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EVA vraid1; redundancy

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Brad Kozak
Valued Contributor

EVA vraid1; redundancy

We're in the process of installing a new system with an EVA4000 as its' storage system. I just have a few questions:

1. Our vendor says that the EVA gives you 30% redundancy; that you could lose 30% of your drives and still be up and running. All of our virtual disks are VRaid1. If I look at the properties of our disk groups, the "Disk drive failure protection" actual level is set to "None". I'm asking myself how can that be? Does the EVA provide redundancy within itself which might explain this 30% drive loss with no interruption.

2. Which brings me to VRaid1. Everything I know about RAID1 (mirroring) is that data on one disk is duplicated onto another separate disk. So, if we lost a mirrored pair, we'd be hosed. If that's the case then how can the EVA continue to run if we did lose a mirrored pair? Is there some other technology (raid 5) working behind the scenes when you select VRaid1 for a virtual disk? Are there any HP docs around which talk about each level of VRaid in technical detail?; i.e. what's *actually* going on behind the scenes for each VRaid level.


Brad Kozak.

Uwe Zessin
Honored Contributor

Re: EVA vraid1; redundancy

The "protection level", a characteristic of a disk group, is only a space reservation, which is used for recovery in case of a disk drive failure. Just think of it as a set of spare disks that is flipped by 90 degrees and distributed over all disk drive in the disk group. It does not really protect any data. If you create a VRAID-0 virtual disk and you loose a disk drive, no setting of the "protection level" can prevent data loss.

For a VRAID-1 virtual disk, the data chunks are stored on two different physical disk drives - the EVA even tries to distribute the mirror over two different disk drive enclosures. If you loose both chunks, you have lost data - there is no additional protection in the background.

That covers VRAID-0 and 1. For VRAID-5, the EVA takes 4 data chunks, calculates the parity chunk and stores the result on 5 different physical disk drives. If two disk drives which are used by this stripe fail, gess what? You have lost data! Again, no setting of the "protection level" can prevent data loss.

So the EVA works very similar to a traditional RAID storage array - it cannot do wonders eiher ;-)