When RAI5 is not enough

[Howell]

How many drives would you put into a RAID5 array before you decided that the probability of more than one drive failing was too great.

HP has decided the number is 14.

And their solution to this problem is: Advanced Data Guarding

 

[Stereodude]

As many as I can get on the RAID card.

In my case I have 6 on my 3ware card. I probably wouldn't do more than 8 myself.

 

[Fushigi]

New iSeries (AS/400) machines can do anywhere from 3 to 18 drives in a single RAID5 set. Older machines, like the ones I manage, max at 10 drives/set.

 

[blakerwry]

That sounds like it would be terribly slow at writes... 2 sets of parity information instead of a single set... knowing all the read/writes that are normally done with raid5 in order to calculate and store parity info this would be bad. Well... I'm guessing that it's actually the same parity information but it's been "sliden over" 1 drive in the array to allow for multiple failures. So you would only have to do the calculation once and the wirtes of the parity information twice.

 

[Handruin]

I think howell's initial point is a very good question. The more drives you add to the RAID 5 array, the greater chance more than one drive may fail at the same time. I would chose 7 as my safe number.

 

[blakerwry]

as for a safe number.. i'd say 5 without a hot spare.. with a hot spare i see no problem for 10. If you need more redundancy get RAID 10 or 50 with spares.

 

[blakerwry]

doh, that should not be 50.... that should be 5+1

 

[Buck]

doh, that should not be 50.... that should be 5+1

Don't worry Blake, it will become 50, just 10 was originally 1+0.

 

[Buck]

Don't worry Blake, it will become 50, just like 10 was originally 1+0.

 

[Mercutio]

There used to be a distinction between 0+1 and 1+0, too.

 

[Buck]

There used to be a distinction between 0+1 and 1+0, too.

And technically there still is. Either you mirror a striped set or your stripe a mirrored set.

 

[Howell]

as for a safe number.. i'd say 5 without a hot spare.. with a hot spare i see no problem for 10. If you need more redundancy get RAID 10 or 50 with spares.

The article talks about their solution being comfortable up to 56 drives and able to take two lost drives with no data loss.

 

[Howell]

There used to be a distinction between 0+1 and 1+0, too.

That's like saying there used to be a distinction between something and something else that's fundamentally different.