kfarlee (4/24/2012)

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While much of this article is accurate, I need to correct a few misconceptions.

I am a PM, working on the storage engine for SQL Server, specifically on access to storage devices. Over the past couple of years, I've been working with the Windows File Server team to see if there was anything we could do about the poor performance of SQL running over SMB.

As it turns out, there was. It seems that SQL had never paid attention to NAS because nobody used it, because the performance sucked. Windows didn't pay attention to workloads like SQL because again, nobody used it, so it wasn't on their radar. We got the File Server team using TPCC as one of their performance benchmarks for the file server, and some interesting things came to light.

The first was a bug which has been in the client side of the protocol stack since Windows 95! It effectively serializes all write traffic. When we started out, SMB was at ~20% of the performance (as measured by TPCC) compared to direct attached storage. By the time we were done, the performance over SMB was at 97% of the performance of direct-attach.

The great thing about this is that it is a client-side fix, which has been released as a Windows QFE to Windows Server 2008 R2, and I believe 2008 as well. Because it's client-side, you get the benefit regardless of what NAS device is on the other end of the wire.

Going beyond that, the File server team has incorporated the RDMA protocol, in which network traffic goes directly from the wire into user memory without being copied to kernel space. This dramatically reduces CPU consumption, which can suck up most of your CPU, even in iSCSI environments. Now, the CPU used by the TCP stack is comparable to Fibre Channel.

There are also a collection of significant enhancements to the protocol, moving the protocol from 1.0 to 2.2.

The Windows architects have published all of these changes, and have been sitting with all of the major NAS vendors, to make sure that they can also take advantage of these improvements.

Bottom line is that just within the past year or so, the landscape has significantly shifted, so that now it is possible to put together a NAS-based configuration which performs on par with a SAN.

Kevin Farlee

SQL Server Storage Engine PM