SSAS 2016 Locking Improvements

I first became aware of the server-wide lock taken out by SSAS when processing finishes – and the issues that this can cause – from this blog post by Andrew Calvett back in 2009. More information on how locking works in SSAS can be found in chapter 26 of “Microsoft SQL Server 2008 Analysis Services Unleashed”, while the most comprehensive discussion of this topic can be found in this post by Jason Howell:

Over the years I’ve worked with several customers who have run into locking problems as a result of users querying while processing or synchronisation are taking place, so as a result I was interested to read the following paragraph in the white paper on “Automated Partition Management For Analysis Services Tabular Models” that was published a few months ago:

Note that commit operations have been optimized considerably for tabular models in SQL Server 2016. This has caused noticeable improvements in locking and blocking for some customers with near-real time processing requirements. Database write-commit locks are required to safely complete tasks such as merging pending changes, persisting files to disk, clearing some cached state, deletion of old files, etc. In previous versions of Analysis Services, a server-level write commit lock was taken while most of these tasks were performed. With SQL Server 2016, the server-level locks are far more limited; they are only taken while producing the delta of transaction updates, and are then immediately released.

This is very good news, and in fact the improvements apply to SSAS Multidimensional 2016 as well as SSAS Tabular 2016. The ever-helpful Akshai Mirchandani of the dev team has given me more details on the changes, so here’s a summary of what happens during a commit operation and what’s new in SSAS 2016:

  • First of all, a database read-commit lock is taken to analyse all the pending changes.
  • Next a database write-commit lock is taken so that the transaction can be committed safely. This is the lock that can be blocked by long-running queries, and this is where the ForceCommitTimeout property comes into play with the result that these long-running queries may get cancelled.
  • This lock is held while the pending changes are merged together.
  • At this point SSAS is ready to do the commit, and where it takes a server-level write-commit lock. This is also the point where the improvements in SSAS 2016 have been made.
    • In previous versions SSAS would update the master.vmp file in place and hold the server-level write-commit lock while that happens and while some other, potentially time-consuming things like clearing cached state and deleting all the old files take place. This could in some cases result in the server-level write-commit lock being held for an extended period.
    • Instead in SSAS 2016 a delta of all the transaction updates are written to a .txn file, and after that the server commit lock is released. The time-consuming tasks mentioned in the previous bullet still take place but after the server-level write-commit lock has been released. This means the server-level write-commit lock is now held for a very short amount of time, and what’s more that amount of time is quite consistent.
  • Finally, all remaining locks such as the database write-commit lock are released.

I haven’t had a chance to test these changes in a production system yet but it sounds like anyone that needs to process or synchronise regularly throughout the day will benefit from upgrading to SSAS 2016.

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