按官方手冊推薦Innodb buffer Hit Ratios的計算是:
100-((iReads / iReadRequests)*100) iReads : mysql->status->Innodb_buffer_pool_reads iReadRequests: mysql->status->Innodb_buffer_pool_read_requests
出處: http://dev.mysql.com/doc/mysql-monitor/2.0/en/mem_graphref.html
搜”Hit Ratios”
推薦有興趣的同學把這個頁面都看一下應該也會有很大收獲.
另外在hackmysql: www.hackmysql.com網站上的: mysqlsqlreport中關於buffer命中計算是:
$ib_bp_read_ratio = sprintf "%.2f",
($stats{'Innodb_buffer_pool_read_requests'} ?
100 - ($stats{'Innodb_buffer_pool_reads'} /
$stats{'Innodb_buffer_pool_read_requests'}) * 100 :0);
即:
ib_bp_hit=100-(Innodb_buffer_pool_reads/Innodb_buffer_pool_read_requests)*100
另外我們知道查看Innodb Buffer Hit Ratios的地方是:
復制代碼 代碼如下:show engine innodb status\G;
Buffer pool hit rate : XXXX/1000;
那個XXX/1000即是buffer pool hit ratios的命中.
這樣也可以從代碼裡看一下這個bp命中計算:
storage/innobase/buf/buf0buf.c # void buf_print_io storage/innodbase/include/buf0buf.h #struct buf_block_struct
在buf0buf.c 中的buf_print_io函數中可以看到:
void
buf_print_io(
…
if (buf_pool->n_page_gets > buf_pool->n_page_gets_old) {
fprintf(file, "Buffer pool hit rate %lu / 1000\n",
(ulong)
(1000 - ((1000 * (buf_pool->n_pages_read
- buf_pool->n_pages_read_old))
/ (buf_pool->n_page_gets
- buf_pool->n_page_gets_old))));
} else {
fputs("No buffer pool page gets since the last printout\n",
file);
}
buf_pool->n_page_gets_old = buf_pool->n_page_gets;
buf_pool->n_pages_read_old = buf_pool->n_pages_read;
…
}
結合:
storage\innobase\include\buf0buf.h中
struct buf_block_struct{
…
ulint n_pages_read; /* number read operations */
…
ulint n_page_gets; /* number of page gets performed;
also successful searches through
the adaptive hash index are
counted as page gets; this field
is NOT protected by the buffer
pool mutex */
…
ulint n_page_gets_old;/* n_page_gets when buf_print was
last time called: used to calculate
hit rate */
…
ulint n_pages_read_old;/* n_pages_read when buf_print was
last time called */
…
從這個來看innodb buffer hit Ratios的命中計算需要本次取的值和上次值做一個減法公式應該為
ib_bp_hit=1000 – (t2.iReads – t1.iReads)/(t2.iReadRequest – t1.iReadRequest)*1000
t(n): 時間點 兩個時間間隔最少是30秒以上,在小意義不大.
iReads: Innodb_buffer_pool_reads iReadRequest: Innodb_buffer_pool_read_requests
對innodb的輸出參數有興趣的可以關注: storage/innobase/buf/Srv0srv.c 中的:
void srv_export_innodb_status()
思考:
對於innodb_buffer_pool_read_requests, innodb_buffer_pool_reads這種累加值,當很大時進行: innodb_buffer_pool_reads/innodb_buffer_pool_read_requests 相來講只能得到從開始到現在的命中率的表現了. 如果想得到現在近五分鐘,近一分鐘或是8點到9點每分鐘的命中率情況,如果還是按著innodb_buffer_pool_reads/innodb_buffer_pool_read_requests 進行計算,只能得到mysqld開起累計在8點-9點的每分鐘的累計平均命中情況.
所以如果想到每(五)分鐘的命中情況,就需要本次取得的值和一(五)分鐘前的值進行相減,然後進行運算.這樣才能得到一個當下的bp命中情況.
兩種方法沒實質的對錯的問題,但相對於源碼中的那種計算方式更容讓發現數據庫的抖動問題.
能解決的問題:
偶而的數據庫性能抖動能直觀的反應出來.
