在前面的blog中，我们知道，redo entries写入log buffer大致的过程如下：
在PGA中生产Redo Entry -> 服务进程获取Redo Copy latch(存在多个---CPU_COUNT*2) -> 服务进程获取redo allocation latch(仅1个) -> 分配log buffer ->
释放redo allocation latch -> 将Redo Entry写入Log Buffer -> 释放Redo Copy latch
由于log buffer是一块“共享”内存，为了避免冲突，它是受到redo allocation latch保护的，每个server process需要先获取到该latch才能分配redo buffer。因此，在OLTP系统中，我们通常可以观察到redo allocation latch的等待事件。
oracle引入shared strand和private strand来实现并行redo buffer分配机制，借此避免高并发下的redo allocation latch等待事件。
1 shared strand
为了减少redo allocation latch等待事件，oracle引入了log buffer的并行机制。其基本原理是，将log buffer划分为多个小的buffer，这些小的buffer被称作shared strand。每一个shared strand受到一个单独的redo allocation latch的保护。多个shared strand的出现，使得原来序列化的redo buffer分配变成了并行的过程，从而减少了redo allocation latch的等待。
shared strand由一些隐藏参数控制：
- 09:39:59 sys@ORCL (^ω^) col name for a25
- 09:42:11 sys@ORCL (^ω^) col value for a10
- 09:42:11 sys@ORCL (^ω^) col description for a55
- 09:42:11 sys@ORCL (^ω^) select a.ksppinm name,b.ksppstvl value,a.ksppdesc description
- 09:42:11 2 from x$ksppi a,x$ksppcv b
- 09:42:11 3 where a.indx = b.indx
- 09:42:11 4 and a.ksppinm like '%_log_parallelism%'
- 09:42:13 5 /
- NAME VALUE DESCRIPTION
- ------------------------- ---------- -------------------------------------------------------
- _log_parallelism 1 Number of log buffer strands
- _log_parallelism_max 2 Maximum number of log buffer strands
- _log_parallelism_dynamic TRUE Enable dynamic strands --控制是否允许shared strand数量在_log_parallelism和_log_parallelism_max之间动态变化
09:39:59 sys@ORCL (^ω^) col name for a25 09:42:11 sys@ORCL (^ω^) col value for a10 09:42:11 sys@ORCL (^ω^) col description for a55 09:42:11 sys@ORCL (^ω^) select a.ksppinm name,b.ksppstvl value,a.ksppdesc description 09:42:11 2 from x$ksppi a,x$ksppcv b 09:42:11 3 where a.indx = b.indx 09:42:11 4 and a.ksppinm like '%_log_parallelism%' 09:42:13 5 / NAME VALUE DESCRIPTION ------------------------- ---------- ------------------------------------------------------- _log_parallelism 1 Number of log buffer strands _log_parallelism_max 2 Maximum number of log buffer strands _log_parallelism_dynamic TRUE Enable dynamic strands --控制是否允许shared strand数量在_log_parallelism和_log_parallelism_max之间动态变化
每一个shared_strand的大小=log_buffer/(shared_strand的数量)：
- 12:55:40 sys@ORCL (^ω^) select indx,strand_size_kcrfa from x$kcrfstrand where last_buf_kcrfa != '00';
- INDX STRAND_SIZE_KCRFA
- ---------- -----------------
- 0 3512320
- 1 3512320
- 12:58:46 sys@ORCL (^ω^) show parameter log_buffer
- NAME_COL_PLUS_S TYPE VALUE_COL_PLUS_
- --------------- --------------- ---------------
- log_buffer integer 7024640
- 12:58:57 sys@ORCL (^ω^) select 7024640/2 from dual;
- 7024640/2
- ----------
- 3512320
12:55:40 sys@ORCL (^ω^) select indx,strand_size_kcrfa from x$kcrfstrand where last_buf_kcrfa != '00';
INDX STRAND_SIZE_KCRFA
---------- -----------------
0 3512320
1 3512320
12:58:46 sys@ORCL (^ω^) show parameter log_buffer
NAME_COL_PLUS_S TYPE VALUE_COL_PLUS_
--------------- --------------- ---------------
log_buffer integer 7024640
12:58:57 sys@ORCL (^ω^) select 7024640/2 from dual;
7024640/2
----------
3512320
