注意：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。
每个Private strand的大小为65K。10g中，shared pool中的Private strands的大小就是活跃会话数乘以65K，而11g中，在shared pool中需要为每个Private strand额外分配4k的管理空间，即：数量*69k。
--10g: SQL> select * from V$sgastat where name like '%strand%'; POOL NAME BYTES ------------ -------------------------- ---------- shared pool private strands 1198080 HELLODBA.COM>select trunc(value * KSPPSTVL / 100) * 65 * 1024 2 from (select value from v$parameter where name = 'transactions') a, 3 (select val.KSPPSTVL 4 from sys.x$ksppi nam, sys.x$ksppsv val 5 where nam.indx = val.indx 6 AND nam.ksppinm = '_log_private_parallelism_mul') b; TRUNC(VALUE*KSPPSTVL/100)*65*1024 ------------------------------------- 1198080 --11g: HELLODBA.COM>select * from V$sgastat where name like '%strand%'; POOL NAME BYTES ------------ -------------------------- ---------- shared pool private strands 706560 HELLODBA.COM>select trunc(value * KSPPSTVL / 100) * (65 + 4) * 1024 2 from (select value from v$parameter where name = 'transactions') a, 3 (select val.KSPPSTVL 4 from sys.x$ksppi nam, sys.x$ksppsv val 5 where nam.indx = val.indx 6 AND nam.ksppinm = '_log_private_parallelism_mul') b; TRUNC(VALUE*KSPPSTVL/100)*(65+4)*1024 ------------------------------------- 706560 Private strand的数量受到2个方面的影响：logfile的大小和活跃事务数量。
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：
HELLODBA.COM>select bytes from v$log where status = 'CURRENT'; BYTES ---------- 52428800 HELLODBA.COM>select trunc(((select bytes from v$log where status = 'CURRENT') - (select to_number(value) from v$parameter where name = 'log_buffer'))* 2 (select to_number(val.KSPPSTVL) 3 from sys.x$ksppi nam, sys.x$ksppsv val 4 where nam.indx = val.indx 5 AND nam.ksppinm = '_log_private_mul') / 100 / 66560) 6 as "calculated private strands" 7 from dual; calculated private strands -------------------------- 5 HELLODBA.COM>select count(1) "actual private strands" from x$kcrfstrand where last_buf_kcrfa = '00'; actual private strands ---------------------- 5 当logfile切换后(和checkpoint一样，切换之前必须要将所有Private strand的内容flush到logfile中，因此我们在alert log中可能会发现日志切换信息之前会有这样的信息："Private strand flush not complete"，这是可以被忽略的)，会重新根据切换后的logfile的大小计算对Private strand的限制：
HELLODBA.COM>alter system switch logfile; System altered. HELLODBA.COM>select bytes from v$log where status = 'CURRENT'; BYTES ---------- 104857600 HELLODBA.COM>select trunc(((select bytes from v$log where status = 'CURRENT') - (select to_number(value) from v$parameter where name = 'log_buffer'))* 2 (select to_number(val.KSPPSTVL) 3 from sys.x$ksppi nam, sys.x$ksppsv val 4 where nam.indx = val.indx 5 AND nam.ksppinm = '_log_private_mul') / 100 / 66560) 6 as "calculated private strands" 7 from dual; calculated private strands -------------------------- 13 HELLODBA.COM>select count(1) "actual private strands" from x$kcrfstrand where last_buf_kcrfa = '00'; actual private strands ---------------------- 13 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。
HELLODBA.COM>show parameter transactions NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ transactions integer 222 transactions_per_rollback_segment integer 5 HELLODBA.COM>select trunc((select to_number(value) from v$parameter where name = 'transactions') * 2 (select to_number(val.KSPPSTVL) 3 from sys.x$ksppi nam, sys.x$ksppsv val 4 where nam.indx = val.indx 5 AND nam.ksppinm = '_log_private_parallelism_mul') / 100 ) 6 as "calculated private strands" 7 from dual; calculated private strands -------------------------- 22 HELLODBA.COM>select count(1) "actual private strands" from x$kcrfstrand where last_buf_kcrfa = '00'; actual private strands ---------------------- 22 注：在预分配Private strand时，会选择上述2个条件限制下最小一个数量。但相应的shared pool的内存分配和redo allocation latch的数量是按照活跃事务数预分配的。
因此，如果logfile足够大，_log_private_parallelism_mul与实际活跃进程百分比基本相符的话，Private strand的引入基本可以消除redo allocation latch的争用问题。
更新时间：2012-07-15
