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Category: Linux

External Memory Pressure With SQL On Linux

Published 2017-10-23 by Kevin Feasel

Anthony Nocentino explains how SQL Server on Linux reacts to memory pressure:

We can use tools like ps, top and htop to look our are virtual and physical memory allocations. We can also look in the /proc virtual file system for our process and look at the status file. In here we’ll find the point in time status of a process, and most importantly the types of memory allocations for a process. We’ll get granular data on the virtual memory allocations and also the resident set size of the process. Here are the interesting values in the status file we’re going to focus on today.

  • VmSize – total current virtual address space of the process

  • VmRSS – total amount of physical memory currently allocated to the process

  • VmSwap – total amount of virtual memory currently paged out to the swap file (disk)

The differences are going to be interesting for people to troubleshoot later, particularly if you look at SOS_SCHEDULER_YIELD and give a knee-jerk reaction that the problem is with CPU.

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The OOM Killer Cometh

Published 2017-10-18 by Kevin Feasel

Venu Cherukupalli shows how to keep Linux’s Out of Memory Killer from taking down SQL Server:

When an index rebuild was kicked off on a large table (around 25GB), the reindex operation terminated, and the availability group had failed over to the other replica.

Upon further investigation, we discovered that the SQL Server process terminated at the time reindex operation was run and this resulted in the failover.

To determine the reason for the unexpected shutdown, we reviewed the Linux System Logs (/var/log/messages on RHEL) & pacemaker logs. From the pacemaker logs and system logs, we saw entries indicating that oom-killer was invoked, and as a result SQL Server process was terminated.

Read on for the two solutions.  I was hoping for a solution that involved making the SQL Server executable immune from oom-killer’s wily ways, but not so much in this post.

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Active Directory On CentOS

Published 2017-10-05 by Kevin Feasel

Drew Furgiuele shows how to configure a box running CentOS to work with Active Directory:

Before we start though, there’s a few things you’re going to need to have already set up:

  • An Active Directory Domain to test in, and rights to administer it. Since we’re going to be creating (and possibly deleting, if there are errors) computer objects and a service account, you’ll need a domain account with adequate permissions.

  • My example assumes you have a Microsoft DNS server running alongside your domain services. It is possible to use a separate DNS server to get this to work, but you might need some additional network configuration (see below). Also, depending on your environment, you might need a reverse lookup zone defined. If you notice long ping times or other weird lookups, I’d set one up in your DNS.

  • A machine (virtual or otherwise) that is running CentOS 7 or later (and this guide was written and tested against CentOS 7). For this demo, we’ll be using the Server (minimal install) installation option.  If you’re new to Linux, you might opt a desktop version (server with a GUI). When you download a CentOS disk image to install it, you get all these options on the default media; you won’t need separate downloads

There are a few more prereqs, so read the whole thing.  This route is easier than Ubuntu, as Drew notes.

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Unattended Installation Of SQL Server 2017 On Linux

Published 2017-10-05 by Kevin Feasel

Denzil Ribeiro walks us through an unattended installation and configuration of SQL Server 2017 on Linux:

SQL 2017 bits are generally available to customers today. One of the most notable milestones in the 2017 release is SQL Server on Linux. Setup has been relatively simple for SQL Server on Linux, but often there are questions around unattended install. For SQL Server on Linux, there are several capabilities that are useful in unattended install scenarios:

  • You can specify environment variables prior to the install that are picked up by the install process, to enable customization of SQL Server settings such as TCP port, Data/Log directories, etc.

  • You can pass command line options to Setup.

  • You can create a script that installs SQL Server and then customizes parameters post-install with the mssql-conf

The sample script link seems like it’s broken, but you can see it all on Denzil’s Github repo.

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New Docker Tag For SQL Server 2017

Published 2017-10-05 by Kevin Feasel

Hamish Watson notes that there are new Docker tags for SQL Server 2017:

I have been using SQL Server 2017 running on Linux for a while now (blog post pending) and use the official images from:

https://hub.docker.com/r/microsoft/mssql-server-linux/

To get the latest I used to run

docker pull microsoft/mssql-server-linux:latest

However today I noticed that the :latest tag had been removed:

Click through to see the tag you probably want to use.

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Cross-Platform Variables In Powershell Core

Published 2017-10-02 by Kevin Feasel

Max Trinidad shows how to use the Is* variables in Powershell Core to write cross-platform code:

Use the cmdlet Get-Variable to find them, and keep in mind, these variables are not found in Windows PowerShell 5.x.

Get-Variable Is*

Although, the results will display four variable, but let’s pay attention to three of them. Below are the variables with their default values:

IsLinux                            False
IsOSX                              False
IsWindows                    True

These three variables can help in identifying which Operating System the script are been executed.  This way just adding the necessary logic, in order to take the correct action.

Read on for a code example showing how to use these variables.

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Docker Stop Versus Docker Kill

Published 2017-10-02 by Kevin Feasel

Andrew Pruski explains why docker kill is so much faster than docker stop:

When running demos and experimenting with containers I always clear down my environment. It’s good practice to leave a clean environment once you’ve finished working.

To do this I blow all my containers away, usually by running the docker stop command.

But there’s a quicker way to stop containers, the docker kill command.

Sending SIGTERM isn’t particularly polite and doesn’t let processes clean up, which could leave your process in an undesirable state during future runs.  But if you’re just re-deploying a container, you don’t really care about the prior state of the now-disposed container.

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Instant File Initialization On Linux

Published 2017-09-25 by Kevin Feasel

Anthony Nocentino explains how instant file initialization works on SQL Server on Linux:

With strace up and running let’s turn on the trace flags to enable output for Instant File Initialization and create database that has a 100MB data file and a 100MB log file. Check out this post from Microsoft for more details on the trace flags. This database create code is straight from their post. I changed the model database’s data and log file sizes to 100MB each. Also, it’s important to note Instance File Initialization is only for data files, log files are zeroed out due to requirements for crash recovery. We’re going to see that in action in a bit…

Read the whole thing.

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The Predicate For SQL On Linux: SQLPAL

Published 2017-09-25 by Kevin Feasel

Slava Oks explains the importance of the SQL Platform Abstraction Layer:

In addition to business matters, Hal’s article highlights key drawbacks which need to be addressed around engineering as well as  product supportability for the endeavor to be successful.   The article affirms that bringing SQL Server to *nix platform is not the hardest task compare to the additional work that the journey would require.  Indeed, in order for the project to be successful the team would have to:

  • Bring other SQL Server inbox products such as SSAS along
  • Implement platform specific features such as CLR, AGs, and much more
  • Guarantee adequate performance and scalability
  • Create new ecosystem around product support, engineering systems  and more.

So throughout the course of the work, I have continued to question the path we have been on and if we succeed at the end or not.  Every time I would go back, reread the article and every time I would come to the same favorable conclusion.  But why?

Slava links to this article from December from the data platform team explaining what SQLPAL is.  In the end, I think the benefits of this model will be much larger than SQL on Linux (which is itself large).

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Transactional Replication And SQL On Linux

Published 2017-09-22 by Kevin Feasel

Phil Grayson shows a way to get transactional replication working on Linux:

Microsoft have stated that transactional replication isn’t supported on Linux and we’re not sure if they intend to in the future. This means that if you try to add the server to a publisher, you get the following message. So you can’t use the GUI and it also means that you can’t use pull as the necessary files won’t be there.

Despite that warning, there is a way to set up a push subscription; click through for that way.

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