It wasn’t causing blocking, but the overhead of reading the massive version store still caused impact. That one read-only analytical query running in the background (using RCSI… readers don’t cause blocking!), caused the version store growth to impact everything else running on the server. I’ve recently witnessed a server have its version store grow to 200+GB, and at the same time watched read queries that use that version store slow to a crawl. When the version store gets really, really large, performance can take a hit. It could just be a very long running SELECT statement that will prevent the version store from being cleaned up, as even read-only queries (ie without a transaction) still need to access the version store to ensure consistent reads. It doesn’t even have to be a transaction. This means that a long running transaction could both grow your transaction log and grow your tempdb. SQL Server will keep track of the oldest open transaction or query, and clean up anything in the version store older than that point in time using a background process. (For a more accurate & detailed description of how this works, check out the series from Paul White ( blog| twitter) that I referenced earlier.)īut wait, if all these updates are writing to the version store, isn’t tempdb going to be huge? In order to prevent you from needing a 500 Petabyte tempdb, SQL Server will clean up row versions that it doesn’t need. Now, when you query under RCSI or Snapshot isolation, SQL Server can notice “whoops, the current version is too new! I better read an older one from the version store!” then it can go read the right one from the version store. Every update in the database generates an entry in the version store. Now that we have the version timestamp on the row, every time we update the row, SQL Server quietly copies the old version of the row into the version store. This completely solves the “Which version is newer? Which version do I want?” problem. This timestamp tells SQL Server when the row was written. SQL Server does this by storing a 14-byte timestamp as the version identifier on the row. I already mentioned that the “stale” version of the row goes into the version store, but how do we tell which version is which? If I update a row twice, and send two stale versions into the version store, I need to know which row is newer. What it doesīefore we talk about how the version store works, there’s one other ingredient we’ll need: the row timestamp/version itself. The overhead of this isn’t huge, but it is extra I/O on tempdb. This also means that all your snapshot-enabled databases will need to write to the tempdb version store–a new shared bottleneck for your separate databases. This means that every database on the instance shares a single version store. The version store isn’t part of a database itself, but rather is in tempdb. When a row is updated or deleted, the “stale” version gets stored in the version store. When using an optimistic locking isolation level, SQL Server introduces row versions, kept in the version store which allow read queries to not block or be blocked. Let’s talk about what it is, what it does, what it is, what it isn’t. If you have SSMS 19.x installed, installing SSMS 19.2 upgrades it to 19.2. If you have a preview version of SSMS 19 installed, uninstall it before installing SSMS 19.2. SSMS 19.2 is the latest general availability (GA) version. I’m not looking for a cell phone, I want a bargain on some used row versions. Free Download for SQL Server Management Studio (SSMS) 19.2. Turns out, the results for “version store” were not what I expected: If I don’t know what something is, I’m going to Google it. Along the way, people and articles have mentioned this “version store” thing. Perhaps you’ve looked into read-only secondary replicas in your Availability Group, or even had some problems with AG secondaries. Or maybe you were trying to figure out the difference between the two snapshot isolation levels. Maybe you’ve read about the Read Committed Snapshot (RCSI) or Snapshot Isolation levels. To create a Microsoft.Sql/servers resource, add the following Bicep to your template.Sure, Microsoft does versioning, but check out this Microbus versioning! Resource groups - See resource group deployment commandsįor a list of changed properties in each API version, see change log.The servers resource type can be deployed to:
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