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alternatives to read committed snapshot isolation

    Question

  • Hi we run 2016 enterprise. I turned read committed snapshot isolation on recently to a db where I'd like to avoid dirty reads, get "transactionally" complete data and avoid deadlocks.  Are there are other alternatives?  What are they and what are their advantages over rcsi?  


    • Edited by db042190 Wednesday, May 16, 2018 8:31 PM went off on a tangent
    Wednesday, May 16, 2018 4:54 PM

Answers

  • The alternative is snapshot isolation. Then you can enable it only for the pieces where you think that the current behaviour is what you want. This requires that you can change the source code.

    The effects on tempdb is the same as with RCSI.

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:23 AM
    Wednesday, May 16, 2018 9:06 PM
  • The alternative of course is NOT using snapshot isolation, thus avoiding the overhead of RCSI, but then you will get deadlocks that must be resolved with explicit lock hints or other hand-coding.

    Josh

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:23 AM
    Wednesday, May 16, 2018 10:58 PM
  • Hi There,

    If you have deadlock issues I strongly recommend to review your code and troubleshoot. 

    For a heavy transactional database "read committed snapshot isolation" is not recommended due to increased load on the database engine. check the below link for more info.

    https://dba.stackexchange.com/questions/5014/what-risks-are-there-if-we-enable-read-committed-snapshot-in-sql-server?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa

    good luck

    kumar

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:24 AM
    Thursday, May 17, 2018 1:10 AM
  • Hi we run 2016 enterprise. I turned read committed snapshot isolation on recently to a db where I'd like to avoid dirty reads, get "transactionally" complete data and avoid deadlocks.  Are there are other alternatives?  What are they and what are their advantages over rcsi?  


    I would like to divert a bit and focus on point when you said "get Transactionally complete data". Actually the wording is not correct RCSI read statements provide you data I must say provides you snapshot of data which it "sees" at beginning of the transaction and does not cares about what happens to it later. Plus since read statements would not take shared lock the data provided would be correct as per snapshot taken during beginning of transaction but may later would have changed and you might be reading outdated data. The compete behavior is explained by Paul White in this Blog .

    Now coming back to alternative may I ask why are you looking for alternative and what you have in your mind when you say I need alternative ?

    Advantages: readers dont block writers and vice versa, you dont need to  make any change in your application to implement RCSI

    Disadvantages: Uses heavily tempdb and system resources. You might see different result(read as unexpected ) when you run multiple update transactions in RCSI.


    Cheers,

    Shashank

    Please mark this reply as answer if it solved your issue or vote as helpful if it helped so that other forum members can benefit from it

    My TechNet Wiki Articles

    MVP


    Thursday, May 17, 2018 7:13 AM
    Moderator
  • Read_committed_snapshot is the database option that affects behavior of the readers in READ COMMITTED isolation level. When you enable this option on database level (requires exclusive db access to change it), all your readers in READ COMMITTED isolation level will start to use row-versioning reading old(committed) versions from the version store rather than being blocked by (S)/(X) locks incompatibility. This option does not require any code changes (assuming you readers are using READ COMMITTED). It would not change behavior of writers (writers still block each other) nor behavior of readers in any other transaction isolation level.

    Snapshot isolation level is full blown transaction isolation level. It needs to be explicitly specified in the code. Enabling snapshot isolation level on DB level does not change behavior of queries in any other isolation levels. In that option you are eliminating all blocking even between writers (assuming they do not update the same rows) although it could lead to 3960 errors (data has been modified by other sessions). 

    Speaking of consistency, RCSI gives you statement level consistency – readers ignores the data changes done after statement has been started. Snapshot – transaction level consistency – session deals with “snapshot” of the data at the moment transaction started. Again, it could lead to error 3960 in the system with volatile data.

    Downsides:
    1. Excessive tempdb usage due to version store activity. Think about session that deletes 1M rows. All those rows must be copied to version store regardless of session transaction isolation level and/or if there are other sessions that running in optimistic isolation levels at the moment when deletion started.
    2. Extra fragmentation – SQL Server adds 14-byte version tag (version store pointer) to the rows in the data files when they are modified. This tag stayed until index is rebuild
    3. Development challenges – again, error 3960 with snapshot isolation level. Another example in both isolation levels – trigger or code based referential integrity. You can always solve it by adding with (READCOMMITTED) hint if needed. 

