innodb_max_dirty_pages_pct_lwm=0 lost its special meaning

In MDEV-23855, I overlooked the fact that the default value 0 of the parameter innodb_max_dirty_pages_pct_lwm has a special meaning: "ignore this parameter, and look at innodb_max_dirty_pages_pct instead". This special value used to partially cancel the effect of the parameter innodb_adaptive_flushing=ON (which is set by default). The special value 0 would cause the function af_get_pct_for_dirty() to always return either 0 or 100.

This regression was originally reported in MDEV-24272, mixed up with another performance regression that only affects the 10.2, 10.3, and 10.4 release series but not 10.5. On a hard disk, running a 5-minute oltp_read_write in sysbench with 16 threads and 8 tables with 100000 rows each, I verified valerii's finding, using the following settings on MariaDB 10.5.6 and 10.5.8:

innodb_log_file_size=4G

innodb_buffer_pool_size=1G

innodb_flush_log_at_trx_commit=2

innodb-flush-method=O_DIRECT

On my 2TB Western Digital SATA 3.0 hard disk (WDC WD20EZRZ-00Z5HB0) that has a write performance of 51.9 MB/s (reported by GNOME Disks when using 1MiB block size), I got the following results:

The last line was produced with the following fix:

diff --git a/storage/innobase/buf/buf0flu.cc b/storage/innobase/buf/buf0flu.cc

--- a/storage/innobase/buf/buf0flu.cc

+++ b/storage/innobase/buf/buf0flu.cc

@@ -2086,6 +2086,12 @@ static os_thread_ret_t DECLARE_THREAD(buf_flush_page_cleaner)(void*)

     const double dirty_pct= double(dirty_blocks) * 100.0 /

       double(UT_LIST_GET_LEN(buf_pool.LRU) + UT_LIST_GET_LEN(buf_pool.free));

+    if (dirty_pct < srv_max_buf_pool_modified_pct)

+      continue;

+

+    if (srv_max_dirty_pages_pct_lwm == 0.0)

+      continue;

+

     if (dirty_pct < srv_max_dirty_pages_pct_lwm)

       continue;

This above patch is only applicable to 10.5.7 and 10.5.8 only; the code was slightly refactored in MDEV-24278 since then.

I believe that a work-around of this regression is to set innodb_max_dirty_pages_pct_lwm to the same value as innodb_max_dirty_pages_pct (default value: 90).

Side note: The parameter innodb_idle_flush_pct has no effect (MDEV-24536).