Oracle-BCT(Block Change Tracking)参数优化配置
背景:
Oracle10g之后引入的BCT(Block Change Tracking)功能，可以用以提高增量备份的效率，减少扫描的块数量时间，但对于开启了BCT功能的数据库，在业务高并发的期间可能会出现性能的缓慢问题，经常可以观察到等待事件block change tracking buffer space，change tracking file synchronous read/write，除了尽可能的应用最新的补丁来规避bug问题，Oracle官方建议可以通过调整BCT的相关隐含参数进行优化以尽可能的规避出现的性能问题。
本文接下来主要讲解BCT涉及的主要优化参数。
BCT隐含参数(Version:Oracle 19.3):
select a.ksppinm name, b.ksppstvl value, a.ksppdesc description
from sys.x$ksppi a, sys.x$ksppcv b
where a.indx = b.indx and a.ksppinm like '%bct%'
order by name
NAME VALUE DESCRIPTION
---------------------------------------- -------------------- ------------------------------------------------------------------------------------------
_bct_bitmaps_per_file 8 number of bitmaps to store for each datafile
_bct_buffer_allocation_max 104857600 maximum size of all change tracking buffer allocations, in bytes
_bct_buffer_allocation_min_extents 1 mininum number of extents to allocate per buffer allocation
_bct_buffer_allocation_size 2097152 size of one change tracking buffer allocation, in bytes
_bct_chunk_size 0 change tracking datafile chunk size, in bytes
_bct_crash_reserve_size 262144 change tracking reserved crash recovery SGA space, in bytes
_bct_file_block_size 0 block size of change tracking file, in bytes
_bct_file_extent_size 0 extent size of change tracking file, in bytes
_bct_fixtab_file change tracking file for fixed tables
_bct_health_check_interval 60 CTWR health check interval (seconds), zero to disable
_bct_initial_private_dba_buffer_size 0 initial number of entries in the private change tracking dba buffers
_bct_mrp_timeout 600 CTWR MRP wait timeout (seconds), zero to wait forever
_bct_public_dba_buffer_dynresize 2 allow dynamic resizing of public dba buffers, zero to disable
_bct_public_dba_buffer_maxsize 0 max buffer size permitted for public dba buffers, in bytes
_bct_public_dba_buffer_size 0 total size of all public change tracking dba buffers, in bytes
_bct_buffer_allocation_max:
参数说明:全部change tracking buffer分配的最大值
默认值为:104857600
建议值:根据Oracle MOS(Doc ID 2094946.1)
推荐为4倍dba_buffer_count_public*dba_entry_count_public*dba_entry_size并且小于large_pool池大小
--可以通过表x$krcstat查询当前值
select dba_buffer_count_public*dba_entry_count_public*dba_entry_size from x$krcstat;参数测试:在Oracle19.3版本测试，_bct_buffer_allocation_max参数无法限制CTWR dba buffer的大小
--设置_bct_buffer_allocation_max为5M
SQL> alter system set "_bct_buffer_allocation_max"=5242880;
System altered.
--当前CTWR dba buffer大小为1.79M
SQL> select inst_id,pool,name,bytes/1024/1024 as MB ,con_id from gv$sgastat where pool='large pool';
INST_ID POOL NAME MB CON_ID
---------- -------------- ---------------------------------------- ---------- ----------
1 large pool free memory 2028.84766 0
1 large pool ASM map operations hashta .375 0
1 large pool krcc extent chunk 1.98046875 0
1 large pool PX msg pool 15 0
1 large pool CTWR dba buffer 1.796875 0
--执行大批量的块更新之后，CTWR dba buffer大小变为14.359MB，超过了5MB，参数并不能限制CTWR dba buffer的大小
SQL> r
1* select inst_id,pool,name,bytes/1024/1024 as MB ,con_id from gv$sgastat where pool='large pool'
INST_ID POOL NAME MB CON_ID
---------- -------------- ---------------------------------------- ---------- ----------
1 large pool free memory 2016.28516 0
1 large pool ASM map operations hashta .375 0
1 large pool krcc extent chunk 1.98046875 0
1 large pool PX msg pool 15 0
1 large pool CTWR dba buffer 14.359375 0
SQL>
NAME VALUE DESCRIPTION
---------------------------------------- -------------------- ------------------------------------------------------------------------------------------
_bct_buffer_allocation_max 5242880 maximum size of all change tracking buffer allocations, in bytes_bct_public_dba_buffer_maxsize:
