为什么我的err日志里面有大量的show engine innodb status的记录，我自己并没有开启innodb_status_output参数。
二、问题分析
通过查看日志，发现如下输出：
2019-03-21T17:00:02.375231Z 1230497 [Warning] InnoDB: Difficult to find free blocks in the buffer pool (338 search iterations)! 0 failed attempts to flush a page! Consider increasing the buffer pool size. It is also possible
that in your Unix version fsync is very slow, or completely frozen inside the OS kernel. Then upgrading to a
newer version of your operating system may help. Look at the number of fsyncs in diagnostic info below.
Pending flushes (fsync) log: 0; buffer pool: 0. 1446962050 OS file reads, 545881917 OS file writes,
376257282 OS fsyncs. Starting InnoDB Monitor to print further diagnostics to the standard output.
日志也写得很清楚。应该是free block不够了Innodb自动开启了。但是我们需要源码验证一下。
三、源码验证
在源码的buf_LRU_handle_lack_of_free_blocks函数中我们看到如下：
if ((current_ms > started_ms + 2000)
&& (current_ms > last_printout_ms + 2000)
&& srv_buf_pool_old_size == srv_buf_pool_size) {
ib::warn() << "Difficult to find free blocks in the buffer pool"
" (" << n_iterations << " search iterations)! "
<< flush_failures << " failed attempts to"
" flush a page! Consider increasing the buffer pool"
" size. It is also possible that in your Unix version"
" fsync is very slow, or completely frozen inside"
" the OS kernel. Then upgrading to a newer version"
" of your operating system may help. Look at the"
" number of fsyncs in diagnostic info below."
" Pending flushes (fsync) log: "
<< fil_n_pending_log_flushes
<< "; buffer pool: "
<< fil_n_pending_tablespace_flushes
<< ". " << os_n_file_reads << " OS file reads, "
<< os_n_file_writes << " OS file writes, "
<< os_n_fsyncs
<< " OS fsyncs. Starting InnoDB Monitor to print"
" further diagnostics to the standard output.";
last_printout_ms = current_ms;
*mon_value_was = srv_print_innodb_monitor;
*started_monitor = true;
srv_print_innodb_monitor = true;
os_event_set(srv_monitor_event);
这里不仅打印出了日志同时设置了参数srv_print_innodb_monitor = true; 并且开始os_event_set(srv_monitor_event);开启了monitor打印线程。那我们看看srv_print_innodb_monitor 对应什么参数呢。如下：
static MYSQL_SYSVAR_BOOL(status_output, srv_print_innodb_monitor,
PLUGIN_VAR_OPCMDARG, "Enable InnoDB monitor output to the error log.", NULL, innodb_status_output_update, FALSE);
实际上就是innodb_status_output被自动开了。当然如果查看调用可以在上层函数buf_LRU_get_free_block中查看到调用，实际上就是在free list找不到空闲的block的时候会做输出。buf_LRU_get_free_block还包含了一个块的分配流程大约如下，可自行参考：
If there is a block in the free list, take it .如果这里找不到就会自动开启innodb_status_output
If no block was in the free list, search from the end of the LRU list and try to free a block there.
No free block was found: try to flush the LRU list.
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