先执行一下以下sql语句，我的测试环境为sql2005
dbcc
traceon(
3604
)
go
dbcc
page(master,
1
,
0
,
2
)
可以看到MDF文件的一些物理结构信息，其中包括重要的头96个字节。也就是第一个页面的文件头。
........
PAGE HEADER:
Page @0x03FA0000
m_pageId = (1:0) m_headerVersion = 1 m_type = 15
m_typeFlagBits = 0x0 m_level = 0 m_flagBits = 0x8
m_objId (AllocUnitId.idobj) = 99 m_indexId (AllocUnitId.idind) = 0 Metadata: AllocUnitId = 6488064
Metadata: PartitionId = 0 Metadata: IndexId = 0 Metadata: ObjectId = 99
m_prevPage = (0:0) m_nextPage = (0:0) pminlen = 0
m_slotCnt = 1 m_freeCnt = 7937 m_freeData = 3059
m_reservedCnt = 0 m_lsn = (149:448:1) m_xactReserved = 0
m_xdesId = (0:0) m_ghostRecCnt = 0 m_tornBits = -1073741694
........
DATA:
Memory Dump @0x62FEC000
62FEC000: 010f0000 08000000 00000000 00000000 †................
62FEC010: 00000000 00000100 63000000 011ff30b †........c.......
62FEC020: 00000000 01000000 95000000 c0010000 †................
62FEC030: 01000000 00000000 00000000 820000c0 †................
62FEC040: 00000000 00000000 00000000 00000000 †................
62FEC050: 00000000 00000000 00000000 00000000 †................
以上蓝色的文字就是文件头的一些信息。如果这些信息损坏将会造成严重的后果。
经过简单的逐个字节分析，中间借助了windows计算器和c#的BitConverter.GetBytes函数。得出了如下文件结构图，其中每行4个字节，一共分析了文件头的前64个字节。
| 00:0F |
m_headerVersion |
m_type |
m_typeFlagBits |
m_level |
| m_flagBits |
m_indexId |
| m_prevPage(2) |
| m_prevPage(1) |
pminlen |
| 10:1F |
m_nextPage(2) |
| m_nextPage(1) |
m_slotCnt |
| AllocUnitId.idobj |
| m_freeCnt |
m_freeData |
| 20:2F |
m_pageId（2） |
| m_pageId（1） |
m_reservedCnt |
| m_lsn(1) |
| m_lsn(2) |
| 30:3F |
m_lsn(3) |
m_xactReserved |
| m_xdesId(2) |
| m_xdesId(1) |
m_ghostRecCnt |
| m_tornBits |
在数据库的头96个字节中第0x40开始直道0x5F应该都是0。
我发现只有测试页的m_pageId 的冒号前面的数字不为1时才在0x40到0x5f写入数据。但是具体代表什么还没有看出来。
姑且认为数据库第一个页面的0x00-0x3f就如上图所示，0x40-0x5f都为0（不正确的话请纠正一下）
这张图有什么用呢，如果你理解了上述参数的意义，用二进制编辑器打开一个头文件损坏的mdf文件就有可能恢复这个已经损坏的数据库。
偶不是dba也不是专业恢复数据的，只是个普通的开发人员，怎么恢复还请有经验人士补充一下。
有情提醒，这些东西非常危险，请不要随意测试，最好找一个没用的数据库来研究。
参数的意义
m_pageId
Thisidentifiesthefilenumberthepageispartofandthepositionwithinthefile.(1:143)meanspage143infile1.
m_headerVersion
Thisisthepageheaderversion.Sinceversion7.0thisvaluehasalwaysbeen1.
m_typea
Thisisthepagetype.Thevaluesyou'relikelytoseeare:
1-datapage.Thisholdsdatarecordsinaheaporclusteredindexleaf-level.
2-indexpage.Thisholdsindexrecordsintheupperlevelsofaclusteredindexandalllevelsofnon-clusteredindexes.
3-textmixpage.AtextpagethatholdssmallchunksofLOBvaluesplusinternalpartsoftexttree.ThesecanbesharedbetweenLOBvaluesinthesamepartitionofanindexorheap.
4-texttreepage.AtextpagethatholdslargechunksofLOBvaluesfromasinglecolumnvalue.
7-sortpage.Apagethatstoresintermediateresultsduringasortoperation.
8-GAMpage.Holdsglobalallocation@R_603_4045@ionaboutextentsinaGAMinterval(everydatafileissplitinto4GBchunks-thenumberofextentsthatcanberepresentedinabitmaponasingledatabasepage).Basicallywhetheranextentisallocatedornot.GAM=GlobalAllocationMap.Thefirstoneispage2ineachfile.Moreontheseinalaterpost.
9-SGAMpage.Holdsglobalallocation@R_603_4045@ionaboutextentsinaGAMinterval.Basicallywhetheranextentisavailableforallocatingmixed-pages.SGAM=SharedGAM.thefirstoneispage3ineachfile.Moreontheseinalaterpost.
