How many LUN/virtual disks?

[Morten Kristiansen]

Hi,

We have installed a EVA-5000 together with a HP-UX 11.11. If I want to have a partition of 100 GB (or more), should I make one single virtual disk on the EVA of 100MB? Or should I make two of 50 or four of 25, striping those in lvm to 100 MB?

On the EVA I'm running RAID 5. And yes I know that RAID 5 is also striping and that EVA stripes over all disks in the diskgroup. But I've heard that it's better for the OS to stripe on more virtual disk than one single disk. Even if the disks are on a SAN.

Is that true? And can someone explain me why?

[Mark Grant · Accepted solution]

The single most important thing to bear in mind, in my view, is that you have a limit of 256 Luns per host. In a service guard cluster, this, in effect, means, 256 luns for the entire cluster. It is therefore possible to run out luns relatively easily. Also, as the operating system can not cope without resizing luns on the fly, this means that, in effect, you can no longer resize filesystems online without a lot of jumping through hoops and therefore you need plenty of space to start with.

The only way to reduce this problem is to use as few luns as possible and to size them correctly to start with. In my view, this means that any marginal performance issues/gains with regard to LVM and the EVA are out gunned by the need for the least number of luns. I would therefore suggest you great a single 100GB vdisk.

I would add that everything I have heard from the HP guys who have come around to sell us the EVA solution suggests that the LVM striping on top of raid 5 is not a particularly useful thing to do.

[Johan Lorimier]

Hi,
iF you have only one fc controler, it's worth to make only one big LUN.
In in the case of several controler and using PVlink, you can use more Lun to split the IO the differents controlers.

Johan