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SQL and Exchange DBs and Logs on SAN

Posted on 2008-10-16
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Hello, I've got a question re. disk performance for SQL and Exchange running on a SAN.

With a SAN with a fair amount (16GB) of cache, and using 4Gb Fibre links, is it still necessary to split the Logs files and Databases over different arrays like it used to be with local storage?

I realise its probably down to the amount of traffic the servers are processing, but I was just wondering from a 'rule of thumb' point of view.

Many thanks.
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Question by:paulo999
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jkarnes12 earned 64 total points
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You always want to keep Database and log files on separate disks due to I/O considerations.  If they are on the same RAID physical disk configurations there will be I/O contentions that will degrade performance dramatically.
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by:MohammedU
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SAN should have different configuration...
Make sure LUNs used datafiles and logfiles from different filegroup set other even you change the location you have the problem.
Check with your SAN adminstrator about SAN configuration.
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by:Duncan Meyers
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As a rule, database and logs should be put on different physical discs as they present a competing I/O load. The reality in many cases, though, is that a smaller Exchange environment (<500 users) does not generate that much I/O - despite Microsoft's sizing calculators - and the logs and databse *could* go on the same physical discs if you were pressed for space. It sounds like you have a pretty chunky storage array if you've got 16GB of cache whcih suggests that you support a large number of users - so I'd stick with best practices and keep logs and database separated if you can. Note that if your database and Exchange servers are virtualised, then things change. A virtualised environment presents a highly random I/O load, so you need to be very careful with sizing calculations.
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by:andyalder
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If you read http://chucksblog.typepad.com/chucks_blog/2008/08/your-storage-mi.html and the followup from the other vendors you'll see it's a minefield. One of the HP replies on their blog was to have 2 storage groups for 2 exchange servers with the logs for server 1 on the same disk group as the data disks for server 2 and vice versa - http://www.communities.hp.com/online/blogs/datastorage/archive/2008/08/29/emc-distortion-about-capacity-efficiency.aspx .

So, according to the big boys it *is* acceptable to have logs on the same disks as data. I tend to agree and where possible put everything on one set of disks unless data separation of logs for data security is required. There is of course contention for the disks but on the flip side there are more disks available for each individual application.

Look at SAME from Oracle for another example, admittedly it's just one database but a complex one with varying I/O profiles and they put everything on a single disk set (although they keep the redo logs seperate for point in time DR restore purposses) - www.oracle.com/technology/deploy/availability/pdf/oow2000_same.pdf
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