我们可以通过以下命令查看系统回滚段,主要用于数据库的内部事务或SYS的事务信息记录，UNDO$的信息记录. ``` sql --oracle11.2.0.4 --segment 信息 select segment_name,segment_type,blocks,extents,header_file,header_block from dba_segments where segment_name='SYSTEM'; SEGMENT_NAME SEGMENT_TYPE BLOCKS EXTENTS HEADER_FILE HEADER_BLOCK ------------ ------------ ---------- ---------- ----------- ------------ SYSTEM ROLLBACK 48 6 1 128 --区信息 col owner for a12 col segment_name for a12 col segment_type for a12 select owner,segment_name,file_id,EXTENT_ID,block_id,BLOCKS from dba_extents where segment_name='SYSTEM'; OWNER SEGMENT_NAME FILE_ID EXTENT_ID BLOCK_ID BLOCKS ------------ ------------ ---------- ---------- ---------- ---------- SYS SYSTEM 1 0 128 8 SYS SYSTEM 1 1 136 8 SYS SYSTEM 1 2 528 8 SYS SYSTEM 1 3 536 8 SYS SYSTEM 1 4 544 8 SYS SYSTEM 1 5 552 8 ``` 通过bbed检查验证,块128为Undo段头，其他存放undo数据。 ``` shell #128 BBED> map /v dba 1,128 File: /u01/app/oracle/oradata/mydb11/system01.dbf (1) Block: 128 Dba:0x00400080 ------------------------------------------------------------ Unlimited Undo Segment Header #136 BBED> map /v dba 1,136 File: /u01/app/oracle/oradata/mydb11/system01.dbf (1) Block: 136 Dba:0x00400088 ------------------------------------------------------------ Undo Data #dump 128 alter system dump datafile 1 block 128; #trace Dump of buffer cache at level 4 for tsn=0 rdba=4194432 BH (0x91ff8b18) file#: 1 rdba: 0x00400080 (1/128) class: 15 ba: 0x91f42000 set: 12 pool: 3 bsz: 8192 bsi: 0 sflg: 2 pwc: 0,28 dbwrid: 0 obj: -1 objn: 0 tsn: 0 afn: 1 hint: f hash: [0xa75ba9b0,0xa75ba9b0] lru: [0x91ff8d40,0x91ff8ad0] obj-flags: object_ckpt_list ckptq: [0x91ff7b40,0xa7c603e8] fileq: [0x8fff14f0,0xa7c60408] objq: [0xa3fe9a88,0xa3fe9a88] objaq: [0xa3fe9a68,0x91fdaad8] st: XCURRENT md: NULL fpin: 'ktuwh72: ktugus:ktuswr1' tch: 1 flags: buffer_dirty redo_since_read LRBA: [0x6.6626.0] LSCN: [0x0.f2a66] HSCN: [0x0.f2a87] HSUB: [1] Block dump from disk: buffer tsn: 0 rdba: 0x00400080 (1/128) scn: 0x0000.000f2a5d seq: 0x01 flg: 0x04 tail: 0x2a5d0e01 frmt: 0x02 chkval: 0xa4a2 type: 0x0e=KTU UNDO HEADER W/UNLIMITED EXTENTS Hex dump of block: st=0, typ_found=1 Dump of memory from 0x00007FF6ADAA3A00 to 0x00007FF6ADAA5A00 ..... Extent Control Header ----------------------------------------------------------------- Extent Header:: spare1: 0 spare2: 0 #extents: 6 #blocks: 47 last map 0x00000000 #maps: 0 offset: 4128 Highwater:: 0x00400220 ext#: 4 blk#: 0 ext size: 8 #blocks in seg. hdr's freelists: 0 #blocks below: 0 mapblk 0x00000000 offset: 4 Unlocked Map Header:: next 0x00000000 #extents: 6 obj#: 0 flag: 0x40000000 Extent Map ----------------------------------------------------------------- 0x00400081 length: 7 0x00400088 length: 8 0x00400210 length: 8 0x00400218 length: 8 0x00400220 length: 8 0x00400228 length: 8 TRN CTL:: seq: 0x001c chd: 0x0002 ctl: 0x0018 inc: 0x00000000 nfb: 0x0001 mgc: 0x8002 xts: 0x0068 flg: 0x0001 opt: 2147483646 (0x7ffffffe) uba: 0x00400220.001c.01 scn: 0x0000.000e184c Version: 0x01 FREE BLOCK POOL:: uba: 0x00400220.001c.0a ext: 0x4 spc: 0x17d2 uba: 0x00000000.001a.02 ext: 0x2 spc: 0x1f16 uba: 0x00000000.0018.3a ext: 0x0 spc: 0x80e uba: 0x00000000.0000.00 ext: 0x0 spc: 0x0 uba: 0x00000000.0000.00 ext: 0x0 spc: 0x0 TRN TBL:: ``` 回滚段块结构说明 - a. (ktubhseq): undo record seq# - this is the seq# of the block that this undo record WILL BE APPLIED TO. This is from the Undo Block. It is NOT the seq# of the undo block itself. - b. (ktudbseq): redo RECORD seq# - this is the seq# number in the block that this redo WILL BE APPLIED TO. This is from the Redo Record. ``` shell struct ktubh { kxid ktubhxid; /* txid of tx currently using or last used this block */ ub2 ktubhseq; /* undo block sequence number */ ub1 ktubhcnt; /* high water mark record index, number of undo entries */ ub1 ktubhirb; /* rollback record index, rec index to start the rollback */ ub1 ktubhicl; /* collecting record index, rec index to start retrieving col info */ ub1 ktubhflg; /* dummy */ ub2 ktubhidx[1]; /* byte offset of record in block, grows at runtime */ }; struct ktudb Kernel Transaction Undo Data operation Block (redo) { ub2 ktudbsiz; /* size of entry */ ub2 ktudbspc; /* verification: space left in undo block */ ub2 ktudbflg; /* flag to indicate the kind of redo operation */ kxid ktudbxid; /* current tx id */ ub2 ktudbseq; /* block sequence number */ ub1 ktudbrec; /* new record index for this change */ }; ```