{"id":4236,"date":"2013-11-05T13:42:37","date_gmt":"2013-11-05T21:42:37","guid":{"rendered":"http:\/\/3.209.169.194\/blogs\/paul\/?p=4236"},"modified":"2013-11-05T14:25:16","modified_gmt":"2013-11-05T22:25:16","slug":"inside-the-storage-engine-how-are-allocation-unit-ids-calculated","status":"publish","type":"post","link":"https:\/\/www.sqlskills.com\/blogs\/paul\/inside-the-storage-engine-how-are-allocation-unit-ids-calculated\/","title":{"rendered":"Inside the Storage Engine: How are allocation unit IDs calculated?"},"content":{"rendered":"

It’s been a long time since I’ve written a post about pure internals, but every so often I get asked how an allocation unit ID is calculated from the m_objId<\/em> and m_indexId<\/em> fields that are stored in the header of every page.<\/p>\n

When DBCC PAGE<\/em> dumps a page header’s contents, it does the necessary calculations and metadata look-ups to be able to tell you the allocation unit ID, partition ID, relational object ID, and relational index ID. Basically everything prefixed with ‘Metadata:’ in the DBCC PAGE<\/em> output below is NOT stored on the page itself:<\/p>\n

\r\nPage @0x00000004ED8A2000\r\n\r\nm_pageId = (1:445)                  m_headerVersion = 1                 m_type = 1\r\nm_typeFlagBits = 0x0                m_level = 0                         m_flagBits = 0xa000\r\nm_objId (AllocUnitId.idObj) = 97    m_indexId (AllocUnitId.idInd) = 256\r\nMetadata: AllocUnitId = 72057594044284928\r\nMetadata: PartitionId = 72057594039304192                                Metadata: IndexId = 0\r\nMetadata: ObjectId = 599673184      m_prevPage = (0:0)                  m_nextPage = (0:0)\r\npminlen = 8                         m_slotCnt = 1                       m_freeCnt = 8069\r\nm_freeData = 121                    m_reservedCnt = 0                   m_lsn = (225:443:22)\r\nm_xactReserved = 0                  m_xdesId = (0:0)                    m_ghostRecCnt = 0\r\nm_tornBits = 0                      DB Frag ID = 1\r\n<\/pre>\n
The formula is as follows:<\/p>\n
    \n
  • Take the m_indexId<\/em> and left-shift by 48, giving value A<\/li>\n
  • Take the m_objId<\/em> and left-shift by 16, giving value B<\/li>\n
  • AllocUnitId<\/em> = A | B (where | is a logical OR operation)<\/li>\n<\/ul>\n
    Using the page above:<\/p>\n
      \n
    • A = 256 << 48 =\u00a072057594037927936<\/span><\/li>\n
    • B = 97 << 16 =\u00a06356992<\/li>\n
    • AllocUnitId =\u00a072057594044284928<\/li>\n<\/ul>\n
      You can do this using SQL Server using the POWER<\/em> function as a left shift of X bits is the same as multiplying by 2-to-the-power-X:<\/p>\n
      \r\nSELECT 256 * CONVERT (BIGINT, POWER (2.0, 48)) | 97 * CONVERT (BIGINT, POWER (2.0, 16));\r\nGO\r\n<\/pre>\n
      And then you can perform the various look-ups using sys.system_internals_allocation_units<\/em> and sys.partitions<\/em> like so:<\/p>\n
      \r\nSELECT\r\n\t[a].[container_id] AS [Partition ID],\r\n\t[p].[object_id] AS [Object ID],\r\n\t[p].[index_id] AS [Index ID]\r\nFROM sys.system_internals_allocation_units [a]\r\nJOIN sys.partitions [p]\r\n\tON [p].[partition_id] = [a].[container_id]\r\nWHERE\r\n\t[a].[allocation_unit_id] = 72057594044284928;\r\nGO\r\n<\/pre>\n
      \r\nPartition ID         Object ID   Index ID\r\n-------------------- ----------- -----------\r\n72057594039304192    599673184   0\r\n<\/pre>\n
      And you can see that the values match the DBCC PAGE<\/em> output.<\/p>\n
      To convert from an allocation unit ID to what you should see in the DBCC PAGE<\/em> output:<\/p>\n
        \n
      • m_indexId<\/em> = AllocUnitId<\/em> >> 48<\/span><\/li>\n
      • m_objId<\/em> = (AllocUnitId<\/em> – (m_indexId<\/em> << 48)) >> 16<\/li>\n<\/ul>\n
        The T-SQL for this involves floating point math as we need to use the reciprocal of POWER<\/em>:<\/p>\n
        \r\nDECLARE @alloc BIGINT = 72057594044284928;\r\nDECLARE @index BIGINT;\r\n\r\nSELECT @index =\r\n\tCONVERT (BIGINT,\r\n\t\tCONVERT (FLOAT, @alloc)\r\n\t\t\t* (1 \/ POWER (2.0, 48)) -- right shift, reciprocal of left shift\r\n\t);\r\nSELECT\r\n\tCONVERT (BIGINT,\r\n\t\tCONVERT (FLOAT, @alloc - (@index * CONVERT (BIGINT, POWER (2.0, 48))))\r\n\t\t\t* (1 \/ POWER (2.0, 16)) -- right shift, reciprocal of left shift\r\n\t) AS [m_objId],\r\n\t@index AS [m_indexId];\r\nGO\r\n<\/pre>\n
         <\/p>\n
        \r\nm_objId              m_indexId\r\n-------------------- --------------------\r\n97                   256\r\n<\/pre>\n
        An example of when you might use this information\/code is during programmatic analysis of a corrupt database that DBCC CHECKDB<\/em> cannot process to allow you to extract data as a last resort.<\/p>\n
        Enjoy!<\/p>\n","protected":false},"excerpt":{"rendered":"
        It’s been a long time since I’ve written a post about pure internals, but every so often I get asked how an allocation unit ID is calculated from the m_objId and m_indexId fields that are stored in the header of every page. When DBCC PAGE dumps a page header’s contents, it does the necessary calculations […]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[48,62],"tags":[],"class_list":["post-4236","post","type-post","status-publish","format-standard","hentry","category-inside-the-storage-engine","category-on-disk-structures"],"yoast_head":"\nInside the Storage Engine: How are allocation unit IDs calculated? - Paul S. 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