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SQLserver数据文件(MDF)的页面文件头结构剖析
先执行一下以下SQL语句，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
This identifies the file number the page is part of and the position within the file. (1:143) means page 143 in file 1.
m_headerVersion
This is the page header version. Since version 7.0 this value has always been 1.
m_typea
This is the page type. The values you’’re likely to see are:
1 - data page. This holds data records in a heap or clustered index leaf-level.
2 - index page. This holds index records in the upper levels of a clustered index and all levels of non-clustered indexes.
3 - text mix page. A text page that holds small chunks of LOB values plus internal parts of text tree. These can be shared between LOB values in the same partition of an index or heap.
4 - text tree page. A text page that holds large chunks of LOB values from a single column value.
7 - sort page. A page that stores intermediate results during a sort operation.
8 - GAM page. Holds global allocation information about extents in a GAM interval (every data file is split into 4GB chunks - the number of extents that can be represented in a bitmap on a single database page). Basically whether an extent is allocated or not. GAM = Global Allocation Map. The first one is page 2 in each file. More on these in a later post.
9 - SGAM page. Holds global allocation information about extents in a GAM interval. Basically whether an extent is available for allocating mixed-pages. SGAM = Shared GAM. the first one is page 3 in each file. More on these in a later post.
10 - IAM page. Holds allocation information about which extents within a GAM interval are allocated to an index or allocation unit, in SQL Server 2000 and 2005 respectively. IAM = Index Allocation Map. More on these in a later post.
11 - PFS page. Holds allocation and free space information about pages within a PFS interval (every data file is also split into approx 64MB chunks - the number of pages that can be represented in a byte-map on a single database page. PFS = Page Free Space. The first one is page 1 in each file. More on these in a later post.
13 - boot page. Holds information about the database. There’’s only one of these in the database. It’’s page 9 in file 1.
15 - file header page. Holds information about the file. There’’s one per file and it’’s page 0 in the file.
16 - diff map page. Holds information about which extents in a GAM interval have changed since the last full or differential backup. The first one is page 6 in each file.
17 - ML map page. Holds information about which extents in a GAM interval have changed while in bulk-logged mode since the last backup. This is what allows you to switch to bulk-logged mode for bulk-loads and index rebuilds without worrying about breaking a backup chain. The first one is page 7 in each file.
m_typeFlagBits
This is mostly unused. For data and index pages it will always be 4. For all other pages it will always be 0 - except PFS pages. If a PFS page has m_typeFlagBits of 1, that means that at least one of the pages in the PFS interval mapped by the PFS page has at least one ghost record.
m_level
This is the level that the page is part of in the b-tree.
Levels are numbered from 0 at the leaf-level and increase to the single-page root level (i.e. the top of the b-tree).
In SQL Server 2000, the leaf level of a clustered index (with data pages) was level 0, and the next level up (with index pages) was also level 0. The level then increased to the root. So to determine whether a page was truly at the leaf level in SQL Server 2000, you need to look at the m_type as well as the m_level.
For all page types apart from index pages, the level is always 0.
m_flagBits
This stores a number of different flags that describe the page. For example, 0x200 means that the page has a page checksum on it (as our example page does) and 0x100 means the page has torn-page protection on it.
Some bits are no longer used in SQL Server 2005.
m_objId
m_indexId
In SQL Server 2000, these identified the actual relational object and index IDs to which the page is allocated. In SQL Server 2005 this is no longer the case. The allocation metadata totally changed so these instead identify what’’s called the allocation unit that the page belongs to (I’’ll do another post that describes these later today).
m_prevPage
m_nextPage
These are pointers to the previous and next pages at this level of the b-tree and store 6-byte page IDs.
The pages in each level of an index are joined in a doubly-linked list according to the logical order (as defined by the index keys) of the index. The pointers do not necessarily point to the immediately adjacent physical pages in the file (because of fragmentation).
The pages on the left-hand side of a b-tree level will have the m_prevPage pointer be NULL, and those on the right-hand side will have the m_nextPage be NULL.
In a heap, or if an index only has a single page, these pointers will both be NULL for all pages.
pminlen
This is the size of the fixed-length portion of the records on the page.
m_slotCnt
This is the count of records on the page.
m_freeCnt
This is the number of bytes of free space in the page.
m_freeData
This is the offset from the start of the page to the first byte after the end of the last record on the page. It doesn’’t matter if there is free space nearer to the start of the page.
m_reservedCnt
This is the number of bytes of free space that has been reserved by active transactions that freed up space on the page. It prevents the free space from being used up and allows the transactions to roll-back correctly. There’’s a very complicated algorithm for changing this value.
m_lsn
This is the Log Sequence Number of the last log record that changed the page.
m_xactReserved
This is the amount that was last added to the m_reservedCnt field.
m_xdesId
This is the internal ID of the most recent transaction that added to the m_reservedCnt field.
m_ghostRecCnt
The is the count of ghost records on the page.
m_tornBits
This holds either the page checksum or the bits that were displaced by the torn-page protection bits - depending on what form of page protection is turnde on for the database. |
