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We have recently acquired 3 Drobo and tried setting them up, each of them contain 4x2TB HDD, but it only shows 5,44TB available, probably because of their beyondRaid technology, this lead to 2 questions :

1) How can I know which disk has the backup? If I want to take it with me each time I leave the office, can I remove 1 ( or 2?) disk each night?

2) Also, if there a way to disable the beyondRaid, and instead activate a Mac mirroring between the disk inside the drobo?

3) If one of the 3 disk fail, I just have to remove it, place another, and the mirror will extract itself to the failed disk right?

What we aim to do is to have Disk1, Disk1A, Disk2, Disk2A in each Drobo, VersionA would be a mirror of the preceding HDD.

Thanks

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It doesn't quite work like that.

1) When you have more than 2 disks in the Drobo, it doesn't just create a mirror/backup of your data. It is helpful to think of it similarly to RAID5. Basically, each chunk of data is split amongst 3 of your 4 drives. Drobo "does math" and calculates a parity bit for each chunk and stores this on the 4th drive. Every time you store data, the "4th drive" changes.

So, there is no actual backup disk. The parity information is split evenly amongst all the disks.

2) There is no way to disable BeyondRaid. The Drobo doesn't present the individual disks to the OS. As far as your Mac is concerned, you just have a single Very Large external hard disk connected.

3) You are mostly right, if one of the 4 disks fails, the green light next to it will turn red. You just pop it out, and put a new one in. You can do this while it is running even continue using the Drobo while it rebuilds.

Unfortunately, it is not possible to setup the drives in a D1, D1A, D2, D2A scheme. That would be similar to RAID1/0 and Drobo functions similarly to RAID5. One of great things about Drobo is that it is Simple. It just works, you plug in drives, and it manages them however it thinks is best. This does lead to some fairly inflexible configurations. There are other NAS devices that perform the same functions as Drobo but allow greater flexibility. But, I found them to either be more expensive or require a bit more time to setup/configure.

I'm sure you've seen their BeyondRaid description page, but if not: http://drobo.com/resources/beyondraid.php

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So if I understand well, there is no way to have an offsite backup with Drobo, except than copying it manually on another drive? –  Dominique Dec 6 '10 at 14:02
    
That is correct. Drobo (and most NAS?) aren't intended to function that way. RAID is only intended to provide redundancy to protect against disk failures (And also speed in certain configurations). It is not intended as a backup solution. –  minamhere Dec 6 '10 at 14:15
    
However, you could set up one of your Drobos at an offsite location and use software to sync their contents. Something like rsync or I have used CrashPlan Pro to automate the offsite process. You could also perform this locally between 2 Drobos. In this case, you could just physically remove ALL the drives from a single Drobo and take them offsite. As long as you keep all the drives from the Pack together, Drobo should be able to put them together when you plug them all back in at the same time. –  minamhere Dec 6 '10 at 14:19
    
And one last comment. Drobo has a new, much more expensive, version. The DroboPro FS has a new feature called DroboSync( drobo.com/products/drobo_sync.php ). It allows you to schedule automatic offsite replications to a second DroboPro FS. The DroboPro and DroboPro FS hold 8 drives and also allow a double parity configuration (similar to RAID 6), that would permit 2 drive failures at the same time without data loss. –  minamhere Dec 6 '10 at 14:22
    
Sadly we can't rely on automatic offsite Sync as we have way too much data for our bandwidth. –  Dominique Dec 6 '10 at 15:30

The Drobo units implement a variation of RAID5 in their BeyondRaid setup. You lose one disk worth of storage space to redundancy. This redundancy is spread across the disks in a non-uniform manor. There is no one disk used for redundancy information, and even if there was, you need more than one disk (how do you think one disk backs-up the data from three others?!)

  1. No, you need 3 out of the 4 disks, and the way their BeyondRaid works, you should never remove a functioning disk unless you're decommissioning the array.

  2. What is "Mac mirroring" - If you're referring to something related to a Macintosh computer, no. Different Drobo units only support various BeyondRaid. It can not be disabled to my knowledge.

  3. Yes; in the worst case you'll need to pull of the GUI and click a "rebuild" or similar button. You should always verify through the management console that the array is in fact rebuilt after replacing a failed disk.

Note: You're getting 5.4TB right now because HD manufacturers count decimal bytes and the rest of the world counts binary bytes. So their "2TB" drives show up as 1.86TB to everything else. One less for redundancy and a bit less for file system overhead and you get 5.4TB.

Note: Using consumer SATA drives in a enterprise environment is asking for failure/data-loss at some point in the long run. YMMV.

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What would you rather use instead of SATA Drives? We need something portable, which can handle TB of video files. –  Dominique Dec 6 '10 at 14:01

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