The Drobo Math

In the previous blog post, I covered the newly introduced Drobo S and Drobo Elite storage systems, they are new product offerings from Data Robotics, you can read about them here.

Today we will jump into installing and configuring a new Drobo.

So after writing this post in its entirety, I decided to change the title of it. Though we will walk through an install of the Drobo during this post, it uncovers the details on writable / usable storage based on what you insert and the amount of overhead it creates with using the BeyondRaid technology.

Here is a video, created while unpacking, installing and configuring the Drobo, you may probably like the sound track of the video.

Song: Jai Ho (Movie: Slumdog Millionaire)

As for the installation and setup, here are some important things to look forward to..

I have to honestly say, this configuration exercise was a piece of cake, but we have uncovered some details around usable space with the Drobo. With the installation of Drobo you do not need to be a storage expert neither a person with a technical background and that makes it a big selling point in the home / SMB space.

For this blog post,  I have used the MAC OSX 10.6.2 installation along with HFS+ file system. But a windows installation with FAT32 file system will be on similar lines.

  • Insert the CD in the CD Rom Drive….let the magic begin…
  • Welcome Screen.
    Screen shot 2009-11-23 at 1.35.49 PM.
  • Accept the licensing agreement and click on Continue, to install the Drobo dashboard.Screen shot 2009-11-23 at 1.39.13 PM.
  • During the installation you will be prompted to register your Drobo with Data Robotics. You can skip this step and register later. If you decide to register now, you will need your name, address, phone, Drobo serial number (found at the bottom of the unit) and some other additional information.Screen shot 2009-11-23 at 1.43.10 PM.
  • If you have any Drobo’s connected to the computer where you are installing the Dashboard, please disconnect them at this point.Screen shot 2009-11-23 at 1.44.20 PM.
  • It will take about couple of minutes to install the necessary files on Mac OSX or Windows.Screen shot 2009-11-23 at 1.48.32 PM.
  • Reconnect the Drobo to the computer. Drives can be installed or uninstalled within the Drobo at this point. Screen shot 2009-11-23 at 2.01.38 PM.
  • As seen in the picture, we have connected the Drobo with no drives in it.  Drobo Volume shows 0 TB usable.Screen shot 2009-11-23 at 2.02.19 PM.
  • Drobo showing no drives present in any slots.  Screen shot 2009-11-23 at 2.03.34 PM.
  • Insert 1 x 1TB drive in the Drobo, alerts showing the Drobo will not be able to protect the drive until another drive is inserted. You can only obtain data protection through atleast 2 drives in the Drobo. Screen shot 2009-11-23 at 2.04.36 PM.
  • Insert 1TB drive with 32MB cache drive in Slot 0, Insert 500 GB with 16MB cache drive in slot 1. Two drives have same speed but different onboard cache and drive sizes.Screen shot 2009-11-23 at 2.05.58 PM

Note: The beauty of the Drobo is it can accept drives with multiple speeds, sizes and cache on board to create a Drobo thin provisioned volume.

  • Going through Data Protection process now.  It may take about 24 mins for a 1TB drive and a 500GB drive to creat the necessary data protection. During this data protection build, the drives and data on the Drobo is not protected from failures (in short your data will stay intact if no additional drives fail during the data protection build).Screen shot 2009-11-23 at 2.06.12 PM.
  • After about 21 mins of wait, the data is finally protected. Screen shot 2009-11-23 at 2.13.12 PM.
  • As you can see we have 449GB of available space to create volumes, there are no volumes created so far. Creating volumes in a Drobo world means installing a file system on the volume and making it available to the attached host. Volumes can be bigger than the actual drive sizes installed in the system. Standard Volume sizes offered within the Drobo are 2TB, 4TB, 8TB and 16TB. With 16TB, is possibly only supported with Drobo Pro and not with a Drobo. Screen shot 2009-11-23 at 2.37.48 PM.
  • On the Tools tab, lets setup some values.Screen shot 2009-11-23 at 2.41.04 PM.
  • Blink Lights” will start flashing lights on the front of the Drobo, “Standby” will put the Drobo in Standby mode for maintenance or possibly to turn it off if not needed. “Format” allows you for format the Drobo in HFS+ or FAT32 formats. “Reset” will reset your Drobo to factory setting. “Rename the Drobo and Volumes” will rename your Drobo and the volume names.
  • Drobo will send alerts based on your selection. I like to leave the option on for “when a situation is important” send me an email. This option will only work when an actual computer is connected to the Drobo. If you plan to leave your Drobo unconnected for longer duration of times, you may not get an notifications for failed drives or components.
  • Register your Drobo if you haven’t so far, Check for updates for the Drobo dashboard and Drobo firmware.
  • Format Volume, Here we are creating a 4TB volume, even though the drives we are inserting in the Drobo do not add up to total 4 TB’s right now. We are creating a max volume size at this point based on what we think we may use within this Drobo (Again remember the concept of the Drobo is to expand your storage on the fly). That means if the Drobo was running at full capacity, we would be able to use 4TB of storage with 4 x 1TB drives. Create a volume name, hit continue and wait for about 5 mins for volume format.
  • Screen shot 2009-11-23 at 2.42.48 PM.
    Screen shot 2009-11-23 at 2.43.13 PM
  • A confirmation screen pops up and an icon of your Drobo named volume shows up. Screen shot 2009-11-23 at 3.17.57 PM
    Screen shot 2009-11-23 at 3.18.14 PM
    Screen shot 2009-11-23 at 3.20.35 PM
    Screen shot 2009-11-23 at 3.20.42 PM
  • Volume created now is a 4TB volume, though the usable space on this volume will only be 455GB. Remember our total volume is 4TB (Max). Screen shot 2009-11-23 at 3.23.41 PM.
  • To setup email alerts, go to the Drobo Dashboard email settings.  Include your TO email address, FROM email address, mail server address, port address (generally 25 or 587)Screen shot 2009-11-23 at 3.24.20 PM.
  • To Create Drobo CopyScreen shot 2009-11-23 at 4.53.44 PM.
  • Advanced settings within Drobo CopyScreen shot 2009-11-23 at 4.54.13 PM.
  • Email settings within  Drobo Copy
  • Screen shot 2009-11-23 at 4.54.45 PM.