    While switching to RCSI could be good emergency technique to remove blocking between readers and writers (if you can live with overhead AND readers are using read committed), I would suggest to find root cause of the blocking. Confirm that you have locking issues – check if there are shared lock waits in wait stats, that there is no lock escalations that block readers, check that queries are optimized, etc.  

    Best Regards,Uri Dimant SQL Server MVP, http://sqlblog.com/blogs/uri_dimant/

    MS SQL optimization: MS SQL Development and Optimization
    MS SQL Consulting: Large scale of database and data cleansing
    Remote DBA Services: Improves MS SQL Database Performance
    SQL Server Integration Services: Business Intelligence

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:25 AM
    Thursday, May 17, 2018 10:46 AM
    Answerer
  • The 14 byte overhead is the rowversion that is stored in the data record of each row. Once READ_COMMITTED_SNAPSHOT or ALLOW_SNAPSHOT_ISOLATION is turned on, the rowversion is added during subsequent INSERT and UPDATE operations. This is in addition to the tempdb row version store.

    Dan Guzman, Data Platform MVP, http://www.dbdelta.com

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:25 AM
    Thursday, May 17, 2018 11:45 AM
    Moderator
  • thx Uri.  So the version store with rcsi (14 bytes etc) is in tempdb, not with/alongside the table (data of record)? 

    That is correct, the version store is in tempdb (clarification: as explained by Dan above, the 14 byte is in addition to the version store. 14 byte is broken as 6 bytes for transaction sequence number i.e. information about the DML transaction that caused the change and 8 bytes for the row identifier that points to the versioned row)

     SQL Server, by default, runs under READ COMMITTED (locking, pessimistic) isolation level. 

    Read Committed comes in two variations, the default one (mentioned above) and RCSI (when you enable RCSI on your DB). They both give the same undesirable behaviors (listed below) with an exception in RCSI which is blocking-free (readers and writers don't interfere with each other in RCSI).

    1. Dirty Reads = No
    2. Non-repeatable Reads =  Yes
    3. Phantom Reads = Yes
    4. Duplicated Reads = Yes
    5. Skipped Rows = Yes

    I have written the following article explaining the behaviors under each isolation level. You might want to give it a read.

    https://social.technet.microsoft.com/wiki/contents/articles/51484.sql-server-concurrency-control-models-acid-properties-and-transaction-isolation-levels.aspx

    Hope this helps.


    Please remember to click "Mark as Answer" if my response answered your question or click "Vote as helpful" if it helped you in any way.


    • Edited by Mohsin_A_Khan Thursday, May 17, 2018 11:58 AM added clarification
    • Marked as answer by db042190 Tuesday, May 22, 2018 11:25 AM
    Thursday, May 17, 2018 11:47 AM
  • Row versions are created and kept in tempdb that leads to increase space usage and IOs there. 

    When you enable snapshot isolation or read-committed-snapshot option on the database, the SQL Server starts adding a 14 byte overhead to each row to keep the information about ransaction sequence number (6)  and row identifier (8)

     One point to keep in mind is that new rows inserted after SI/RCSI, will have this 14 byte overhead even though there is no row version to point to.,


    Best Regards,Uri Dimant SQL Server MVP, http://sqlblog.com/blogs/uri_dimant/

    MS SQL optimization: MS SQL Development and Optimization
    MS SQL Consulting: Large scale of database and data cleansing
    Remote DBA Services: Improves MS SQL Database Performance
    SQL Server Integration Services: Business Intelligence

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:26 AM
    Thursday, May 17, 2018 11:54 AM
    Answerer
  • I don't really understand your concerns.

    If you have concurrent processes, and you write good transactional code, you will have no dirty reads.

    You may have some blocking, but you WANT blocking, because you don't want dirty reads.

    There's no such thing as a free lunch!

    Josh

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:27 AM
    Monday, May 21, 2018 4:41 PM
  • My problem is that rcsi seems to get bad press.  So much so that I want to understand what else I can do.      

