Re: Ask the Expert: Performance Calculations on Clariion/VNX
As a rule of thumb it's an understandable rule, but if you really want to be sure, you should calculate what you need!
Suppose you have a Database server that has 100% writes on the log disk and 85R/15W on the database drive, but the amount of write IOps to the log drive is only 100 as seen on the host. Now it doesn't matter whether or not you use RAID10 or RAID5, because a 2 drive RAID10 (or RAID1) on 15 RPM can handle 2 x 180 IOps, so the 200 host write IOps (2 x 100 because of the WP=2) is no problem at all. On 4RAID5 (4+1) the RG can deliver 5 x 180 = 900 IOps while only 200 is needed. So RAID5 is also not a problem if you have IOps to spare. If all IOps of a RG are already accounted for and you have some space left, do not think you can safely hand this out to new servers as your calculations say that you don't have any IOps left, so eventually you will get some performance problem somewhere.
The consideration I can think of which you are thinking of is that you might have a dedicated RAID10 RG of several disks (for example 6+6) which is only used for logs of more than 1 server. Add up all the required host write IOps and see if your RAID10 RG can handle that and if the space is sufficient. If you have a dedicated pool with RAID10 RGs especially for log drives I think you're safe, because you can easily expand the pool to provide more space or IOps. With the new Flare 32 the existing LUNs will even be redistributed accross all drives after the addition of new drives.
Calculation is always the best method, but if you have lots of servers and the amount of space for the logs and the IOps they need are in balance (you don't have room left when the IOps are all accounted for) you just might have a best practice for your company to do what you do now.