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Karthick P.K on SQL Server

SQL Server: Ghost records

Posted by Karthick P.K on August 30, 2012

 

What is Ghost record?

When a record is deleted from a clustered index data page or non-clustered index leaf page or a versioned heap page or a forwarded record is recalled, the record is logically removed by marking them as deleted but not physically removed from the page immediately.

Pages which are marked as deleted but actually not deleted physically are called Ghost Records.

 

Why?

Allowing delete or update operation run faster and to ensure faster rollback if the transaction is cancelled or roll backed.

 

Who will remove the records which are marked for deletion?

Ghostcleanuptask: SQL Server Ghostcleanuptask thread physically removes the records which are marked as deleted.

 

How Ghost cleanup task works?

1.       Ghostcleanuptask thread wakes up every 10 seconds.

2.       Sweep databases one by one starting from master.

3.       Skip the database if it is not able to take a shared lock for database (LCK_M_S) or database is not in Open read/write state.

4.       Scans the PFS pages of the current database to get the pages which has ghost records.

PFS Page: A PFS page occurs once in 8088 pages. SSQL Server will attempt to place a PFS page on the first page of every PFS interval(8088Pages). The only time a PFS page is not the first page in its interval is in the first interval for a file.

In this case, the file header page is first, and the PFS page is second. (Page ID starts from 0 so the first PFS page is at Page ID 1)

clip_image002_thumb

5.       Remove the records which are marked as deleted (ghosted) physically.

 

 

The GHOST_VERSION_RECORD is used in cases where versioned heap records, or BLOB fragments are deleted and when versioned forwarded heap rows are recalled back to their originating page. The records must be ghosted rather than deleted so the versioning chain can be maintained. The GHOST_VERSION_RECORDs cannot be expunged until all transactions that have references to the original data record have finished.

 

 

Let us see how the ghost records are marked for deletion and deleted later using below example.

 

Note: We will be using few trace flags for testing purpose. Use this trace flags and below steps only in your testing servers.

Trace flag 3604 Directs SQL Server to send output some of the command’s to client executing the command. Ex.  When we execute  DBCC PAGE or DBCC TRACESTATUS(-1) we will not see results in SSMS or any other client unless Trace flag 3604 is on

Trace flag   661 Disables ghost record cleanup task.  When this trace flag is enabled Ghost cleanup task will not run. 

Trace flag   662 along with 3605 prints the output of Ghost cleanup task to SQL Server error log ( DBCC TRACEON (662, 3605,-1))

 

 

 

Step1:

//Create a database named Ghost test

create database Ghosttest

go

use Ghosttest

go

 

Step 2:

//Create a table named ‘a’ with clustered and non-clustered index.

CREATE TABLE [dbo].[a](

      [a] [char](3000) NULL,

      [b] [char](3000) NULL,

      [c] [int] NULL

) ON [PRIMARY]

 

CREATE Clustered INDEX [CIX_a] ON [dbo].[a]

(

      [c] ASC

)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

 

 

 

CREATE  INDEX [IX_a] ON [dbo].[a]

(

      [c] ASC

)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

 

 

 

Go

 

Step 3:

//Insert 1000 records in to the table.

insert into  a values (‘Ghost’,‘Record’,1);

go 1000

 

Step 4:

//Enable trace flag 3604 to see the output of DBCC command from client like SSMS.

 

dbcc traceon (3604,-1)

 

go

 

Step 5:

// Disable Ghost record cleanup task to see how the ghost records are marked in data/index pages.   (Don’t enable in production)

 

dbcc traceon (661,-1)

go

 

Step 6:

//Delete the 1000 records which we inserted in table ‘a’ to generate ghost records (Records marked for deletion but not physically deleted).

 

delete from a

go

 

Step 7:

//Ghost cleanup task first scans the PFS pages in  current database to get the pages which has ghost records. Let us dump the PFS pages to see how the pages with ghost records are marked

// Watch the text which are in red or search for Has Ghost in the output of DBCC page.