    The potential problems with RCSI is mainly that code that runs validation in conjunction with update reads stale data. That does not seem to be a concern in the scenario you describe, so go for it.

    The other concern is that you need more space in tempdb. Well, just size tempdb accordingly.

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:27 AM
    Monday, May 21, 2018 9:31 PM

All replies

  • The alternative is snapshot isolation. Then you can enable it only for the pieces where you think that the current behaviour is what you want. This requires that you can change the source code.

    The effects on tempdb is the same as with RCSI.

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:23 AM
    Wednesday, May 16, 2018 9:06 PM
  • The alternative of course is NOT using snapshot isolation, thus avoiding the overhead of RCSI, but then you will get deadlocks that must be resolved with explicit lock hints or other hand-coding.

    Josh

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:23 AM
    Wednesday, May 16, 2018 10:58 PM
  • Hi There,

    If you have deadlock issues I strongly recommend to review your code and troubleshoot. 

    For a heavy transactional database "read committed snapshot isolation" is not recommended due to increased load on the database engine. check the below link for more info.

    https://dba.stackexchange.com/questions/5014/what-risks-are-there-if-we-enable-read-committed-snapshot-in-sql-server?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa

    good luck

    kumar

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:24 AM
    Thursday, May 17, 2018 1:10 AM
  • Hi we run 2016 enterprise. I turned read committed snapshot isolation on recently to a db where I'd like to avoid dirty reads, get "transactionally" complete data and avoid deadlocks.  Are there are other alternatives?  What are they and what are their advantages over rcsi?  


    I would like to divert a bit and focus on point when you said "get Transactionally complete data". Actually the wording is not correct RCSI read statements provide you data I must say provides you snapshot of data which it "sees" at beginning of the transaction and does not cares about what happens to it later. Plus since read statements would not take shared lock the data provided would be correct as per snapshot taken during beginning of transaction but may later would have changed and you might be reading outdated data. The compete behavior is explained by Paul White in this Blog .

    Now coming back to alternative may I ask why are you looking for alternative and what you have in your mind when you say I need alternative ?

    Advantages: readers dont block writers and vice versa, you dont need to  make any change in your application to implement RCSI

    Disadvantages: Uses heavily tempdb and system resources. You might see different result(read as unexpected ) when you run multiple update transactions in RCSI.


    Cheers,

    Shashank

    Please mark this reply as answer if it solved your issue or vote as helpful if it helped so that other forum members can benefit from it

    My TechNet Wiki Articles

    MVP


    Thursday, May 17, 2018 7:13 AM
    Moderator
  • thx Erland.  what do you mean by pieces (queries?)?  and doesn't rcsi always entail a 14 byte overhead for every record?
    Thursday, May 17, 2018 10:38 AM
  • thx Kumar.  Its a new db (so no history of deadlocks) and not heavily transactional.  This approach was put in day 1.
    Thursday, May 17, 2018 10:40 AM
  • thx shanky_621.  I don't need an alternative.  Just asking what they are.  And the potential for reading old data was understood going in.  Which I think is the same potential if using snapshot isolation which requires changing the app? 
    Thursday, May 17, 2018 10:45 AM
  • Read_committed_snapshot is the database option that affects behavior of the readers in READ COMMITTED isolation level. When you enable this option on database level (requires exclusive db access to change it), all your readers in READ COMMITTED isolation level will start to use row-versioning reading old(committed) versions from the version store rather than being blocked by (S)/(X) locks incompatibility. This option does not require any code changes (assuming you readers are using READ COMMITTED). It would not change behavior of writers (writers still block each other) nor behavior of readers in any other transaction isolation level.

    Snapshot isolation level is full blown transaction isolation level. It needs to be explicitly specified in the code. Enabling snapshot isolation level on DB level does not change behavior of queries in any other isolation levels. In that option you are eliminating all blocking even between writers (assuming they do not update the same rows) although it could lead to 3960 errors (data has been modified by other sessions). 

    Speaking of consistency, RCSI gives you statement level consistency – readers ignores the data changes done after statement has been started. Snapshot – transaction level consistency – session deals with “snapshot” of the data at the moment transaction started. Again, it could lead to error 3960 in the system with volatile data.

