_doublewrite_path was deprecated.
However, MySQL did not implement parallel doublewrite buffer encryption at this time, so Percona reimplemented parallel doublewrite buffer encryption on top of the MySQL parallel doublewrite buffer implementation. Percona preserved the meaning and functionality of the innodb_parallel_dblwr_encrypt variable.
Percona-Server-8.0.12-1.alpha to Percona-Server-8.0.19-10 inclusive Percona Server for MySQL had its own implementation of the parallel doublewrite buffer which was enabled by setting the innodb_parallel_doublewrite_path variable.
Enabling the innodb_parallel_dblwr_encrypt controlled whether the parallel doublewrite pages were encrypted or not. In case the parallel doublewrite buffer was disabled (innodb_parallel_doublewrite_path was set to empty string),the doublewrite buffer pages were located in the system tablespace (ibdata1). The system tablespace itself could be encrypted by setting innodb_sys_tablespace_encrypt, which also encrypted the doublewrite buffer pages.

For Percona Server for MySQL versions below Percona Server for MySQL version 8.0.23-14, Percona encrypts the doublewrite buffer using innodb_parallel_dblwr_encrypt.

innodb_parallel_dblwr_encrypt

Option Description
Command-line –innodb-parallel-dblwr-encrypt
Scope Global
Dynamic Yes
Data type Boolean
Default OFF

Percona Server for MySQL 8.0.23-14 has deprecated this variable and the variable has no effect.

This variable controls whether the parallel doublewrite buffer pages were encrypted or not. The encryption used the key of the tablespace to which the page belongs.

Starting from Percona Server for MySQL 8.0.23-14 the value of this variable is ignored. Pages from the encrypted tablespaces are always written to the doublewrite buffer as encrypted, and pages from unencrypted tablespaces are always written unencrypted.

The innodb_parallel_dblwr_encrypt is accepted but has no effect. An explicit attempt to change the value generates the following warning in the error log file:

Error message 
**Setting Percona-specific INNODB_PARALLEL_DBLWR_ENCRYPT is deprecated and has no effect.**