关于shared strand的数量设置，16个cpu之内最大默认为2，当系统中存在redo allocation latch等待时，每增加16个cpu可以考虑增加1个strand，最大不应该超过8。
并且_log_parallelism_max不允许大于cpu_count。
2 private strand
为了进一步降低redo buffer的冲突，10g引入了private strand机制，这是从shared pool中分配出来的一块内存空间。每一个Private strand受到一个单独的redo allocation latch保护，每个Private strand作为“私有的”strand只会服务于一个活动事务。获取到了Private strand的用户事务不是在PGA中而是在Private strand生成Redo。当flush private strand或者commit时，Private strand被批量写入log文件中。如果新事务申请不到Private strand的redo allocation latch，则会继续遵循旧的redo buffer机制，申请写入shared strand中。对于未能获取到Private strand的redo allocation latch的事务，在事务结束前，即使已经有其它事务释放了Private strand，也不会再申请Private strand了。
整个事务过程大致如下：
事务开始 -> 申请Private strand的redo allocation latch (申请失败则申请Shared Strand的redo allocation latch) -> 在Private strand中生产Redo Enrey ->
Flush/Commit ->申请Redo Copy Latch -> 服务进程将Redo Entry批量写入Log File -> 释放Redo Copy Latch -> 释放Private strand的redo allocation latch
- 12:59:30 sys@ORCL (^ω^) select * from v$sgastat where name like '%strand%';
- POOL NAME BYTES
- ------------------------ --------------- ----------
- shared pool private strands 1198080
- 13:12:31 sys@ORCL (^ω^) select indx,strand_size_kcrfa from x$kcrfstrand where last_buf_kcrfa = '00';
- INDX STRAND_SIZE_KCRFA
- ---------- -----------------
- 2 66560
- 3 66560
- 4 66560
- 5 66560
- 6 66560
- 7 66560
- 8 66560
- 9 66560
- 10 66560
- 11 66560
- 12 66560
- 13 66560
- 14 66560
- 15 66560
- 16 66560
- 17 66560
- 18 66560
- 19 66560
- 已选择18行。
- 14:53:42 sys@ORCL (^ω^) select 66560*18 from dual;
- 66560*18
- ----------
- 1198080
12:59:30 sys@ORCL (^ω^) select * from v$sgastat where name like '%strand%';
POOL NAME BYTES
------------------------ --------------- ----------
shared pool private strands 1198080
13:12:31 sys@ORCL (^ω^) select indx,strand_size_kcrfa from x$kcrfstrand where last_buf_kcrfa = '00';
INDX STRAND_SIZE_KCRFA
---------- -----------------
2 66560
3 66560
4 66560
5 66560
6 66560
7 66560
8 66560
9 66560
10 66560
11 66560
12 66560
13 66560
14 66560
15 66560
16 66560
17 66560
18 66560
19 66560
已选择18行。
14:53:42 sys@ORCL (^ω^) select 66560*18 from dual;
66560*18
----------
1198080
每个Private strand的大小为65K。Private strand的数量受到2个方面的影响：min（logfile的大小，活跃事务数量）
2.1 logfile的大小
参数_log_private_mul指定了使用多少logfile空间预分配给Private strand，默认为5。我们可以根据当前logfile的大小（要除去预分配给log buffer的空间）计算出这一约束条件下能够预分配多少个Private strand。
- 15:27:25 sys@ORCL (^ω^) select bytes from v$log where status='CURRENT';
- BYTES
- ----------
- 52428800
- 15:32:30 sys@ORCL (^ω^) select trunc(((select bytes from v$log where status = 'CURRENT') - (select to_number(value) from v$parameter where name = 'log_buffer'))*
- 15:35:16 2 (select to_number(val.KSPPSTVL) from sys.x$ksppi nam, sys.x$ksppsv val where nam.indx = val.indx AND nam.ksppinm = '_log_private_mul') / 100 / 66560)
- 15:35:16 3 as "calculated private strands"
- 15:35:16 4 from dual;
- calculated private strands
- --------------------------
- 34
- 15:35:22 sys@ORCL (^ω^) select count(1) "actual private strands" from x$kcrfstrand where last_buf_kcrfa = '00';
- actual private strands
- ----------------------
- 18
- 15:36:03 sys@ORCL (^ω^) alter system switch logfile;
- 系统已更改。
- 15:36:22 sys@ORCL (^ω^) select bytes from v$log where status='CURRENT';