    Downsides:
    1. Excessive tempdb usage due to version store activity. Think about session that deletes 1M rows. All those rows must be copied to version store regardless of session transaction isolation level and/or if there are other sessions that running in optimistic isolation levels at the moment when deletion started.
    2. Extra fragmentation – SQL Server adds 14-byte version tag (version store pointer) to the rows in the data files when they are modified. This tag stayed until index is rebuild
    3. Development challenges – again, error 3960 with snapshot isolation level. Another example in both isolation levels – trigger or code based referential integrity. You can always solve it by adding with (READCOMMITTED) hint if needed. 

    While switching to RCSI could be good emergency technique to remove blocking between readers and writers (if you can live with overhead AND readers are using read committed), I would suggest to find root cause of the blocking. Confirm that you have locking issues – check if there are shared lock waits in wait stats, that there is no lock escalations that block readers, check that queries are optimized, etc.  

    Best Regards,Uri Dimant SQL Server MVP, http://sqlblog.com/blogs/uri_dimant/

    MS SQL optimization: MS SQL Development and Optimization
    MS SQL Consulting: Large scale of database and data cleansing
    Remote DBA Services: Improves MS SQL Database Performance
    SQL Server Integration Services: Business Intelligence

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:25 AM
    Thursday, May 17, 2018 10:46 AM
    Answerer
  • thx Uri.  So the version store with rcsi (14 bytes etc) is in tempdb, not with/alongside the table (data of record)? 
    • Edited by db042190 Thursday, May 17, 2018 11:28 AM qualify rcsi
    Thursday, May 17, 2018 11:27 AM
  • The 14 byte overhead is the rowversion that is stored in the data record of each row. Once READ_COMMITTED_SNAPSHOT or ALLOW_SNAPSHOT_ISOLATION is turned on, the rowversion is added during subsequent INSERT and UPDATE operations. This is in addition to the tempdb row version store.

    Dan Guzman, Data Platform MVP, http://www.dbdelta.com

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:25 AM
    Thursday, May 17, 2018 11:45 AM
    Moderator
  • thx Uri.  So the version store with rcsi (14 bytes etc) is in tempdb, not with/alongside the table (data of record)? 

    That is correct, the version store is in tempdb (clarification: as explained by Dan above, the 14 byte is in addition to the version store. 14 byte is broken as 6 bytes for transaction sequence number i.e. information about the DML transaction that caused the change and 8 bytes for the row identifier that points to the versioned row)

     SQL Server, by default, runs under READ COMMITTED (locking, pessimistic) isolation level. 

    Read Committed comes in two variations, the default one (mentioned above) and RCSI (when you enable RCSI on your DB). They both give the same undesirable behaviors (listed below) with an exception in RCSI which is blocking-free (readers and writers don't interfere with each other in RCSI).

    1. Dirty Reads = No
    2. Non-repeatable Reads =  Yes
    3. Phantom Reads = Yes
    4. Duplicated Reads = Yes
    5. Skipped Rows = Yes

    I have written the following article explaining the behaviors under each isolation level. You might want to give it a read.

    https://social.technet.microsoft.com/wiki/contents/articles/51484.sql-server-concurrency-control-models-acid-properties-and-transaction-isolation-levels.aspx

    Hope this helps.


    Please remember to click "Mark as Answer" if my response answered your question or click "Vote as helpful" if it helped you in any way.


    • Edited by Mohsin_A_Khan Thursday, May 17, 2018 11:58 AM added clarification
    • Marked as answer by db042190 Tuesday, May 22, 2018 11:25 AM
    Thursday, May 17, 2018 11:47 AM
  • thx guys, so sql needs overhead in both the record and tempdb to pull off rcsi.  Is the overhead in tempdb at least short lived?
    Thursday, May 17, 2018 11:53 AM
  • Row versions are created and kept in tempdb that leads to increase space usage and IOs there. 

