引入一段文档对于checksum的说明
A number calculated by the database from all the bytes stored in a data or redo block. If the DB_BLOCK_CHECKSUM initialization parameter is enabled, then the database calculates the checksum for every data file or online redo log block and stores it in the block header when writing to disk. The database can use the checksum value to check consistency.
数据块中的checksum值储存在每个块的第16个字节，可以通过bbed查看
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 BBED> set dba 5,131 DBA 0x01400083 (20971651 5,131) BBED> p kcbh struct kcbh, 20 bytes @0 ub1 type_kcbh @0 0x06 ub1 frmt_kcbh @1 0xa2 ub1 spare1_kcbh @2 0x00 ub1 spare2_kcbh @3 0x00 ub4 rdba_kcbh @4 0x01400083 ub4 bas_kcbh @8 0x0013f022 ub2 wrp_kcbh @12 0x0000 ub1 seq_kcbh @14 0x01 ub1 flg_kcbh @15 0x04 (KCBHFCKV) ub2 chkval_kcbh @16 0x2dd5 ub2 spare3_kcbh @18 0x0000
flg_kcbh：
KCBHFNEW BIT 0×01 // new block – zeroed data area
KCBHFDLC BIT 0×02 // Delayed Logging Change advanced SCN/seq
KCBHFCKV BIT 0×04 // ChecK Value saved – block xor’s to zero
KCBHFTMP BIT 0×08 // Temporary block.
0×06=0×02+0×04 KCBHFDLC+KCBHFCKV
而这个值就是用来控制是否对block进行checksum验证，可以通过DB_BLOCK_CHECKSUM参数来配置，如果设为typical和full时，读取块的时候就会重新计算checksum值，并与写出的时候的值做比对，设置其他值的时候就不会校验。
db_block_checksum设置为typical
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 SYS@xb> show parameter DB_BLOCK_CHECKSUM NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ db_block_checksum string TYPICAL SYS@xb> create table t_check(id number,name varchar2(30)) tablespace tbsxb; 表已创建。 insert into t_check values(1,'www.xbdba.com'); 已创建 1 行。 SYS@xb> SYS@xb> commit; 提交完成。 select dbms_rowid.rowid_object(rowid) object_id, dbms_rowid.rowid_relative_fno(rowid) file_id, dbms_rowid.rowid_block_number(rowid) block_id, dbms_rowid.rowid_row_number(rowid) num from t_check; OBJECT_ID FILE_ID BLOCK_ID NUM ---------- ---------- ---------- ---------- 87883 5 244 0
dump这个块
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 SYS@xb> alter system dump datafile 5 block 244; 系统已更改。 SYS@xb> oradebug setmypid; 已处理的语句 SYS@xb> oradebug tracefile_name; /u01/app/oracle/diag/rdbms/xb/xb/trace/xb_ora_28067.trc Block dump from disk: buffer tsn: 9 rdba: 0x014000f4 (5/244) scn: 0x0000.00155de9 seq: 0x02 flg: 0x04 tail: 0x5de90602 frmt: 0x02 chkval: 0xbfe6 type: 0x06=trans data Hex dump of block: st=0, typ_found=1
这里可以看到当设置为typical时，block查看到flg为0x04，checksum值为0xbfe6
用BBED查看
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 BBED> set dba 5,244 DBA 0x014000f4 (20971764 5,244) BBED> p kcbh struct kcbh, 20 bytes @0 ub1 type_kcbh @0 0x06 ub1 frmt_kcbh @1 0xa2 ub1 spare1_kcbh @2 0x00 ub1 spare2_kcbh @3 0x00 ub4 rdba_kcbh @4 0x014000f4 ub4 bas_kcbh @8 0x00155de9 ub2 wrp_kcbh @12 0x0000 ub1 seq_kcbh @14 0x02 ub1 flg_kcbh @15 0x04 (KCBHFCKV) ub2 chkval_kcbh @16 0xbfe6 ub2 spare3_kcbh @18 0x0000
比对可以看到flg_kcbh就是block dump里的flg，chkval_kcbh就是chkval
如果我们直接在bbed里做一些不同的操作，checksum值是否需要重新计算？
1 2 3 4 5 6 7 8 9 10 11 12 13 BBED> d /v offset 8150 File: /u01/app/oracle/oradata/xb/tbsxb01.dbf (5) Block: 244 Offsets: 8150 to 8191 Dba:0x014000f4 ------------------------------------------------------- 8d45e048 89c241ff 95890600 004d89e3 l .E.H..A......M.. 41ff9509 0a000048 89c64989 f64983e6 l A......H..I..I.. f8e929ff ffff0206 e95d l ..)......] >16 bytes per line> BBED> sum Check value for File 5, Block 244: current = 0xbfe6, required = 0xbfe6