DBCC PAGE(‘Ghosttest’,1,1,3)

Go

 

PAGE: (1:1)

 

 

BUFFER:

 

 

BUF @0x00000000FEC0ED80

 

bpage = 0x00000000CA024000           bhash = 0x0000000000000000           bpageno = (1:1)

bdbid = 9                            breferences = 0                      bcputicks = 0

bsampleCount = 0                     bUse1 = 62975                        bstat = 0xc0010b

blog = 0x79797979                    bnext = 0x0000000000000000          

 

PAGE HEADER:

 

 

Page @0x00000000CA024000

 

m_pageId = (1:1)                     m_headerVersion = 1                  m_type = 11

m_typeFlagBits = 0x1                 m_level = 0                          m_flagBits = 0x0

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 = 2                        m_freeData = 8188

m_reservedCnt = 0                    m_lsn = (50:335:27)                  m_xactReserved = 0

m_xdesId = (0:0)                     m_ghostRecCnt = 0                    m_tornBits = 412090936

 

Allocation Status

 

GAM (1:2) = ALLOCATED                SGAM (1:3) = NOT ALLOCATED           PFS (1:1) = 0x44 ALLOCATED 100_PCT_FULL

DIFF (1:6) = CHANGED                 ML (1:7) = NOT MIN_LOGGED           

 

PFS: Page Alloc Status  @0x000000002205A000

 

(1:0)        – (1:3)        =     ALLOCATED 100_PCT_FULL                             

(1:4)        – (1:5)        = NOT ALLOCATED   0_PCT_FULL                             

(1:6)        – (1:7)        =     ALLOCATED 100_PCT_FULL                             

(1:8)        -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:9)        -              =     ALLOCATED 100_PCT_FULL                     Mixed Ext

(1:10)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:11)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:12)       -              =     ALLOCATED 100_PCT_FULL           IAM Page  Mixed Ext

(1:13)       – (1:14)       =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:15)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:16)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:17)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:18)       -              =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:19)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:20)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:21)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:22)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:23)       – (1:25)       =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:26)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:27)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:28)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:29)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:30)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:31)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:32)       -              =     ALLOCATED  50_PCT_FULL                     Mixed Ext

(1:33)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:34)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:35)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:36)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:37)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:38)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:39)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:40)       – (1:41)       =     ALLOCATED   0_PCT_FULL                             

(1:42)       -              = NOT ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:43)       -              = NOT ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:44)       -              = NOT ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:45)       – (1:46)       = NOT ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:47)       -              = NOT ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:48)       – (1:50)       =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:51)       -              =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:52)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:53)       -              =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:54)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:55)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:56)       – (1:61)       =     ALLOCATED   0_PCT_FULL                             

(1:62)       -              = NOT ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:63)       -              = NOT ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:64)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:65)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:66)       – (1:71)       =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:72)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:73)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:74)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:75)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:76)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:77)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:78)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:79)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:80)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:81)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:82)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:83)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:84)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:85)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:86)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:87)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:88)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:89)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:90)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:91)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:92)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:93)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:94)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:95)       -              =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:96)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:97)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:98)       -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:99)       -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:100)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:101)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:102)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:103)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:104)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:105)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:106)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:107)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:108)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:109)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:110)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:111)      – (1:116)      =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:117)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:118)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:119)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:120)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:121)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:122)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:123)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:124)      -              =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:125)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:126)      -              =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:127)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:128)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:129)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:130)      -              =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:131)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:132)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:133)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:134)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:135)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:136)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:137)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:138)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:139)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:140)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:141)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:142)      – (1:143)      =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:144)      -              =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:145)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:146)      -              =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:147)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:148)      -              =     ALLOCATED  50_PCT_FULL                     Mixed Ext

(1:149)      -              =     ALLOCATED   0_PCT_FULL           IAM Page  Mixed Ext

(1:150)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:151)      -              =     ALLOCATED 100_PCT_FULL                     Mixed Ext

(1:152)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:153)      – (1:158)      =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:159)      -              =     ALLOCATED   0_PCT_FULL                     Mixed Ext

(1:160)      – (1:383)      =     ALLOCATED   0_PCT_FULL Has Ghost                   

(1:384)      – (1:385)      =     ALLOCATED   0_PCT_FULL Has Ghost           Mixed Ext

(1:386)      – (1:391)      = NOT ALLOCATED   0_PCT_FULL                             

(1:392)      – (1:497)      =     ALLOCATED   0_PCT_FULL Has Ghost                   

(1:498)      – (1:499)      =     ALLOCATED   0_PCT_FULL                             

(1:500)      – (1:655)      =     ALLOCATED   0_PCT_FULL Has Ghost                   

(1:656)      -              =     ALLOCATED   0_PCT_FULL                             

(1:657)      – (1:663)      =     ALLOCATED   0_PCT_FULL Has Ghost                   

(1:664)      – (1:671)      = NOT ALLOCATED   0_PCT_FULL                             

 

Step 8:

//Dump one of the page which is marked as having ghost record in PFS page. I have picked Page 158 for example.