10-IAMpage.Holdsallocation@R_603_4045@ionaboutwhichextentswithinaGAMintervalareallocatedtoanindexorallocationunit,insqlServer2000and2005respectively.IAM=IndexAllocationMap.Moreontheseinalaterpost.
11-PFSpage.Holdsallocationandfreespace@R_603_4045@ionaboutpageswithinapfsinterval(everydatafileisalsosplitintoapprox64MBchunks-thenumberofpagesthatcanberepresentedinabyte-maponasingledatabasepage.PFS=PageFreeSpace.Thefirstoneispage1ineachfile.Moreontheseinalaterpost.
13-bootpage.Holds@R_603_404[email protected]'sonlyoneoftheseinthedatabase.It'spage9infile1.
15-fileheaderpage.Holds@R_603_404[email protected]'soneperfileandit'spage0inthefile.
16-diffmappage.Holds@R_603_4045@ionaboutwhichextentsinaGAMintervalhavechangedsincethelastfullordifferentialbackup.Thefirstoneispage6ineachfile.
17-MLmappage.Holds@R_603_4045@ionaboutwhichextentsinaGAMintervalhavechangedwhileinbulk-loggedmodesincethelastbackup.Thisiswhatallowsyoutoswitchtobulk-loggedmodeforbulk-loadsandindexrebuildswithoutworryingaboutbreakingabackupchain.Thefirstoneispage7ineachfile.
m_typeFlagBits
Thisismostlyunused.Fordataandindexpagesitwillalwaysbe4.Forallotherpagesitwillalwaysbe0-exceptPFSpages.Ifapfspagehasm_typeFlagBitsof1,thatmeansthatatleastoneofthepagesinthePFSintervalmappedbythePFSpagehasatleastoneghostrecord.
m_level
Thisisthelevelthatthepageispartofintheb-tree.
Levelsarenumberedfrom0attheleaf-levelandincreasetothesingle-pagerootlevel(i.e.thetopoftheb-tree).
InsqlServer2000,theleaflevelofaclusteredindex(withdatapages)waslevel0,andthenextlevelup(withindexpages)wasalsolevel0.Thelevelthenincreasedtotheroot.sotodeterminewhetherapagewastrulyattheleaflevelinsqlServer2000,youneedtolookatthem_typeaswellasthem_level.
Forallpagetypesapartfromindexpages,thelevelisalways0.
m_flagBits
Thisstoresanumberofdifferentflagsthatdescribethepage.Forexample,0x200meansthatthepagehasapagechecksumonit(asourexamplepagedoes)and0x100meansthepagehastorn-pageprotectiononit.
SomebitsarenolongerusedinsqlServer2005.
m_objId
m_indexId
InsqlServer2000,theseidentifiedtheactualrelationalobjectandindexIDstowhichthepageisallocated.InsqlServer2005thisisnolongerthecase.TheallocationMetadatatotallychangedsotheseinsteadidentifywhat'scalledtheallocationunitthatthepagebelongsto(I'lldoanotherpostthatdescribestheselatertoday).
m_prevPage
m_nextPage
ThesearepointerstotheprevIoUsandnextpagesatthisleveloftheb-treeandstore6-bytepageIDs.
Thepagesineachlevelofanindexarejoinedinadoubly-linkedlistaccordingtothelogicalorder(asdefinedbytheindexkeys)oftheindex.Thepointersdonotnecessarilypointtotheimmediatelyadjacentphysicalpagesinthefile(becauSEOffragmentation).
Thepagesontheleft-handsideofab-treelevelwillhavethem_prevPagepointerbeNULL,andthoSEOntheright-handsidewillhavethem_nextPagebeNULL.
Inaheap,orifanindexonlyhasasinglepage,thesepointerswillbothbeNULLforallpages.
pminlen
Thisisthesizeofthefixed-lengthportionoftherecordsonthepage.
m_slotCnt
Thisisthecountofrecordsonthepage.
m_freeCnt
Thisisthenumberofbytesoffreespaceinthepage.
m_freeData
Thisistheoffsetfromthestartofthepagetothefirstbyteaftertheendofthelastrecordonthepage.Itdoesn'tmatterifthereisfreespacenearertothestartofthepage.
m_reservedCnt
Thisisthenumberofbytesoffreespacethathasbeenreservedbyactivetransactionsthatfreedupspaceonthepage.Itpreventsthefreespacefrombeingusedupandallowsthetransactionstoroll-backcorrectly.There'saverycomplicatedalgorithmforchangingthisvalue.
m_lsn
ThisistheLogSequenceNumberofthelastlogrecordthatchangedthepage.
m_xactReserved
Thisistheamountthatwaslastaddedtothem_reservedCntfield.
m_xdesId
ThisistheinternalIDofthemostrecenttransactionthataddedtothem_reservedCntfield.
m_ghostRecCnt
Theisthecountofghostrecordsonthepage.
m_tornBits
Thisholdseitherthepagechecksumorthebitsthatweredisplacedbythetorn-pageprotectionbits-dependingonwhatformofpageprotectionisturndeonforthedatabase.
