Inadequate background LRU flushing for write workloads with InnoDB compression

The provided fix candidate for MDEV-5949 significantly improved the situation but not fixed it fully. Now the performance remains quite a stable for whole 6h LinkBench run. However, still performance numbers measured using MariaDB 10.0.9 containing the fix candidate contain a significant difference between XtraDB and InnoDB.

XtraDB: Requests/second = 15700
InnoDB: Requests/second = 20485

These number without using multi-threaded flush or atomic writes and doublewrite on. I will attach full outputs from both LinkBench measure phases and used configuration file and used load and run commands for LinkBench.