参数说明:允许分配全部public change tracking dba buffer的最大值
默认值为:0，自动分配
建议值:
_bct_public_dba_buffer_size<=建议值<=2*_bct_public_dba_buffer_size 并且小于large_pool池大小
参数测试:在Oracle19.3版本测试，_bct_public_dba_buffer_maxsize参数控制CTWR dba buffer的最大值
--参数都设置为512K
alter system set "_bct_buffer_allocation_max"=5242880;
alter system set "_bct_public_dba_buffer_maxsize"=524288;
alter system set "_bct_public_dba_buffer_size"=524288;
NAME VALUE DESCRIPTION
---------------------------------------- ---------------------------------------- ------------------------------------------------------------
_bct_buffer_allocation_max 524288 maximum size of all change tracking buffer allocations, in b
_bct_public_dba_buffer_maxsize 524288 max buffer size permitted for public dba buffers, in bytes
_bct_public_dba_buffer_size 524288 total size of all public change tracking dba buffers, in byt
--当前CTWR dba buffer为1MB，接近2倍_bct_public_dba_buffer_size
SQL> select inst_id,pool,name,bytes/1024/1024 as MB ,con_id from gv$sgastat where pool='large pool';
INST_ID POOL NAME MB CON_ID
---------- -------------- ---------------------------------------- ---------- ----------
1 large pool free memory 189.636719 0
1 large pool ASM map operations hashta .375 0
1 large pool krcc extent chunk 1.98046875 0
1 large pool PX msg pool 15 0
1 large pool CTWR dba buffer 1.0078125 0
--进行大批量的块操作，可以看到产生大量的block change tracking buffer space等待事件
SAMPLE_TIME EVENT SESSION_ID BLOCKING_SESSION SESSION PROGRAM TIME_WAITED/1000
---------------------------------------- ---------------------------------------- ---------- ---------------- ------- ---------------------------------------- ----------------
19-MAR-23 03.51.12.745 PM control file sequential read 497 WAITING oracle@rac19a (CKPT) .297
19-MAR-23 03.54.42.856 PM change tracking file synchronous read 510 WAITING rman@rac19a (TNS V1-V3) .233
19-MAR-23 03.58.58.979 PM 10 ON CPU oracle@rac19a (CTWR) 0
19-MAR-23 03.59.11.009 PM change tracking file synchronous read 750 WAITING sqlplus@rac19a (TNS V1-V3) 5.282
19-MAR-23 03.59.19.020 PM block change tracking buffer space 507 WAITING sqlplus@rac19a (TNS V1-V3) 999.912
19-MAR-23 03.59.19.020 PM block change tracking buffer space 382 WAITING sqlplus@rac19a (TNS V1-V3) 999.727
19-MAR-23 03.59.19.020 PM block change tracking buffer space 626 WAITING oracle@rac19a (W002) 999.692
19-MAR-23 03.59.19.020 PM block change tracking buffer space 750 WAITING sqlplus@rac19a (TNS V1-V3) 999.642
19-MAR-23 03.59.19.020 PM block change tracking buffer space 139 WAITING sqlplus@rac19a (TNS V1-V3) 999.587
19-MAR-23 03.59.20.020 PM block change tracking buffer space 626 WAITING oracle@rac19a (W002) 780.752
19-MAR-23 03.59.20.020 PM block change tracking buffer space 507 WAITING sqlplus@rac19a (TNS V1-V3) 962.714
19-MAR-23 03.59.20.020 PM block change tracking buffer space 382 WAITING sqlplus@rac19a (TNS V1-V3) 793.489
19-MAR-23 03.59.20.020 PM block change tracking buffer space 139 WAITING sqlplus@rac19a (TNS V1-V3) 968.197
19-MAR-23 03.59.20.020 PM block change tracking buffer space 750 WAITING sqlplus@rac19a (TNS V1-V3) 966.412
--当前CTWR dba buffer为1MB没有任何动态调整
--调整_bct_public_dba_buffer_maxsize为100MB
alter system set "_bct_public_dba_buffer_maxsize"=104857600;
--当前CTWR dba buffer动态调整为CTWR dba buffer，block change tracking buffer space等待事件消失
1* select inst_id,pool,name,bytes/1024/1024 as MB ,con_id from gv$sgastat where pool='large pool'
INST_ID POOL NAME MB CON_ID
---------- -------------- ---------------------------------------- ---------- ----------
1 large pool free memory 2016.28516 0
1 large pool ASM map operations hashta .375 0
1 large pool krcc extent chunk 1.98046875 0
