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Category: Administration

Data Inconsistency in Postgres HA Clusters

Umair Shahid gives us an overview:

While PostgreSQL is known for its robustness, scalability, and reliability, data inconsistency can occur in PostgreSQL clusters, which can cause issues and impact the overall performance of the system. In this blog, we’ll define data inconsistency in PostgreSQL clusters, discuss the challenges it poses, its causes, and provide some tips on how to prevent and resolve it if it occurs.

Click through for the article.

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MVCC and Vacuuming in Postgres

Ryan Booz explains one area where Postgres’s implementation differs from most other vendors:

All relational databases handle transaction isolation in some way, typically with an implementation of Multi-version Concurrency Control (MVCC). Plain ‘ol, mainline SQL Server uses a form of MVCC, but all older rows (currently retained for ongoing transactions) are stored in TempDB. Oracle and MySQL also do something similar, storing (essentially) diffs of the modified data outside of the table that is merged at runtime for ongoing transactions that still need to see the older data.

Among these databases, PostgreSQL stands alone in the specific way MVCC is implemented. Rather than storing some form of the older data outside of the current table for transactions to query/merge/etc. at runtime, PostgreSQL always creates the newly modified row in-table alongside the existing, older versions that are still needed for running transactions. Yes, every UPDATE creates a new row of data in the table, even if you just change one column.

Read on to understand some of the implications of this and how it affects the way we manage databases.

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Keeping VLF Counts Low (Enough)

Eitan Blumin has a script for us:

In this ultimate guide, learn how to fix high VLF counts and shrink transaction logs in SQL Server to improve performance. Discover best practices for managing transaction log sizes and VLFs, and get your hands on a robust script to help you out. Boost your SQL Server’s performance and optimize your database administration with these tips.

Click through for the guide, and for a script Eitan has put together.

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Holding and Pruning WhoIsActive Results

Andrea Allred has a script for us:

Last month I talked about how I started collecting data from sp_whoisactive. The long term goal was to analyze and tune those long running processes that are sometimes hard to quantify in Query Store. We had started collecting the data in a table (whoisactiveLog), but wanted to make a simple table that our Engineers could refer to and find the long running processes. We also wanted to archive off the whoisactiveLog into another table and save that for 70 days in case it was needed for further research on the tuning of processes. Each night, we have a break in our processes where we can do this maintenance so we decided we would empty the whoisactiveLog table completely.

There’s also a check for session length in there.

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Delayed Durability and Shutdown Behaviors

Guy Glantser does some explaining:

In the world of database management systems, ensuring data durability is of paramount importance. It involves guaranteeing that committed transactions are not lost in the event of unexpected failures or system crashes. SQL Server offers a feature known as Delayed Durability to address this challenge. Delayed Durability provides a mechanism for optimizing transaction logging and improving performance without compromising data integrity. This article aims to delve into the concept of Delayed Durability in SQL Server, specifically focusing on its behavior during expected shutdown scenarios.

Guy’s post surprised me, and I recommend reading it if you’ve thought about enabling the feature.

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Finding SQL Server Installation Media on Azure VMs

Bob Pusateri does a search:

I was recently wanting to test out some PolyBase features in SQL Server. Azure being my test environment of choice these days, I spun up an Azure SQL Virtual Machine, but I quickly found that PolyBase wasn’t installed. To add it I would need the install media of course, but how does one get that in an Azure SQL Virtual Machine?

Everybody should test out PolyBase, but that’s because I’m wildly biased. Anyhow, Bob shows us where we can find the installation media.

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Notes on Postgres Backups

Muhammad Ali hits us with it:

Backing up your PostgreSQL database is a critical task for ensuring the safety and availability of your data. In the event of a hardware failure, software error, or other disaster, having a recent backup of your database can mean the difference between a brief outage and a catastrophic data loss. In this blog post, we’ll cover best practices for backing up PostgreSQL database.

Click through for some notes on various backup utilities (pg_dump, pg_dumpall, pg_basebackup), when you might want to use each, and a few more topics.

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The Myth of the DBA-Free Cloud

Matthew McGiffen lays out an explanation:

I was chatting with a cloud consultant who was advising on a large scale migration to AWS. He told me that one of the advantages of going for a PaaS offering (Platform as a Service) was that DBAs were no longer required as backups and restores were handled for you. PaaS services for SQL Server include AWS RDS and Azure SQL Database or Azure SQL Managed Instance.

I found it quite a funny conversation, partly as I don’t think he realised being a DBA was part of my job role, but also because I don’t know a single DBA who spends a significant amount of their time doing backups and restores.

I still remember (through others—I wasn’t in this space yet) the advertising campaign that SQL Server 2005 would completely eliminate the need for a DBA because everything would just work on its own, even sweet database tuning using the Database Tuning Advisor. The same thing applies today: even those DBA-free databases eventually need somebody to optimize them along various dimensions, ensure they are running smoothly, and correct issues if they are not. Perhaps we could call this role the Administrator of a Database or AoD, so as not to scare the DBA-free database vendors. “No, we don’t have DBAs—we just need you to have a few AoDs on staff.”

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TDE and Database Backups

Matthew McGiffen shares some advice:

Database backups continue to work without change when you have TDE enabled. The only difference is that the backups contain encrypted data that cannot be read without the certificate and private key. There are a couple of points that are worth discussing though.

Click through for several notes, including a warning to those still on SQL Server 2016 and woefully under-patched.

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