    When you enable snapshot isolation or read-committed-snapshot option on the database, the SQL Server starts adding a 14 byte overhead to each row to keep the information about ransaction sequence number (6)  and row identifier (8)

     One point to keep in mind is that new rows inserted after SI/RCSI, will have this 14 byte overhead even though there is no row version to point to.,


    Best Regards,Uri Dimant SQL Server MVP, http://sqlblog.com/blogs/uri_dimant/

    MS SQL optimization: MS SQL Development and Optimization
    MS SQL Consulting: Large scale of database and data cleansing
    Remote DBA Services: Improves MS SQL Database Performance
    SQL Server Integration Services: Business Intelligence

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:26 AM
    Thursday, May 17, 2018 11:54 AM
    Answerer
  • thx shanky_621.  I don't need an alternative.  Just asking what they are.  And the potential for reading old data was understood going in.  Which I think is the same potential if using snapshot isolation which requires changing the app? 
    In any case you dont need to change anything in application code, just make sure you are using snapshot isolation level in queries

    Cheers,

    Shashank

    Please mark this reply as answer if it solved your issue or vote as helpful if it helped so that other forum members can benefit from it

    My TechNet Wiki Articles

    MVP

    Thursday, May 17, 2018 11:56 AM
    Moderator
  • thx Uri, so for a particular record's one update that occurred today, and before the index is rebuilt tonite, that version sits in tempdb all day?
    Thursday, May 17, 2018 12:07 PM
  • thx Erland.  what do you mean by pieces (queries?)?  and doesn't rcsi always entail a 14 byte overhead for every record?

    "pieces" = parts of application where you want snapshot isolation. Since I don't know anything about your application, I'm using a generic terminology. That could be single queries, stored procedures, parts of client code or whatnots.

    Thursday, May 17, 2018 9:01 PM
  • thx, we traditionally see problems in adhoc queries where folks forget things like this.  Any thoughts?  It sounds to me like rcsi isn't desirable in most peoples minds but perhaps less risky when compared to snapshot isolation.    
    Monday, May 21, 2018 11:33 AM
  • What exactly is the problem you are trying to solve?  You would get a better answer if you describe your problem.

    Monday, May 21, 2018 12:17 PM
    Moderator
  • thx Tom.  I have a warehousing clustered column store with over 100 columns.  It is supplemented by a traditional table that contains comments and a pointer back to the column store.  The two tables are in a new db where I turned rcsi on day one.  Deletes never occur.  Only updates (to version obsolete records) and inserts.  I don't want dirty reads.  I don't want blocking.  I cannot afford to show transactionally incomplete data.  With ever increasing frequencies, the data is incrementally loaded from the oltp.  Adhoc queries occur now and then.   Reporting clients (eg ssrs reports, excel pivots, tableau slicing, power bi etc)  are expected to keep growing.

    My problem is that rcsi seems to get bad press.  So much so that I want to understand what else I can do.

          
    Monday, May 21, 2018 1:01 PM
  • I don't really understand your concerns.

    If you have concurrent processes, and you write good transactional code, you will have no dirty reads.

    You may have some blocking, but you WANT blocking, because you don't want dirty reads.

    There's no such thing as a free lunch!

    Josh

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:27 AM
    Monday, May 21, 2018 4:41 PM
  • My problem is that rcsi seems to get bad press.  So much so that I want to understand what else I can do.      

    The potential problems with RCSI is mainly that code that runs validation in conjunction with update reads stale data. That does not seem to be a concern in the scenario you describe, so go for it.

    The other concern is that you need more space in tempdb. Well, just size tempdb accordingly.

    • Marked as answer by db042190 Tuesday, May 22, 2018 11:27 AM
    Monday, May 21, 2018 9:31 PM
  • thx JR and Erland.  I thought about it a bit more last eve and have to conclude what Erland said, go for it.  Agreed, its not free.  But the fact is that at present I have folks running adhoc queries against my data.  So I cant totally control how well formed the code is.  Even if I could, the snapshot isolation alternative still seems to have more downside than rcsi.  I was hoping to get a better understanding from this post of the actual impact on tempdb etc but I fell a little short.  I may try to get that knowledge with a separate post or find an article.  RCSI definitely makes people's eyes roll when mentioned but I'm guessing that its ok under certain conditions.    
    Tuesday, May 22, 2018 11:23 AM