1 large pool PX msg pool 15 0
1 large pool CTWR dba buffer 14.359375 0_bct_public_dba_buffer_size:
参数说明:全部public change tracking dba buffer的总值
默认值为:0，自动分配
建议值:根据Oracle MOS(Doc ID 2094946.1)
推荐为2倍dba_buffer_count_public*dba_entry_count_public*dba_entry_size并且小于large_pool池大小，减少block change tracking buffer space的等待产生
--可以通过表x$krcstat查询当前值
select dba_buffer_count_public*dba_entry_count_public*dba_entry_size from x$krcstat;参数测试:在Oracle19.3版本测试，_bct_public_dba_buffer_size参数影响CTWR dba buffer的当前大小
2倍dba_buffer_count_public*dba_entry_count_public*dba_entry_size<CTWR dba buffer<=2倍_bct_public_dba_buffer_size
--设置_bct_public_dba_buffer_size参数之前，要先设置_bct_public_dba_buffer_maxsize>=_bct_public_dba_buffer_size
否则可能会出现错误02097, 00000, "parameter cannot be modified because specified value is invalid"
// *Cause: Though the initialization parameter is modifiable, the modified
// value is not acceptable to the parameter
--当前CTWR dba buffer 14MB
SQL> select inst_id,pool,name,bytes/1024/1024 as MB ,con_id from gv$sgastat where pool='large pool'
INST_ID POOL NAME MB CON_ID
---------- -------------- -------------------------- ---------- ----------
1 large pool free memory 2016.28516 0
1 large pool ASM map operations hashta .375 0
1 large pool krcc extent chunk 1.98046875 0
1 large pool PX msg pool 15 0
1 large pool CTWR dba buffer 14.359375 0
--查看dba_buffer_count_public*dba_entry_count_public*dba_entry_size当前大小
SQL> select dba_buffer_count_public*dba_entry_count_public*dba_entry_size from x$krcstat;
DBA_BUFFER_COUNT_PUBLIC*DBA_ENTRY_COUNT_PUBLIC*DBA_ENTRY_SIZE
-------------------------------------------------------------
7528320
--设置为2倍DBA_BUFFER_COUNT_PUBLIC*DBA_ENTRY_COUNT_PUBLIC*DBA_ENTRY_SIZE
SQL> alter system set "_bct_public_dba_buffer_maxsize"=15056640;
System altered.
SQL> alter system set "_bct_public_dba_buffer_size"=15056640;
System altered.
--当前CTWR dba buffer为28MB，基本等于两倍_bct_public_dba_buffer_size
SQL> select inst_id,pool,name,bytes ,con_id from gv$sgastat where pool='large pool';
INST_ID POOL NAME BYTES CON_ID
---------- -------------- -------------------------- ---------- ----------
1 large pool free memory 2099171328 0
1 large pool ASM map operations hashta 393216 0
1 large pool krcc extent chunk 2076672 0
1 large pool PX msg pool 15728640 0
1 large pool CTWR dba buffer 30113792 0
--设置为4倍DBA_BUFFER_COUNT_PUBLIC*DBA_ENTRY_COUNT_PUBLIC*DBA_ENTRY_SIZE
SQL> alter system set "_bct_public_dba_buffer_maxsize"=60227584;
System altered.
SQL> alter system set "_bct_public_dba_buffer_size"=60227584;
System altered.
--当前CTWR dba buffer为60MB，大于当前两倍dba_buffer_count_public*dba_entry_count_public*dba_entry_size
--小于2倍_bct_public_dba_buffer_size
SQL> select inst_id,pool,name,bytes ,con_id from gv$sgastat where pool='large pool';
INST_ID POOL NAME BYTES CON_ID
---------- -------------- -------------------------- ---------- ----------
1 large pool free memory 2065534976 0
1 large pool ASM map operations hashta 393216 0
1 large pool krcc extent chunk 2076672 0
1 large pool PX msg pool 15728640 0
1 large pool CTWR dba buffer 63750144 0
SQL> select dba_buffer_count_public*dba_entry_count_public*dba_entry_size from x$krcstat;
DBA_BUFFER_COUNT_PUBLIC*DBA_ENTRY_COUNT_PUBLIC*DBA_ENTRY_SIZE
-------------------------------------------------------------
31874688 参数说明:CTWR健康心跳检查间隔
默认值为:60秒，19.17及以后版本默认为0，禁用健康心跳检查
建议值:存在Bug33239980导致由于CTWR健康心跳检查导致CKPT进程被堵塞的问题，如果CTWR存在性能问题，建议设置为0，禁用健康心跳检查或者应用最新的RU补丁
_bct_bitmaps_per_file:
参数说明:bct块追踪文件里面每个数据文件保留的块追踪位图版本
默认值为:8
建议值:8
参数测试:每一次全量备份或者增量备份，BCT文件都会重新创建一个新的版本用于记录新的块变化，如果版本的个数超过8，旧的版本会被覆盖，这可能会导致新的增量备份无法查看之前增量备份的历史版本，这时增量备份会恢复到全数据库块扫描，备份效率变低，所以至少每周进行一次全量备份，每天进行一次增量备份，这样的备份策略对于BCT比较合理