Monthly Archives: November 2016

SQL Server on Linux – How I think they did it!

OK, so everyone wants to know how Microsoft did it…how they got SQL Server running on Linux. In this article, I’m going to try to figure out how.

Update: Since the publication of this post, Microsoft has published a blog post detailing the implementation here

There’s a couple of approaches they could take…a direct port or some abstraction layer…A direct port would have been hard, basically any OS interaction would have had to been looked at and that would have been time consuming and risk prone. Who comes along to save the day? Abstraction. The word you hear about a million times when you take Operating Systems classes in undergrad and grad computer science courses. :) 

Well things are finally starting to come to light on how it was done. I had a Twitter conversation this weekend with Slava Oks, who is a leader on the project team and several other very active people in the SQL Community Klaus AschenbrennerEwald Cress, and Lonny Niederstadt. This got my gears turning…to find out…how they did it!

What do we know so far?

So here’s what we know, there’s some level of abstraction going on using a layer called SQL Platform Abstraction Layer (SQLPAL) and also some directly ported code via SQLOSv2. From a design standpoint this it a pretty good compromise. Check out Figure 1, here you can see SQLPAL sits between the Database Engine and the underlying operating system. Whichever one it may be, Windows, Linux and oh yeah “other OS in Future” :)

SQLServer on Linux

Figure 1 – SQL Server on Linux – source @SQLRockstar

Background information

So to understand how we got here, it’s worth looking at the Drawbridge project from Microsoft Research. Drawbridge is basically application, or more specifically, process virtualization with a contained OS inside that process space. This is called a picoprocess. Since the process is abstracted away from the underlying operating system, the process will need some part of an OS inside its address space. This is called the Library OS. With that abstracted away…each process has a consistent view of it’s own operating environment. In figure 2, you can see the Library OS and it’s roots into ntoskrnl.dll, which is an NT user-mode kernel. This provides a consistent OS interface for the application. Essentially program code doesn’t need to change.

Now it’s up to the picoprocess as a whole to provide some abstraction back to the actual operating system and that’s where the Platform Abstraction Layer (PAL) comes in. All that’s left is to provide an application binary interface for the picoprocess and you have a completely self-contained process without the need to interact directly the host operating system. This is amazing stuff!


Figure 2 – Drawbridge Architecture – Source MS Research


SQLPAL – SQL Server Platform Abstraction Layer

So, I wanted to see this in action. In the Windows world, hard core SQL people are familiar with attaching a debugger to a SQL process and loading debug symbols to get a view into what’s going on inside of SQL Server. Well in Linux, we can do the same, and it’s a LOT easier. On Linux, there’s a tool called strace, which will give you a view into your programs execution and any interactions it has with the OS. So I launched SQL Server and strace and here’s what I found.

So to launch strace and SQL Server, we add the SQL Server binary as a parameter to strace. Caution, do not do this as root as it may cause a permission issue with log files generated by the sqlservr process. Use sudo to change to the msssql user.

[mssql@rhel1 ~]$ strace /opt/mssql/bin/sqlservr

The first thing you’ll see is a call to execve, which is a LINUX system call to start a new process. A regular old Linux process. So that means that sqlservr is a program binary compiled for Linux.

execve(“/opt/mssql/bin/sqlservr”, [“/opt/mssql/bin/sqlservr”], [/* 24 vars */]) = 0

At this point we see it loading all the local natively compiled libraries required for the program. Here’s one example, open is a system call to open a file, subsequent reads will occur when needed. There are many more libraries loaded.

open(“/lib64/”, O_RDONLY|O_CLOEXEC) = 3

Now, we see something interesting, a load of a library called This file is in the /opt/mssql/lib/ directory and is shipped in the SQL Server package. So my guess is that this is the application binary interface for SQL Server’s picoprocess.

open(“/opt/mssql/bin/../lib/”, O_RDONLY|O_CLOEXEC) = 3

Now we see a transition into Drawbridge like functionality, with the system.sfp open. This looks like it’s responsible for setting up the OS like substrate for the application’s execution environment. 

open(“/opt/mssql/lib/system.sfp”, O_RDONLY) = 3

During the load of system.sfp, we see several libraries, registry and DLL loads that look like they’re responsible for setting up the kernel level abstraction.

pread(3, “Win8.dbmanifest\0”, 16, 4704) = 16

Reading in the registry? Man that’s never going away :)

pread(3, “\0”, 12, 4753)     = 12

Reading in the NtOsKrn.dll, the NT user-mode kernel

pread(3, “NtOsKrnl.dll\0”, 13, 5123)    = 13

Next SFP we see load is system.common.sfp. This looks to be a second stage boot process, perhaps Drawbridge’s library OS? 

open(“/opt/mssql/lib/system.common.sfp”, O_RDONLY) = 4

During this phase we see many other DLLs loading. Looks like we’re setting up an environment…here’s an example of something loaded at this time. Clearly higher level OS provided functionality.

pread(4, “kerberos.dll\0”, 13, 15055)   = 13

After a few more SFP files are opened for certificates and NetFX4, and then we end up at sqlservr.sfp. And inside here, it loads things familiar to deep dive SQL Server pros…first we see the program binary load sqlservr.exe, SqlDK.dll, sqllang.dll, SQLOS.dll, and sqlmin.dll. I omitted some output for readability.

