The Myth Around 32-bit SQL Server Instances on 64-bit Operating Systems, and AWE

Argenis — Sun, 30 Dec 2012 23:36:00 GMT

(I’m taking a break from my DBA Best Practices blog series to talk about this today – the series continues after the Holidays!)

I love to be proved wrong. Really, I do.

Recently we had a discussion in an distribution list where somebody asked whether a SQL Server 32-bit instance could address more than 4Gb of RAM when running on top of a 64-bit OS. One of the really smart guys in Microsoft SQL Server Support, Karthick P.K. [Blog|Blog|Twitter] replied “sure, just add more RAM and enable AWE – SQL will use that memory”. I was much convinced that this was incorrect, so I jumped in and said that AWE does nothing under those circumstances. After all, that is what I had read in the past in many different articles. I even had some SQL Server MVPs who have been around the block forever agreeing with me.

But the good news is that I was wrong.

AWE in fact does nothing on 64-bit instances of SQL Server. But on WOW64 - Windows on Windows 64-bit, the subsystem of Windows x64 that allows you to run 32-bit processes and instances of SQL Server, AWE does allow you to address memory above 4Gb. Let me show you.

Side note: AWE is now deprecated - removed in SQL Server 2012. The last version of SQL Server that supports AWE is SQL Server 2008 R2. Because of this, a SQL Server 2012 x86 instance won’t be able to enjoy over 4Gb of memory – even if running on WOW64.

I setup a vanilla VM in my lab with a 64-bit OS with 8Gb of memory. MSINFO32 looks like this on the VM:

I installed a 32-bit SQL Server 2008 R2 SP2 instance on the VM. @@VERSION looked like this:

I did not enable AWE. I did grant the “Perform Volume Maintenance Tasks” and “Lock Pages in Memory” privileges using secpol.msc (not pictured!). I wanted to see what memory consumption would look like without AWE enabled.

This screenshot shows the permissions granted to the SQL Server Service Account (NETWORK SERVICE, in this case) – used “whoami /priv”, a quick and dirty way to get the privileges of the service account using xp_cmdshell. Ugh, xp_cmdshell. It’s okay, it’s just a test.

I went ahead and created a test database with just one big table – big enough to use all the buffer pool memory on the instance when a size-of-data operation is executed on it. I did set Max Server Memory to 6Gb.

Here’s the CREATE table script:

CREATE TABLE [dbo].[testTable](
    [id] [int] IDENTITY(1,1) NOT NULL,
    [wideColumn] [nvarchar](4000) NULL,
        PRIMARY KEY CLUSTERED 
        (
            [id] ASC
        )WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[testTable] ADD DEFAULT (replicate(N'A',(4000))) FOR [wideColumn]
GO

Next, I went ahead and inserted 1M rows on the table:

INSERT INTO [dbo].[testTable] DEFAULT VALUES
GO 1000000

If you do the math very quickly, you will agree with me that this not so little table will be more than 6Gb in size. Let’s check anyway:

Cool! The table is large enough for the purpose of this test. But how does memory utilization look like after that 1M row insert?

That’s less than 4Gb. Next I turned on AWE, and restarted the instance. Then I queried the ERRORLOG to confirm that AWE was enabled:

Okay, since I had just restarted the instance my buffer pool was gone – needed to force a size of data operation on the large table. Rebuilding the clustered index did just fine (I know, that’s probably overkill). Then I checked sys.dm_os_process_memory again:

And there we go! A lot more than 4Gb being used now that AWE is in play. I had set Max Server Memory to 6Gb, as I detailed earlier.

It looks like there’s quite a lot of incorrect documentation regarding this, so I wanted to clear out any confusion – and learn something new along the way. Don’t you love SQL Server?

Happy Holidays and Happy New Year!

-Argenis

P.S.: If you are running IA64 (Itanium) - then this doesn't apply to you. AWE does not work at all on IA64.

Argenis Fernandez : x64, AWE

The Myth Around 32-bit SQL Server Instances on 64-bit Operating Systems, and AWE