Dan Harris wrote:


On Aug 9, 2005, at 3:51 PM, John A Meinel wrote:

Dan Harris wrote:

On Aug 10, 2005, at 12:49 AM, Steve Poe wrote:

Dan,

Do you mean you did RAID 1 + 0 (RAID 10) or RAID 0 + 1? Just a
clarification, since RAID 0 is still a single-point of failure  even if
RAID1 is on top of RAID0.

Well, you tell me if I stated incorrectly. There are two raid enclosures with 7 drives in each. Each is on its own bus on a dual- channel controller. Each box has a stripe across its drives and the enclosures are mirrors of each other. I understand the controller could be a single point of failure, but I'm not sure I understand your concern about the RAID structure itself.


In this configuration, if you have a drive fail on both controllers, the entire RAID dies. Lets label them A1-7, B1-7, because you stripe within a set, if a single one of A dies, and a single one of B dies, you have lost your entire mirror.

The correct way of doing it, is to have A1 be a mirror of B1, and then stripe above that. Since you are using 2 7-disk enclosures, I'm not sure how you can do it well, since it is not an even number of disks. Though if you are using software RAID, there should be no problem.

The difference is that in this scenario, *all* of the A drives can die, and you haven't lost any data. The only thing you can't lose is a matched pair (eg losing both A1 and B1 will cause complete data loss)

I believe the correct notation for this last form is RAID 1 + 0 (RAID10) since you have a set of RAID1 drives, with a RAID0 on-top of them.


I have read up on the difference now. I don't understand why it's a "single point of failure". Technically any array could be a "single point" depending on your level of abstraction. In retrospect, I probably should have gone 8 drives in each and used RAID 10 instead for the better fault-tolerance, but it's online now and will require some planning to see if I want to reconfigure that in the future. I wish HP's engineer would have promoted that method instead of 0+1..

I wouldn't say that it is a single point of failure, but I *can* say that it is much more likely to fail. (2 drives rather than on average n drives)

If your devices will hold 8 drives, you could simply do 1 8-drive, and one 6-drive. And then do RAID1 with pairs, and RAID0 across the resultant 7 RAID1 sets.

I'm really surprised that someone promoted RAID 0+1 over RAID10. I think I've heard that there is a possible slight performance improvement, but really the failure mode makes it a poor tradeoff.

John
=:->


-Dan

Attachment: signature.asc
Description: OpenPGP digital signature

Reply via email to