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I've recently begun an exercise in looking into SQL best practices in terms of performance. I've read that it is recommended to put MS SQL data files (mdf) and log files (ldf) on separate volumes.

And this is how my organization has been doing it, when SQL was all on local server hard disks.

Now that the company has purchased an Equallogic SAN, I'm wondering if this still necessary. Dell support tells me that any volume created on the SAN will be spanned across all drives in the RAID set. In this case, that is 14 spindles. Dell says there is no performance advantage to separating those mdfs and ldfs since the volume will already have max I/O across all those drives. Creating two volumes isn't increasing the number of spindles in use...

Any thoughts or suggestions?

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2 Answers

If performance is such a serious problem you can make different raid groups - however you will loose capacity from the total disks.

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There are some small benefits that you can get. The biggest one is the ability to easily move the logs to another array without having to disconnect all the databases then move the files. You'd be able to simply shut down the SQL Server instance, move the files to another drive, switch the drive letters and you are done.

As you'll have multiple drive queues in Windows you could potentially see queuing benefits by having two volumes.

The place where you will see the most benefit the quickest is with your multi-pathing. As you'll be using the native MPIO driver you are limited to only using one iSCSI nic (or HBA) per drive letter at one time. That limits you to basically one 10 Gig NIC (assuming you are using 10 Gigs NICs and 10 Gig switches here). When you have multiple volumes (drive letters or mount points) each one of them picks a NIC to use as it's active. Some will share and some will pick different ones. This is all especially important if you have 1 Gig NICs or 1 Gig Network switches.

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