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• ???Flashback Log???????Redo Log?

  - by Liu Maclean(???)

  ????????????????????redo log?   RVWR( Recovery Writer)?3s??flashback generate buffer??block before image?????????? ?????block change???RVWR??block before image ?flashback log? ?????????,Oracle???????????before image????????,????????flashback database logs?????   ???????????,????? ??????????????????,???????????before image?????shared pool??flashback log buffer?,RVWR??????flashback log buffer??????????? ?DBWR???????????????,DBWR?????buffer header??FBA(Flashback Byte Address)?flashback log buffer?????????? ???? ?????? ??? ????????????? , RVWR???????????(flashback markers)?flashback database logs?? ????(flashback markers)?????????????Oracle??flashback ??????????  ??????????, Oracle ??????(flashback markers)????????????flashback database log???????????block image; ??Oracle ???????(forward recovery)?????????????????SCN?????? flashback markers for example: **** Record at fba: (lno 1 thr 1 seq 1 bno 4 bof 8184) **** RECORD HEADER: Type: 3 (Skip) Size: 8132 RECORD DATA (Skip): **** Record at fba: (lno 1 thr 1 seq 1 bno 4 bof 52) **** RECORD HEADER: Type: 7 (Begin Crash Recovery Record) Size: 36 RECORD DATA (Begin Crash Recovery Record): Previous logical record fba: (lno 1 thr 1 seq 1 bno 3 bof 316) Record scn: 0x0000.00000000 [0.0] **** Record at fba: (lno 1 thr 1 seq 1 bno 3 bof 8184) **** RECORD HEADER: Type: 3 (Skip) Size: 7868 RECORD DATA (Skip): **** Record at fba: (lno 1 thr 1 seq 1 bno 3 bof 316) **** RECORD HEADER: Type: 2 (Marker) Size: 300 RECORD DATA (Marker): Previous logical record fba: (lno 0 thr 0 seq 0 bno 0 bof 0) Record scn: 0x0000.00000000 [0.0] Marker scn: 0x0000.0060e024 [0.6348836] 06/13/2012 15:56:35 Flag 0x0 Flashback threads: 1, Enabled redo threads 1 Recovery Start Checkpoint: scn: 0x0000.0060e024 [0.6348836] 06/13/2012 15:56:12 thread:1 rba:(0x80.180.10) Flashback thread Markers: Thread:1 status:0 fba: (lno 1 thr 1 seq 1 bno 2 bof 8184) Redo Thread Checkpoint Info: Thread:1 rba:(0x80.180.10) **** Record at fba: (lno 1 thr 1 seq 1 bno 2 bof 8184) **** RECORD HEADER: Type: 3 (Skip) Size: 8168 RECORD DATA (Skip): End-Of-Thread reached ????????????????block change ????before image????????flashback log?? ?????block change???flashback log record ????????? redo log???!????flashback log ???????before image ? redo log??? change vector ?  Oracle?????????????????????????????????????,??????I/O??????????????: ??hot block??,Oracle???????????block image?????; Oracle ?????????(flashback barriers)???????????????,flashback barriers???????(???15??),??????????(flashback barriers)????(flashback markers)????????? ????, ??????change?????, ???????????????????????????, ?15????????????????????flashback log????????before image?????????????,?????????????????????,?????????????? ????????,??????????????(flashback barriers), flashback barriers???????,?????15????? ?????flashback barriers????????(flashback markers)???????????????,???????????????????(????barriers?????)??????block image ,????????????????????????????????? ??????????flashback log????redo log????! ????,????????????????, ?????????? SQL> select * from v$version; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production PL/SQL Release 11.2.0.3.0 - Production CORE 11.2.0.3.0 Production TNS for Linux: Version 11.2.0.3.0 - Production NLSRTL Version 11.2.0.3.0 - Production SQL> select * from global_name; GLOBAL_NAME -------------------------------------------------------------------------------- www.oracledatabase12g.com SQL> create table flash_maclean (t1 varchar2(200)) tablespace users; Table created. SQL> insert into flash_maclean values('MACLEAN LOVE HANNA'); 1 row created. SQL> commit; Commit complete. SQL> startup force; ORACLE instance started. Total System Global Area 939495424 bytes Fixed Size 2233960 bytes Variable Size 713034136 bytes Database Buffers 218103808 bytes Redo Buffers 6123520 bytes Database mounted. Database opened. SQL> update flash_maclean set t1='HANNA LOVE MACLEAN'; 1 row updated. commit; Commit complete. SQL> alter system checkpoint; System altered. SQL> select dbms_rowid.rowid_block_number(rowid),dbms_rowid.rowid_relative_fno(rowid) from flash_maclean; DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID) DBMS_ROWID.ROWID_RELATIVE_FNO(ROWID) ------------------------------------ ------------------------------------ 140431 4 datafile 4 block 140431 ??RDBA rdba: 0x0102248f (4/140431) SQL> ! ps -ef|grep rvwr|grep -v grep oracle 26695 1 0 15:56 ? 