- BYTES
- ----------
- 52428800
- 15:36:46 sys@ORCL (^ω^) select trunc(((select bytes from v$log where status = 'CURRENT') - (select to_number(value) from v$parameter where name = 'log_buffer'))*
- 15:36:59 2 (select to_number(val.KSPPSTVL) from sys.x$ksppi nam, sys.x$ksppsv val where nam.indx = val.indx AND nam.ksppinm = '_log_private_mul') / 100 / 66560)
- 15:37:00 3 as "calculated private strands"
- 15:37:00 4 from dual;
- calculated private strands
- --------------------------
- 34
- 15:37:01 sys@ORCL (^ω^) select count(1) "actual private strands" from x$kcrfstrand where last_buf_kcrfa = '00';
- actual private strands
- ----------------------
- 18
- 15:37:19 sys@ORCL (^ω^)
15:27:25 sys@ORCL (^ω^) select bytes from v$log where status='CURRENT';
BYTES
----------
52428800
15:32:30 sys@ORCL (^ω^) select trunc(((select bytes from v$log where status = 'CURRENT') - (select to_number(value) from v$parameter where name = 'log_buffer'))*
15:35:16 2 (select to_number(val.KSPPSTVL) from sys.x$ksppi nam, sys.x$ksppsv val where nam.indx = val.indx AND nam.ksppinm = '_log_private_mul') / 100 / 66560)
15:35:16 3 as "calculated private strands"
15:35:16 4 from dual;
calculated private strands
--------------------------
34
15:35:22 sys@ORCL (^ω^) select count(1) "actual private strands" from x$kcrfstrand where last_buf_kcrfa = '00';
actual private strands
----------------------
18
15:36:03 sys@ORCL (^ω^) alter system switch logfile;
系统已更改。
15:36:22 sys@ORCL (^ω^) select bytes from v$log where status='CURRENT';
BYTES
----------
52428800
15:36:46 sys@ORCL (^ω^) select trunc(((select bytes from v$log where status = 'CURRENT') - (select to_number(value) from v$parameter where name = 'log_buffer'))*
15:36:59 2 (select to_number(val.KSPPSTVL) from sys.x$ksppi nam, sys.x$ksppsv val where nam.indx = val.indx AND nam.ksppinm = '_log_private_mul') / 100 / 66560)
15:37:00 3 as "calculated private strands"
15:37:00 4 from dual;
calculated private strands
--------------------------
34
15:37:01 sys@ORCL (^ω^) select count(1) "actual private strands" from x$kcrfstrand where last_buf_kcrfa = '00';
actual private strands
----------------------
18
15:37:19 sys@ORCL (^ω^)
2.2 活跃事务数量
参数_log_private_parallelism_mul用于推算活跃事务数量在最大事务数量中的百分比，默认为10。Private strand的数量不能大于活跃事务的数量。
- 15:37:19 sys@ORCL (^ω^) show parameter transactions
- NAME_COL_PLUS_S TYPE VALUE_COL_PLUS_
- --------------- --------------- ---------------
- transactions integer 187
- transactions_pe integer 5
- r_rollback_segm
- ent
- 15:42:47 sys@ORCL (^ω^) select trunc((select to_number(value) from v$parameter where name = 'transactions') *
- 15:44:33 2 (select to_number(val.KSPPSTVL)
- 15:44:33 3 from sys.x$ksppi nam, sys.x$ksppsv val
- 15:44:33 4 where nam.indx = val.indx
- 15:44:33 5 AND nam.ksppinm = '_log_private_parallelism_mul') / 100 )
- 15:44:33 6 as "calculated private strands"
- 15:44:33 7 from dual;
- calculated private strands
- --------------------------
- 18
- 15:44:35 sys@ORCL (^ω^) select count(1) "actual private strands" from x$kcrfstrand where last_buf_kcrfa = '00';
- actual private strands
- ----------------------
- 18