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Understanding File System IO, Lessons Learned From SQL Server

I do more than just SQL Server. I enjoy programming. In my former life I have worked with C/C++ and Assembler. As I spent more and more time with SQL Server my programming took a back seat career wise. Having that background though really helps me day in and day out understanding why SQL Server does some of the things it does at the system level.

Fast forward several years and I’ve moved away from C/C++ and spent the last few years learning C#.

Now that I work mostly in C# I do look up solutions for my C# dilemmas on sites like http://www.codeplex.com and http://www.codeproject.com. I love the internet for this very reason, hit a road block do a search and let the collective knowledge of others speed you on your way. But, it can be a trap if you don’t do your own homework.

I write mostly command line or service based tools these days not having any real talent for GUI’s to speak of. Being a person obsessed with performance I build these things to be multi-threaded, especially with today’s computers having multiple cores and hyper threading it just makes since to take advantage of the processing power. This is all fine and dandy until you want to have multiple threads access a single file and all your threads hang out waiting for access.

So, I do what I always do, ask by best friend Google what the heck is going on. As usual, he gave me several quality links and everything pointed to the underlying file not being set in asynchronous mode. Now having done a lot of C++ I knew about asynchronous IO, buffered and un-buffered. I could have made unmanaged code calls to open or create the file and pass the safe handle back, but just like it sounds it is kind of a pain to setup and if you are going down that path you might as well code it all up in C++ anyway.

Doing a little reading on MSDN I found all the little bits I needed to set everything to rights. I set up everything to do asynchronous IO and I started my test run again. It ran just like it had before slow and painful. Again, I had Mr. Google go out and look for a solution for me, sometimes being lazy is a bad thing, and he came back with several hits where people had also had similar issues. I knew I wasn’t the only one! The general solution? Something I consider very, very .Net, use a background thread and a delegate to keep the file access from halting your main thread, so your app “feels” responsive. It is still doing synchronous IO. Your main thread goes along but all file access is still bottle-necked on a single reader/writer thread. Sure, it solves the issue of program “freezing” up on file access but doesn’t really solve the problem of slow file access that I am really trying to fix.

I know that SQL Server uses asynchronous un-buffered IO to get performance from the file system. I did some refresh reading on the MSDN site again and struck gold. Writes to the file system may OR may not be asynchronous depending on several factors. One of which is, if the file must be extended everything goes back to synchronous IO while it extends the file. Well, since I was working with a filestream and a newly created file every time I was pretty much guaranteeing that I would be synchronous no matter what. At this point I dropped back to C++. I started to code it up when I realized I was doing things differently in my C++ version.

I was manually creating the file and doing an initial allocation growing it out to the size the file buffer and the file length on close if need be.

I started up my C++ version of the code and watched all the IO calls using Sysinternal’s Process Monitor. I watched my C++ version, and lo, it was doing asynchronous IO in the very beginning then switching to synchronous IO as the file started growing. I fired up my instance of SQL Server and watched as the asynchronous IO trucked right along…. until a file growth happened and everything went synchronous for the duration of the growth.


So, taking that little extra knowledge I manually created my file in C# set an initial default size and wouldn’t you know asynchronous IO kicked right in until it had to grow the file. I had to do a little extra coding watching for how much free space was in the file when I get close I now pause any IO,  manually the file by some amount and then start up the writes again keeping things from going into a synchronous mode without me knowing.

So, there you go my little adventure and how my old skills combined with knowing how  SQL Server works helped me solve this problem. Never assume that your new skills and old skills won’t overlap.