可以看到如果只涉及dump操作，不涉及修改，重新计算的sum值与current值一样，所以并不影响。
进行修改操作
1 2 3 4 5 6 7 8 9 10 11 12 13 BBED> m /x 9d offset 8150 File: /u01/app/oracle/oradata/xb/tbsxb01.dbf (5) Block: 244 Offsets: 8150 to 8191 Dba:0x014000f4 ------------------------------------------------------------------------ 9d45e048 89c241ff 95890600 004d89e3 41ff9509 0a000048 89c64989 f64983e6 f8e929ff ffff0206 e95d >32 bytes per line> BBED> sum Check value for File 5, Block 244: current = 0xbfe6, required = 0xbff6
修改了第一个offset以后，重新计算的值就变成了0xbff6
将修改的内容保存以后，可以看到chkval_kcbh也变成了重新计算的值
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 BBED> sum apply Check value for File 5, Block 244: current = 0xbff6, required = 0xbff6 BBED> p kcbh struct kcbh, 20 bytes @0 ub1 type_kcbh @0 0x06 ub1 frmt_kcbh @1 0xa2 ub1 spare1_kcbh @2 0x00 ub1 spare2_kcbh @3 0x00 ub4 rdba_kcbh @4 0x014000f4 ub4 bas_kcbh @8 0x00155de9 ub2 wrp_kcbh @12 0x0000 ub1 seq_kcbh @14 0x02 ub1 flg_kcbh @15 0x04 (KCBHFCKV) ub2 chkval_kcbh @16 0xbff6 ub2 spare3_kcbh @18 0x0000
将新的block进行dump
1 2 3 4 5 Block dump from disk: buffer tsn: 9 rdba: 0x014000f4 (5/244) scn: 0x0000.00155de9 seq: 0x02 flg: 0x04 tail: 0x5de90602 frmt: 0x02 chkval: 0xbff6 type: 0x06=trans data Hex dump of block: st=0, typ_found=1
可以看到dump信息里的chkval依然与bbed的chkval_kcbh一致，flg信息同样一致
根据文档所说的，flg标志位代表是否要计算sum，那如果手动修改bbed的flg值，修改数据还会重新计算sum吗
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 BBED> d /v offset 15 File: /u01/app/oracle/oradata/xb/tbsxb01.dbf (5) Block: 244 Offsets: 15 to 526 Dba:0x014000f4 ------------------------------------------------------- 04f6bf00 0001000f 00495701 00e95d15 l .........IW...]. 000000e8 1f021f32 00f00040 01000000 l .......2...@.... BBED> m /x 02 offset 15 File: /u01/app/oracle/oradata/xb/tbsxb01.dbf (5) Block: 244 Offsets: 15 to 526 Dba:0x014000f4 ------------------------------------------------------------------------ 02f6bf00 0001000f 00495701 00e95d15 000000e8 1f021f32 00f00040 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 BBED> p kcbh struct kcbh, 20 bytes @0 ub1 type_kcbh @0 0x06 ub1 frmt_kcbh @1 0xa2 ub1 spare1_kcbh @2 0x00 ub1 spare2_kcbh @3 0x00 ub4 rdba_kcbh @4 0x014000f4 ub4 bas_kcbh @8 0x00155de9 ub2 wrp_kcbh @12 0x0000 ub1 seq_kcbh @14 0x02 ub1 flg_kcbh @15 0x02 (KCBHFDLC) ub2 chkval_kcbh @16 0xbff6 ub2 spare3_kcbh @18 0x0000 BBED> sum Check value for File 5, Block 244: current = 0xbff6, required = 0xbff6 BBED> d /v offset 8150 File: /u01/app/oracle/oradata/xb/tbsxb01.dbf (5) Block: 244 Offsets: 8150 to 8191 Dba:0x014000f4 ------------------------------------------------------- 9d45e048 89c241ff 95890600 004d89e3 l .E.H..A......M.. 41ff9509 0a000048 89c64989 f64983e6 l A......H..I..I.. f8e929ff ffff0206 e95d l ..)......] >16 bytes per line> BBED> m /x 8d offset 8150 File: /u01/app/oracle/oradata/xb/tbsxb01.dbf (5) Block: 244 Offsets: 8150 to 8191 Dba:0x014000f4 ------------------------------------------------------------------------ 8d45e048 89c241ff 95890600 004d89e3 41ff9509 0a000048 89c64989 f64983e6 f8e929ff ffff0206 e95d >32 bytes per line> BBED> sum Check value for File 5, Block 244: current = 0xbff6, required = 0xbff6
