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Besides the obvious things like:

* battery backed
* cache memory

Are there any other specifications you should look for when choosing a up-to-date raid controller? Read something eg about

* NCQ - Native Command Queueing
* Ability to replace the controller without loosing the raid-config and the data that way



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up vote 1 down vote accepted

Firstly the most important thing is to make sure it's on the vSphere HCL.

Secondly is this the controller that will manage just your boot disks or the disks that will hold your actual VMs? You mention NCQ, suggesting you're looking at SATA disks, if it's just for boot disk then you'll be fine with these but if it's to hold your actual VM files then I'd strongly urge you to go for a SCSI/SAS-based disk controller and disks solution. ESX uses scsi device locking heavily to manage disks and that function's absence on SATA disks forces it to be significantly less efficient, lowering your performance.

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Just wondering: Is the locking per iSCSI adapter (ESX), per controller, per LUN, ...? If it is per LUN then creating more LUN woul be advisable? – ToreTrygg Feb 15 '10 at 13:17
it's on a LUN-by-LUN basis, and yes you're right - think of it this way, the more .vmdk arbitration caused by having more than one VM per LUN the lower the performance. I tend to stick to a '4 VMs per LUN' ratio, but I'm happy for this to grow a little on production systems and a lot on non-production system, think I have 12 in some reference LUNs. The key point is that although ESX 4.0U1 can use SATA disks for VMFS volumes it's very new and really not advised, go for a SAS/SCSI controller and disks or FC/iSCSI/NFS over SATA ok ;) – Chopper3 Feb 15 '10 at 13:29

Most important thing with a RAID controller (or any other hardware for that matter), IMO, is that it's certified to work with and is supported with the rest of your hardware. Otherwise you risk finding yourself in the old each-vendor-blaming-the-other tangle. Look for this first and worry about other specs after.

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