00:00:00 ora_rvwr_G11R23 SQL> oradebug setospid 26695 Oracle pid: 20, Unix process pid: 26695, image: [email protected] (RVWR) SQL> ORADEBUG DUMP FBTAIL 1; Statement processed. To dump the last 2000 flashback records , ??ORADEBUG DUMP FBTAIL 1????????2000?????? SQL> oradebug tracefile_name /s01/orabase/diag/rdbms/g11r23/G11R23/trace/G11R23_rvwr_26695.trc ? TRACE?????????block? before image **** Record at fba: (lno 1 thr 1 seq 1 bno 55 bof 2564) **** RECORD HEADER: Type: 1 (Block Image) Size: 28 RECORD DATA (Block Image): file#: 4 rdba: 0x0102248f Next scn: 0x0000.00000000 [0.0] Flag: 0x0 Block Size: 8192 BLOCK IMAGE: buffer rdba: 0x0102248f scn: 0x0000.00609044 seq: 0x01 flg: 0x06 tail: 0x90440601 frmt: 0x02 chkval: 0xc626 type: 0x06=trans data Hex dump of block: st=0, typ_found=1 Dump of memory from 0x00002B1D94183C00 to 0x00002B1D94185C00 2B1D94183C00 0000A206 0102248F 00609044 06010000 [.....$..D.`.....] 2B1D94183C10 0000C626 00000001 00014AD4 0060903A [&........J..:.`.] 2B1D94183C20 00000000 00320002 01022488 00090006 [......2..$......] 2B1D94183C30 00000CC8 00C00340 000D0542 00008000 [[email protected].......] 2B1D94183C40 006040BC 000F000A 00000920 00C002E4 [.@`..... .......] 2B1D94183C50 0017048F 00002001 00609044 00000000 [..... ..D.`.....] 2B1D94183C60 00000000 00010100 0014FFFF 1F6E1F77 [............w.n.] 2B1D94183C70 00001F6E 1F770001 00000000 00000000 [n.....w.........] 2B1D94183C80 00000000 00000000 00000000 00000000 [................] Repeat 500 times 2B1D94185BD0 00000000 00000000 2C000000 4D120102 [...........,...M] 2B1D94185BE0 454C4341 4C204E41 2045564F 4E4E4148 [ACLEAN LOVE HANN] 2B1D94185BF0 01002C41 43414D07 4E41454C 90440601 [A,...MACLEAN..D.] Block header dump: 0x0102248f Object id on Block? Y seg/obj: 0x14ad4 csc: 0x00.60903a itc: 2 flg: E typ: 1 - DATA brn: 0 bdba: 0x1022488 ver: 0x01 opc: 0 inc: 0 exflg: 0 Itl Xid Uba Flag Lck Scn/Fsc 0x01 0x0006.009.00000cc8 0x00c00340.0542.0d C--- 0 scn 0x0000.006040bc 0x02 0x000a.00f.00000920 0x00c002e4.048f.17 --U- 1 fsc 0x0000.00609044 bdba: 0x0102248f data_block_dump,data header at 0x2b1d94183c64 =============== tsiz: 0x1f98 hsiz: 0x14 pbl: 0x2b1d94183c64 76543210 flag=-------- ntab=1 nrow=1 frre=-1 fsbo=0x14 fseo=0x1f77 avsp=0x1f6e tosp=0x1f6e 0xe:pti[0] nrow=1 offs=0 0x12:pri[0] offs=0x1f77 block_row_dump: tab 0, row 0, @0x1f77 tl: 22 fb: --H-FL-- lb: 0x2 cc: 1 col 0: [18] 4d 41 43 4c 45 41 4e 20 4c 4f 56 45 20 48 41 4e 4e 41 end_of_block_dump SQL> select dump('MACLEAN LOVE HANNA',16) from dual; DUMP('MACLEANLOVEHANNA',16) -------------------------------------------------------------------- Typ=96 Len=18: 4d,41,43,4c,45,41,4e,20,4c,4f,56,45,20,48,41,4e,4e,41 ???????????????????????,??flashback log??before image????????? create table flash_maclean1 (t1 int) tablespace users; SQL> select vs.name, ms.value 2 from v$mystat ms, v$sysstat vs 3 where vs.statistic# = ms.statistic# 4 and vs.name in ('redo size','db block changes'); NAME VALUE ---------------------------------------------------------------- ---------- db block changes 0 redo size 0 SQL> select name,value from v$sysstat where name like 'flashback log%'; NAME VALUE ---------------------------------------------------------------- ---------- flashback log writes 49 flashback log write bytes 9306112 SQL> begin 2 for i in 1..5000 loop 3 update flash_maclean1 set t1=t1+1; 4 commit; 5 end loop; 6 end; 7 / PL/SQL procedure successfully completed. SQL> select vs.name, ms.value 2 from v$mystat ms, v$sysstat vs 3 where vs.statistic# = ms.statistic# 4 and vs.name in ('redo size','db block changes'); NAME VALUE ---------------------------------------------------------------- ---------- db block changes 20006 redo size 3071288 SQL> select name,value from v$sysstat where name like 'flashback log%'; NAME VALUE ---------------------------------------------------------------- ---------- flashback log writes 52 flashback log write bytes 10338304 ??????????? ??hot block,???20006 ?block changes???? ??? 3000k ?redo log ? ??1000k? flashback log ?