可以看到当手动把flg_kcbh修改成02 也就是KCBHFDLC以后，对数据块进行任何修改都不会触发checksum的重新计算。那是否可以通过参数的方式来控制这个值呢，那就是db_block_checksum
修改db_block_checksum为false
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 BBED> m /x 04 offset 15 File: /u01/app/oracle/oradata/xb/tbsxb01.dbf (5) Block: 244 Offsets: 15 to 526 Dba:0x014000f4 ------------------------------------------------------------------------ 04f6bf00 0001000f 00495701 00e95d15 000000e8 1f021f32 00f00040 01000000 BBED> p kcbh struct kcbh, 20 bytes @0 ub1 type_kcbh @0 0x06 ub1 frmt_kcbh @1 0xa2 ub1 spare1_kcbh @2 0x00 ub1 spare2_kcbh @3 0x00 ub4 rdba_kcbh @4 0x014000f4 ub4 bas_kcbh @8 0x00155de9 ub2 wrp_kcbh @12 0x0000 ub1 seq_kcbh @14 0x02 ub1 flg_kcbh @15 0x04 (KCBHFCKV) ub2 chkval_kcbh @16 0xbff6 ub2 spare3_kcbh @18 0x0000 SYS@xb> alter system set db_block_checksum=false; 系统已更改。 SYS@xb> drop table t_check purge; 表已删除。 SYS@xb> create table t_check(id number,name varchar2(30)) tablespace tbsxb; 表已创建。 SYS@xb> insert into t_check values(1,'www.xbdba.com'); 已创建 1 行。 SYS@xb> commit; 提交完成。 select dbms_rowid.rowid_object(rowid) object_id, dbms_rowid.rowid_relative_fno(rowid) file_id, dbms_rowid.rowid_block_number(rowid) block_id, dbms_rowid.rowid_row_number(rowid) num 5 from t_check; OBJECT_ID FILE_ID BLOCK_ID NUM ---------- ---------- ---------- ---------- 87884 5 243 0 SYS@xb> alter system checkpoint; 系统已更改。
block dump
1 2 3 4 5 6 7 8 9 10 11 12 Block dump from disk: buffer tsn: 9 rdba: 0x014000f3 (5/243) scn: 0x0000.001588bc seq: 0x01 flg: 0x02 tail: 0x88bc0601 frmt: 0x02 chkval: 0x0000 type: 0x06=trans data Hex dump of block: st=0, typ_found=1 block_row_dump: tab 0, row 0, @0x1f84 tl: 20 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 2] c1 02 col 1: [13] 77 77 77 2e 78 62 64 62 61 2e 63 6f 6d end_of_block_dump
看这里显示flg为0x02 chkval为0x0000 即为空，所以认定并为对block写入进行checksum操作
bbed的结果值也是一样
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 BBED> set dba 5,243 DBA 0x014000f3 (20971763 5,243) BBED> p kcbh struct kcbh, 20 bytes @0 ub1 type_kcbh @0 0x06 ub1 frmt_kcbh @1 0xa2 ub1 spare1_kcbh @2 0x00 ub1 spare2_kcbh @3 0x00 ub4 rdba_kcbh @4 0x014000f3 ub4 bas_kcbh @8 0x001588bc ub2 wrp_kcbh @12 0x0000 ub1 seq_kcbh @14 0x01 ub1 flg_kcbh @15 0x02 (KCBHFDLC) ub2 chkval_kcbh @16 0x0000 ub2 spare3_kcbh @18 0x0000
在这种条件下对block进行bbed修改
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 BBED> d /v offset 8150 File: /u01/app/oracle/oradata/xb/tbsxb01.dbf (5) Block: 243 Offsets: 8150 to 8191 Dba:0x014000f3 ------------------------------------------------------- 01020000 00000041 114d0000 0041114d l .......A.M...A.M 00002c01 0202c102 0d777777 2e786264 l ..,......www.xbd 62612e63 6f6d0106 bc88 l ba.com.... >16 bytes per line> BBED> m /x 02 offset 8150 File: /u01/app/oracle/oradata/xb/tbsxb01.dbf (5) Block: 243 Offsets: 8150 to 8191 Dba:0x014000f3 ------------------------------------------------------------------------ 02020000 00000041 114d0000 0041114d 00002c01 0202c102 0d777777 2e786264 62612e63 6f6d0106 bc88 >32 bytes per line> BBED> sum Check value for File 5, Block 243: current = 0x0000, required = 0x0000
即使修改了数据块，current和required值依然都是0，所以可以认为如果db_check_sum值置为false以后，对块的写入操作并不会计算和修改checksum