//Look at m_ghostRecCnt

 

dbcc page(‘Ghosttest’,1,158,3)

go

 

 

PAGE: (1:158)

 

 

BUFFER:

 

 

BUF @0x00000000FEC02CC0

 

bpage = 0x00000000C9E22000           bhash = 0x0000000000000000           bpageno = (1:158)

bdbid = 9                            breferences = 0                      bcputicks = 933

bsampleCount = 1                     bUse1 = 64101                        bstat = 0xc0010b

blog = 0x2159bb79                    bnext = 0x0000000000000000          

 

PAGE HEADER:

 

 

Page @0x00000000C9E22000

 

m_pageId = (1:158)                   m_headerVersion = 1                  m_type = 1

m_typeFlagBits = 0x4                 m_level = 0                          m_flagBits = 0x0

m_objId (AllocUnitId.idObj) = 28     m_indexId (AllocUnitId.idInd) = 256 

Metadata: AllocUnitId = 72057594039762944                                

Metadata: PartitionId = 72057594038845440                                 Metadata: IndexId = 1

Metadata: ObjectId = 2105058535      m_prevPage = (1:157)                 m_nextPage = (1:160)

pminlen = 3508                       m_slotCnt = 2                        m_freeCnt = 1054

m_freeData = 7134                    m_reservedCnt = 0                    m_lsn = (37:160:37)

m_xactReserved = 0                   m_xdesId = (0:1983)                  m_ghostRecCnt = 2

m_tornBits = 602498023              

 

Allocation Status

 

GAM (1:2) = ALLOCATED                SGAM (1:3) = NOT ALLOCATED          

PFS (1:1) = 0x68 MIXED_EXT ALLOCATED   0_PCT_FULL                         DIFF (1:6) = CHANGED

ML (1:7) = NOT MIN_LOGGED           

 

Slot 0 Offset 0x60 Length 3519

 

Record Type = GHOST_DATA_RECORD      Record Attributes =  NULL_BITMAP VARIABLE_COLUMNS

Record Size = 3519                  

Memory Dump @0x000000001CEBA060

 

 

//There are 2 ghost records in page 158 (m_ghostRecCnt=2)

 

Step 8:

//Additionally we can query sys.dm_db_index_physical_stats to find the number of ghost records.

 

SELECT object_name(object_id) as Name, record_count, GHOST_RECORD_COUNT,Version_ghost_record_count,INDEX_TYPE_DESC, ALLOC_UNIT_TYPE_DESC

FROM sys.dm_db_index_physical_stats (DB_ID(N‘Ghosttest’), NULL, NULL, NULL , ‘DETAILED’) where index_level=0

go

clip_image004_thumb

 

 

Step 8:

//Let us enable trace flag 662 and 3605  to print the output of ghost cleanup task to SQL error log

  

dbcc traceon (662,3605,-1)

go

 

Step 9:

//Let us disable trace flag 661 to start the Ghost cleanup task again and let us disable 3604  which we enabled to print output of DBCC command in client (SSMS).

 

dbcc traceoff (661,3604,-1)

go

 

Purging page Dbid 9, File 1, Page 144

Purging page Dbid 9, File 1, Page 146

Purging page Dbid 9, File 1, Page 153

Purging page Dbid 9, File 1, Page 154

Purging page Dbid 9, File 1, Page 155

Purging page Dbid 9, File 1, Page 156

Purging page Dbid 9, File 1, Page 157

Purging page Dbid 9, File 1, Page 158

Purging page Dbid 9, File 1, Page 160

Purging page Dbid 9, File 1, Page 161

 

We will see the pages purged by the ghost cleanup task in the SQL Server error log.

Step 10:

//  We can also capture Ghost cleanup task in profiler by selecting SQL Transaction and filter by object name like  “GhostCleanupTask”

image

 

 

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Thank you,

Karthick P.K |My Facebook Page |My Site| Blog space| Twitter

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12 Responses to “SQL Server: Ghost records”

  1. Brahmananda Reddy said

    nice post karthick…keep them coming

  2. Rondorr said

    Amazing post.Thanks for your explanation about Ghost records.

  3. pinaldave said

    Very interesting read

  4. Ramkumar said

    Nice Article. Well explained. Keep up the good work

  5. Fantastic blog! Trace 661 does not a restart of sqlserver instance.

  6. Denny said

    Amazing article where do you learn all this?

  7. Jacky said

    Hi
    Why heap doesn’t support Ghost records?

  8. Good post, Karthick. Looking forward to reading more of your blog. -Kev (@kekline)

  9. […] SQL Server: Ghost records […]

  10. Shanky said

    HI,
    You said, Ghostcleanuptask thread wakes up every 10 seconds.But it runs every 5 seconds AFAIK

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