Dual disk redundancy


I just purchased a new Drobo S and was wondering if it’s recommend to have dual disk redundancy feature turned on? I have 2 - 1TB drives and 3 - 2TB drives so turning on dual disk redundancy does decrease my storage.

Thanks for your opinions.

dual disk redundancy always decreases your storage

its up to you - it makes your storage significantly more secure, at the cost of space

if one of your disk dies you will replace it with a new one

then drobo will being to rebuild.

the rebuild process is intensive on your disks, and if a disk dies during the rebuild you will lose everything

with dual disk redundancy it can survive a second disk failing during the rebuild.

its a question of how much do you value your data?

Well I do have all my important data stored on other drives and I’m using Drobo to back-up that data. So if either one goes I have a second copy. I am also looking at using online backup for off-site storage.

How often do you think a drive will go bad in a Drobo unit? I’ve got quite a few drives and I have not had one go bad in a few years.

Thanks for your input.


If the DroboS was my only copy of data, then I would want to use the Dual Redundancy. It is not my only copy though.
As wtih Docchris’s post, I would find the feature more important depending on the need for security.
I would want to use the feature if I had to ensure that most up-time of the Drobo, or if I had one of the 8-12 bay units.

As Docchris mentioned, the intense I/O activity during a lengthy relayout/rebuild makes it all the more probable that a second borderline disk will fail during that period.
In that case, you loose all your data since single disk redundancy can only accommodate a single disk failure. Thus the benefit of dual disk redundancy benefit.
But the space penalty is huge (at best 2/5 = 40%) so if you have additional backup you can take the risk.

One thing that I have noticed, that while the Drobo v2 will take about 24 hours per 1TB to rebuild, the DroboS seems able to to rebuild up to 4.5TB in 10-12 hours. While the rebuild may be stressful upon the disk, it is of much less duration, time-wise.

yeah but going faster is probably more stressful on the disk - more thrashing - i wouldn’t necessarily expect the failure rate to change

but yes - back to the OP - if drobo is your backup, then single disk redundancy is fine.

if ti is does lose a second disk it will be a nusiance to put your data back on it, but not impossible

OK, thanks for the info.


In my experience…

Drives do go bad.

Drives tend to go bad either very early in their life, or very late. This jives with something someone else here posted (might’ve been rdo, NeilR, or one of the other smart folks)

Drives seem to go bad more often nowadays than they did 5+ years ago.

While probability says it’s an equivalent chance of failure, statistics say the more drives you have, the more likely you are to experience an overlapping failure.

So, unless you really need the extra storage, I would enable Dual-Disk Redundancy. You can always turn it off later.

…and it is always good to have a backup, meaning a completely separate, static copy of your important data.

do you have to rebuild your data disks (and lose all data) to enable dual disk redundancy?
or do you just click on the option, push OK and it does it automatically?

just tick the option and push ok and it flashes its lights like it is rebuilding - but really it is just generating the extra data it needs (it takes as long as rebuild). all of your data is nice and safe throughout

turning dual disk off is instant

Another decision point may be performance. I have read some comments on this forum about dual disk redundancy (DDR) reducing either read and/or write rates (I think/hope writes only?) but I have not refreshed my memory lately as to the extent.

Personally I think it is a huge mistake to rely on the Drobo - or ANY other disk array - as the only copy of my data. There are too many things that can go wrong and many of those things are totally outside the control of the Drobo. So nothing I am saying is a slam against the Drobo. A Drobo may very well be the most reliable disk array available- I don’t know and can’t have an educated opinion on that. My experience is great over 2.5 years with a V2 but I never had a disk actually fail out of the array. I do burn my disks in before installing them in a Drobo and I have lost a new one during burn in.

(I would not even rely solely on professionally managed internet cloud backup services- there have been a few very publicly discussed failures and loss of data already)

I have seen enough reports here of failed Drobo arrays - often starting with a single disk failure that ends badly - to assume that losing 2 drives is common enough to be cause for concern. And with an S the odds must increase with a 5th drive that is a candidate to fail during a rebuild. Instead of 3 chances for a 2nd failure you now have 4.

I keep multiple backups of everything stored on my Drobo. I very recently ran into an opportunity to acquire an S (V1 USB 2.0 version). Although I do back it up it is not current every minute of the day and given the slowdown in performance during a rebuild I don’t think that is the right time to try to force an upgrade of backups. Plus I occasionally travel with my Drobo and then I can’t be backed up to the minute and really rely on it. So even for me there is some value in a little extra margin of safety with dual disk redundancy and I may pop for a drive to do that. My main concern is the performance issue because I don’t have a pressing need to fill the S with 3TB drives yet.

If I didn’t back up my 5 bay Drobo I would absolutely run DDR. If you’re going to roll the dice on these things, it’s best to rig the dice ever so slightly more in your favor.

I understand the OP has thought this through; I added most of the above just for general public consumption.

the performance hit is minimal, below 10% which is really noticeable in day to day usage

Thanks Chris! I’m only $69.95 away from dual disk redundant nirvana as soon as Newegg throws out a deal on drives :slight_smile:

Actually I’m also thinking about upgrading to black drives for a little better performance. The S might benefit from nicer drives. Decisions, decisions :-). And those blacks (considering replacing all of what I have to make it worthwhile) are quite expensive- about double for a marginal gain…

I think I am getting about 45MB/s read/write performance on the S and I would not expect to get the high end of what others get simply because I don’t with simple JBOD drives. At 45MB/s most or all of my problems with the V2 are solved, I think, although I just haven’t had the time to fully test everything.

I’m fairly sure that the S wont benefit from faster drives, and definitely not noticably.

the DroboPro apparenlty benchmarks MARGINALLY faster with faster drives, but they originally said not to bother using them as the real world usage showed no benefit

You saved me some $$$ Chris, I owe you a drink :slight_smile:

As soon as the 4TB drives come on the market the 3TB drives should be dog meat and I’ll really be set :slight_smile:

I’d actually like to see WD start selling Newegg OEM 3TB drives. How many RocketRaid cards does a guy need? :slight_smile:

Dual disk redundancy should ONLY be used as a tool to provide greater uptime of data and less chance of having to spend your time dealing with data restores. Using it as a protection against data loss is better than nothing, but still a poor strategy.

If the Drobo is your ONLY copy of data you can’t afford to lose, you need to get a backup strategy ASAP before Murphy pays a visit.

Exactly! Disk redundancy will not protect from file corruption or
accidental deletion.