Q

The art of adding LUNs

I want to install an OS/paging file on the SAN and boot the system of the SAN. I created a RAID1 (73gb) group. To achieve good performance how many LUNs can I create in the RAID1 group for the OS?

NT likes 4GB volumes for the C: drive for boot volumes in the SAN. I have had problems with partition sizes over 4GB when booting into the SAN. If you will be partitioning a single RAID1 set for the boot partitions, make sure you spread the I/O load over multiple storage ports for access to the LUN. I would use the standard 7:1 ratio for the fan in ratio on the storage ports. To answer the question on how many servers should have access to the same physical spindles for boot disks and page files, would depend on how much memory was in each server. Remember, that if you want to be able to create crash dump files, you need enough disk space for the amount of memory in each server.

Also, if you have too little memory in your servers and your servers start thrashing (swapping out to pagefile to much), you could overload the boot spindles. There is no hard and fast answer to this question, and the answer is the standard consulting answer, "It depends."

Start off with four servers per mirrorset. If things still look OK, add one more server till you see an impact then stop there. By the way, you will need to test this during peak hours to find out if everything is still humming along.

Chris

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This was first published in January 2003

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