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  • 1. Optimizing Procedural Code Understanding Logging, Recovery, Locking and Performance Presentation HIGHLIGHT The Anatomy of a Data Modification Kimberly L. Tripp President/Founder
  • 2.
    • User sends UPDATE
      • Update is highly selective (only 5 rows)
      • Indexes exist to aid in finding these rows efficiently
      • The update is a SINGLE statement batch NOT enclosed in BEGIN TRAN…COMMIT TRAN block therefore this is IMPLICIT transaction
    • Server receives the request and locates the data in cache OR reads the data from disk into cache
      • Since this is highly selective only the necessary pages are read into cache (maybe a few extra but that’s not important here)
      • Let’s use an example where the 5 rows being modified are located on 3 data pages
    The Anatomy of a Data Modification
  • 3. What it Looks Like – Data UPDATE… Data Log Server… Cache
  • 4.
    • SQL Server proceeds to lock the necessary data
      • Locks are necessary to give us a consistent point FOR ALL rows from which to start
      • If any other transaction(s) have ANY of these rows locked we will wait until ALL locks have been acquired before we can proceed.
      • In the case of this update (because it’s highly selective and because indexes exist to make this possible) SQL Server will use row level locking.
      • The rows are locked but there are also “intent” locks at higher levels to make sure other larger locks (like page or table level locks) are not attempted (and fail)
      • There are a few locks that have already occurred – within indexes, etc. to read the data – but they are not significant here
      • This sounds complex but it’s not too bad…
    The Anatomy of a Data Modification
  • 5. What it Looks Like – Locks Cache Page Page Page Row Row Row Row Row Update Lock Update Lock Update Lock Update Lock Update Lock
  • 6.
    • SQL Server can now begin to make the modifications – for EVERY row the process will include:
      • Change the lock to a stricter lock (eXclusive lock)
        • An update lock helps to allow better concurrency by being compatible with shared locks (readers). Readers can read the pre-modified data as it is still transactionally consistent and has not YET been modified
        • The eXclusive lock is required in order to change the data because once modified no other reads will be allowed to see this transient/un-committed state of the data
      • Make the modification to the data row (yes, in cache)
      • Log the modification to the transaction log pages (also in cache)
    The Anatomy of a Data Modification
  • 7. What it Looks Like – Modifications Cache Page Page Page Row Row Row Row Row Update Lock Exclusive Lock x x Update Lock Exclusive Lock x x Update Lock Exclusive Lock x x Update Lock Exclusive Lock x x Update Lock Exclusive Lock x x L
  • 8.
    • Finally, the transaction is complete – this is the MOST critical step
      • All rows have been modified
      • There are no other statements in this transaction – i.e. this is an implicit transaction in “Autocommit” mode. NOTE: the session setting SET IMPLICIT_TRANSACTIONS ON cannot be turned on and rarely should be! Makes for sloppy developers!)
      • Steps to ensure durability are:
        • Write all log pages to transaction log ON DISK
        • Release the locks
        • Send a message to the user:
        • (5 Rows Affected)
    The Anatomy of a Data Modification
  • 9. What it Looks Like Write-ahead Logging L Data Log Server… Cache ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Sequential writes Change Change Change Change … Log 5 Rows Affected After the log entries are made and the locks are released…
  • 10.
    • The transaction log portion of the database ON DISK – is up to date
    • The data in CACHE – is up to date
    • But when does the data get written from cache to disk?
      • CHECKPOINT
    • It’s important to realize that it’s not the sole purpose of checkpoint to just to write committed pages… Instead a checkpoint writes ALL pages which have changed since they were brought into cache – regardless of the state of the transaction that changed them!
    The Anatomy of a Data Modification
  • 11. Transaction Recovery and Checkpoints Transactions… Action Required if restart recovery None Checkpoint System Failure 1 2 3 4 5 Roll forward Roll back Roll forward Roll back L D L/D L/D L L Time
  • 12.
    • Data Portion mostly random reads – except at checkpoint
    • Log Portion mostly sequential writes
    • Separate physical disks minimizes contention at the drive level – first choice in tuning
    • Log is critical!
      • Log is written AHEAD of the data portion
      • Log is the ONLY place where transactional consistency is known (i.e. guaranteed)
      • Once a checkpoint occurs SQL Server doesn’t need the information in the log – for committed ( a.k.a. inactive ) transactions ( the log could even be cleared )
      • However… without the transaction log a database cannot function ( i.e. marked suspect )
      • Then what?
    Key Points