mixing 146 GB and 300 GB disks in the same disk group

[Paul Henderson_2]

Hello Folks,
I just got in a fifth shelf of storage for my EVA5000. The current four shelves have 146GB disks, but this new shelf has 300GB disks.

Ideally, I'd just like to add this fifth shelf of storage to my current "Default Disk Group", but was told by the HP engineer that if I did so, the 300GB disks would only be used up to the 146GB level, effectively rendering half the 300GB disk unusable.

I find that amazing, to put it lightly.

Note: I am using a "Disk Failure Protection" level of "Double", and am using vRAID5 (stripe-ing but not mirroring).

I'd appreciate any guidance from you gurus about this. I'd hate to lose half my disks.

The fallback is to create a new disk group, I suppose, just for the 300GB disks, but I'd prefer not to.

Thanks!

[Mark Poeschl_2 · Accepted solution]

There's a very good discussion of the pros and cons of the decision you're making in the EVA Best Practices White Paper at:

http://h20000.www2.hp.com/bc/docs/support/SupportManual/lpg29448/lpg29448.pdf

Bottom line is that unless you have a specific plan to manage the new disk group as handling all your less critical (from a performance standpoint) data, you should just go ahead and stay with a single disk group.

I'm not sure what the HP engineer was trying to tell you, but from a capacity point of view what you will get with a single mixed-capacity disk group at "Double" failure protection level is 4x300 GB set aside as spare capacity vs. 4x146 GB in your current configuration. Thus if you add a fully populated shelf of 300 GB drives to your disk group you will see the group's raw capacity increase by .93 x (10x300 + 4x146) GB. This formula takes into account the 7%H/W "tax" discussed in other posts in this forum.

[Susan Carleton]

I also am confused. My experience with the EVAs would say that you will NOT lose half your drive size on the 300GBs. You will lose out (somewhat) as mentioned above in that your sparing level is set at Double, which grabs four times the largest physical drive size worth of space - or 1.2TB in your case.

I would HIGHLY recommend that you take some time and rearrange your drives slightly, such that the 300GB new drives are vertically striped next to the 146GB drives, such that all of your 300GB drives are not on the same shelf. Considering that you will have twice as much data on that shelf, as VRAID stripes across the same percentage of the drive, you may end up with hotspots otherwise.

[Uwe Zessin]

The HP engineer must have confused the EVA with some other storage array...

EVA's disk leveling will make sure that all disk drives within a disk group have the same percentual amount of data loaded. I've analyzed a few arrays and it is usually within 1-2% max. difference.

It will work, but each bigger disk will have more data on it and thus draw more I/Os.


The other downside (a lot of space goes away due to double distributed sparing) has already been explained.