As Sergei Golubchik found out, XtraDB modifies the value without telling the server, and it does so conditionally. So, for big buffer pools it's still BACKOFF, while for small ones it's LEGACY, and we have a discrepancy in SYSTEM_VARIABLES:

At what size of a buffer pool will it choose LEGACY?

When we manually set innodb_empty_free_list_algorithm=LEGACY we no longer get the '[Warning] InnoDB: Difficult to find free blocks in the buffer' in the mysql error log. We changed the parameter to LEGACY on more than 10 databases and the warning went away. Can you explain the cause of this warning, its significance and how to make it go away?

For the record, a message was changed in MDEV-14776. As noted in MDEV-16617, the message used to be 3 lines when it was introduced in MySQL 3.23.44:

InnoDB: Warning: difficult to find free blocks from

InnoDB: the buffer pool (%lu search iterations)! Consider

InnoDB: increasing the buffer pool size.

The XtraDB parameter innodb_empty_free_list_algorithm=BACKOFF looks like it would cause buf_LRU_get_free_block() to sleep, relying on page cleaners to free up more blocks, and emitting the message if it fails to find free blocks due to that modification. XtraDB is not supported by MariaDB 10.2 or later, and the parameter never existed in InnoDB.

The default value of the parameter is BACKOFF since its introduction in MariaDB Server 10.0.10.

Given that MariaDB 10.0 has already reached and 10.1 will soon be at its end of life, I am reluctant to change the default value of this parameter.

Note: Before MDEV-19176 was fixed, InnoDB recovery could hang, emitting this message in an infinite loop.

MariaDB Server 10.1 reached its end of life in October 2020. The last release was 10.1.48.