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SQL Server, SANs and Virtualisation


SQL Server, SANs and Virtualisation

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Perry Whittle
Perry Whittle
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Comments posted to this topic are about the item SQL Server, SANs and Virtualisation

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Jonathan Kehayias
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Good job covering an important topic when it comes to good configuration of your hardware layer for a solid ESX implementation.

Now prepare for the flame war over Virtual vs Physical and SQL Server. It is bound to happen given your topic. Wink

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Jonathan Kehayias
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One thing that I just thought of that you didn't mention in your article is that you can also use your redundant paths to the SAN to balance your IO by setting the preferred path for specific LUN's to the secondary path in the event that you have one path become saturated. This can be done in the Virtual Infrastructure manager, and you can use putty sessions to the ESX host to monitor the true IO metrics for the HBA's to know if they are queuing commands heavily or not, so the old adage that you can't track down bottlenecks in a virtual environment no longer apply as they used to.

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henrik staun poulsen
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Perry,

Most people or consultants that I've spoken to has mentioned that it is important to have 64 kb sectors for SQL Server.

How is this athieved on SANs?

They also claim that the sectors must be "alligned".
What is this, and how does it affect speed?

How do we check the above statements?

TIA
Henrik Staun Poulsen
Stovi Software



magarity
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The most important thing to have when using a SAN is a good SAN administrator. The things are convoluted pieces of equipment with a lot of options and while they work with only a little setup out of the box, tweaking it properly (especially for database servers!) is NOT something one can pick up from the Quick Start installation guide. I've had to DBA a SQL Server hooked up to a SAN when there was a mediocre admin and when there was a great admin. The admin's technical expertise makes 10x the difference in your performance on the exact same SAN hardware.
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Nice Article,

One point of note however; your article seems to focus strictly on Fibre, is this because SQL Server is best suited to Fibre? what about SANS built on ISCSI (equallogic etc)? do these have the same capabilities?

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henrik staun poulsen
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Hi mararity,

How do you tell the difference between mediocre and great Admin?
We have two people working with the SANs, but are they any good???

I would like to be able to verify that we get the best performance that is likely possible, but how?

Best regards
Henrik Staun Poulsen



kucerarj
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Both Fibre Channel and iSCSI use SCSI commands, the only difference is the media. There are a few more tasks that need to be done with FC vs iSCSI but the basic rules still apply. Currently FC connections have 4x more bandwidth than typical iSCSI (4Gb vs 1Gb). But 10Gb ethernet is starting to roll out. 8Gb FC is starting to come out as well. There is a little more overhead with iSCSI vs FC so iSCSI adds about 1-3ms overhead on latency. Another technology that is starting to roll out is FCOE (Fibre Channel over Ethernet). It's Fibre Channel protocol over ethernet media. It should provide the cost benefits of ethernet without the overhead of iSCSI.



Jonathan Kehayias
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henrik staun poulsen (1/8/2009)
Hi mararity,

How do you tell the difference between mediocre and great Admin?
We have two people working with the SANs, but are they any good???

I would like to be able to verify that we get the best performance that is likely possible, but how?

Best regards
Henrik Staun Poulsen


Herik,

You could have a great SAN admin who just doesn't know enough about SQL workloads and have performance issues. Keep in mind that SAN's can be dedicated to SQL, but often are used for enterprise storage across multiple technologies, so they are often built and configured for the other things not SQL in the environment.

SQLIO is a benchmarking tool released by Microsoft that you can use to stress your IO subsystem and test various configuration changes to see the impact they have on different types of IO. This shouldn't be used on a active production SQL Server, but in the planning stages of a new implementation where you can reconstruct the LUN's in various configurations of stripe and block sizes and RAID configurations to find the best fit configuration for SQL in your environment. Other things that make a difference is having the correct offset in the Windows OS so that your drives are aligned properly and can make the most efficient use of the disk subsystem. There is a post in the Administration forum by Kendal van Dyke who has been playing with this topic recently that contains some more information.

http://www.sqlservercentral.com/Forums/Topic624961-146-1.aspx

Some other references on the subject are:

http://blogs.msdn.com/jimmymay/archive/2008/12/04/disk-partition-alignment-sector-alignment-for-sql-server-part-4-essentials-cheat-sheet.aspx
http://sqlblog.com/blogs/linchi_shea/archive/2007/02/01/performance-impact-of-disk-misalignment.aspx

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Conan Whalen-McKain-370433
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I was hoping to recieve from your article a more precise breakdown of how you are making virtualization work. I have been the guy trying to use VMWare server to make it work and just found it to not handle at all well. Could you perhaps discuss more precisely how this works and what the product core is you are using?
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