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• Why not use tables for layout in HTML?

  - by Bno

  It seems to be the general opinion that tables should not be used for layout in HTML. Why? I have never (or rarely to be honest) seen good arguments for this. The usual answers are: It's good to separate content from layoutBut this is a fallacious argument; Cliche Thinking. I guess it's true that using the table element for layout has little to do with tabular data. So what? Does my boss care? Do my users care?Perhaps me or my fellow developers who have to maintain a web page care... Is a table less maintainable? I think using a table is easier than using divs and CSS.By the way... why is using a div or a span good separation of content from layout and a table not? Getting a good layout with only divs often requires a lot of nested divs. Readability of the codeI think it's the other way around. Most people understand html, few understand CSS. It's better for SEO not to use tablesWhy? Can anybody show some evidence that it is? Or a statement from Google that tables are discouraged from an SEO perspective? Tables are slower.An extra tbody element has to be inserted. This is peanuts for modern web browsers. Show me some benchmarks where the use of a table significantly slows down a page. A layout overhaul is easier without tables, see css Zen Garden.Most web sites that need an upgrade need new content (html) as well. Scenarios where a new version of a web site only needs a new CSS file are not very likely. Zen Garden is a nice web site, but a bit theoretical. Not to mention its misuse of CSS. I am really interested in good arguments to use divs + CSS instead of tables.

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