open(“/opt/mssql/lib/sqlservr.sfp”, O_RDONLY) = 7


pread(7, “sqlservr.exe\0”, 13, 13398)   = 13


pread(7, “SqlDK.dll\0”, 10, 14079)      = 10


pread(7, “sqllang.dll\0″, 12, 14382)    = 12


pread(7, “SQLOS.dll\0”, 10, 14418)      = 10


pread(7, “sqlmin.dll\0”, 11, 14511)     = 11

And finally, we end up with application output, something we’ve all seen…SQL Server starting up.

nanosleep({999999999, 0}, 2016-11-17 14:11:37.53 Server      Microsoft SQL Server vNext (CTP1) – (X64) 

Nov  1 2016 23:24:39 

Copyright (c) Microsoft Corporation

on Linux (Red Hat Enterprise Linux)

Oh, and now it makes much more sense why SQL Server on Linux is using Windows like file pathing inside the application, right? Well, think it through, SQL Server is interacting with an operating system that it thinks is still Windows, via the platform abstraction layer.

2016-11-17 14:11:37.53 Server      Logging SQL Server messages in file ‘C:\var\opt\mssql\log\errorlog’.


So in that Twitter conversation I had with Slava and others, we learned it’s not straight PAL, but a SQL Server specific PAL. This allows the product team to provide another path to the underlying OS for performance sensitive code. Look back at figure 1, you’ll see two paths from SQL Sever into SQLPAL. One uses the Win32 APIs, likely provided by Drawbridge (or some variant), and the other is perhaps natively compiled code…really that’s just a guess on my part. 

Final thoughts

All, this is a pretty awesome time we’re getting into…Microsoft embracing Linux, SQL on Linux, PowerShell on Linux. I’ve said this many times…Windows, Linux…it’s just an OS. I would like to thank Slava for his insight and also the product team for a fantastic preview release. It’s amazing how seamless this really is.

In a sidebar conversation with Ewald, he made the point that as SQL Server professionals that our investment in the understanding of SQL Server’s internals will persist with this implementation. Which I think is a huge relief for those that have invested years into understanding it’s internals! 

Please leave some comments on what your thoughts are on how this works. If you want to contact me directly, you can reach me at or @nocentino



Well, if you made it this far…awesome! I want you to know, I don’t have any inside knowledge of how this was developed. I basically sat down and traced the code with the techniques I showed here.  


Building Open Source PowerShell

Open Source PowerShell is available on several operating systems, that really what’s special about the whole project! To get PowerShell to function on these various systems we need to build (compile) the software in that environment. This is what will produce the actual executable program that is powershell.

To facilitate the build process the PowerShell team has documented how to do this for the currently available platforms, Linux, MacOS and Windows. In this post I want to talk about why this is important, point you to the resources available online to help you build Open Source PowerShell and tell you my experiences building PowerShell on the Windows, macOS and Linux!

Why would one want to build PowerShell?

Well for me, I’m and internals geek and I want to be able to debug running PowerShell code so I can follow the flow of control during program execution. This will enable me to learn the internals of certain commands. A great way to see what’s happening on the inside.

Another reason is perhaps you want to contribute, you yourself can download the code…make a change and submit it to the PowerShell team for review. Pretty cool stuff at the “New Microsoft”. Following these steps you’ll be able to have a functioning environment to develop in.

Getting Started with building PowerShell

In general building complex software projects is not a trivial task, but the PowerShell team has done an exceptional job making this as easy as possible for everyone. The documented build processes leverage PowerShell scripts for installing the appropriate dependencies on your system and then managing the build process itself. At a high level, it’s really five easy steps. For more details on building for your platform, check out the links at the bottom of this post.

  1. Download the code from GitHub
  2. Install PowerShell
  3. Import the build module – build.psm1
  4. Install the build dependencies (toolchain setup) with Start-PSBootStrap
  5. Build PowerShell with Start-PSBootBuild (once this is finished, you’ll have a powershell executable)

My notes on building PowerShell

  • On the Linux side of things, it was VERY easy. The PowerShell team includes installation of all build dependencies and package installation for things like make and g++ inside Start-PSBootStrap then built powershell with Start-PSBootBuild
  • Well Windows was pretty easy too, but I had to install a few things manually. First I installed Visual Studio 2015, added the required C++ components, installed Chocolatey, installed cmake, downloaded the PowerShell source, ran Start-PSBootStrap to get the build dependencies, then built PowerShell with  Start-PSBootBuild
  • macOS was a little tougher, updated my installation of XCode, I installed Brew, installed cmake, downloaded the source, ran Start-PSBootStrap to get the build dependencies, then built PowerShell with Start-PSBootBuild. It failed with this error (which has since been corrected)
    •       deprecated in macOS 10.12 – syscall(2) is unsupported; please switch to a supported interface. For SYS_kdebug_trace use kdebug_signpost().


          tid = syscall(SYS_thread_selfid);


      /usr/include/unistd.h:733:6: note: ‘syscall’ has been explicitly marked deprecated here

      int      syscall(int, …);


    • Basically what’s going on here is there’s a deprecated system call on macOS 10.12 which causes the completion to fail. To get the build to work, I changed the function to just return 0. Doing this will likely break something, so I’m not suggesting you do this. I just did this to get the build to work.  I’ve submitted this issue to the PowerShell team via GitHub here

What’s next?

Well, now that I’m able to get Open Source PowerShell built on three major operating systems I’m going to take some time using debugging techniques on each to see what’s going on under the hood inside of PowerShell when I execute certain commands. And of course, up first…Get-Process ;)

Resources for building Open Source PowerShell 

Here are some resources for you to get started working with the PowerShell Projects.