One obvious disadvantage of the Drobo I see is, the drive writable size with two drives is still limited to the smallest drive in place. With a 1TB drive and a 500GB drive, we have a usable space of 455 GBs. With additional two drives we install, the limitation of the smallest drive will be ignored. Again remember our total max volume size is 4TB, so your host see 4TB of storage but can only save uptoo 455 GB of storage on the disk unless you are able to add other disk in the Drobo.

Lets see by adding other drives if things change.

Lets add a 640GB drive into the mix now. I suspect the total usable storage will now be close to 1.0 TB and not 1.1TB. Lets see….


  • Another drive inserted alert Screen shot 2009-11-23 at 4.59.56 PM.
  • It now seems we have 1.02 TB of usable data. Screen shot 2009-11-23 at 5.01.03 PM
    Screen shot 2009-11-23 at 5.02.15 PM.
  • Added a new 1TB drive in the system, lets see what results we get with this new drive. Screen shot 2009-11-23 at 5.06.20 PM.
  • 1.93GB of data is now usable.Screen shot 2009-11-23 at 5.07.28 PM
    Screen shot 2009-11-23 at 5.08.35 PM.

Total inserted storage in our case is 3.14 TB, total usable storage is 1.93TB, Total volume size is 4TB, giving us 61% usable storage ration to our inserted storage. About 941 GB of data is used for protection, or call it parity or call is some bit that is used to recover the data.

With traditional RAID 5 (3+1), you will have around 25% overhead.

With traditional RAID 6 (6+2), you will have around 25% overhead.

With Drobo (4) drives, we have around 39% overhead, the cost of BeyondRAID technology.

  • By Rearranging and reformatting the drives, Slot 0: 1TB, Slot 1: 1TB, Slot 2: 640GB, Slot 3: 500GB, the usable space now is 1.94TB, with 2TB of maximum volume size
  • By Rearranging and reformatting the drives, Slot 0: 1TB, Slot 1: 1TB, Slot 2: 640GB, Slot 3: 500GB, the usable space now is 1.93TB with 4TB maximum volume size.
  • Screen shot 2009-11-23 at 6.16.14 PM
    Screen shot 2009-11-23 at 6.16.38 PM
  • Quick Notes:.
  • Volume size of 4TB that we created in this case cannot be changed without a reformat of the drives. So select the write volume size from the get go.
  • End result is at least 39% overhead, If you look at it, its kinda high….
  • Need to test Drobo with 4 x 1 TB drives to see what is the usable storage…i suspect even though you have created a 4TB volume, it is not 4TB usable space.
  • The DROBO offers easy installation, easy setup, easy configuration, easy use and data in place upgrades if well planned.
  • Stay tuned for a deep dive session on how the Drobo works internally….Note: Will update the post around 11:00 Am tomorrow with my finding on overhead with 2 x 1TB drives in the system. So 4TB volume configured, 2 x 1 TB inserted, how much is the usable / writable storage based on those stats. Please comment if you figure out before I update. I suspect it to be around 610GB – 650GB’s total?
  • Oh….the 2 x 1 TB drives produced a completely different result….920GB usable, so here we only have about 8% overhead. This does mean, that even though the Drobo is marketed as a device that accepts drives of multiple sizes, there is an additional overhead with putting drives of multiple sizes in there.Here is the test result…Screen shot 2009-11-24 at 11.47.10 AM
  • Another test revealed with 4 x 2TB drives in the system, the end user was able to obtain about 5.5TB of usable storage, that is about 68% usable space or 32% overhead.
  • One rule of thumb I get out of this exercise, what ever you do, try to keep the drive sizes common between all the drives you use within the Drobo for maximum usable storage or less overhead.
  • Edited 11/24.2009 at 6:00 PM.
  • Some additional test results with the Drobo
  • If you plug in 4 x 1TB drive you get 2.7TB of usable space that is 32.5% Overhead (protection and reserved space).
  • If you plug in 2 x 1TB drive you get 920 GB, that is 54% overhead (protection and reserved space). And again, I am not recommending you use this scenario, using less than 4 drives or different drive sizes causes added penalties.
  • If you were replacing one drive out of the set of 4 x 1TB drive with a 2TB drive, so 1 x 2TB and 3 x 1 TB, no additional storage space is available.
  • If you had 4 x 2TB drives and just replaced two of those 2TB drives with 2 x 500GB drives, your usable storage falls from 5.5TB to 2.7TB 46% overhead.

May be I am looking at this the completely wrong way, but the marketing buzz word around use the drives you have, mix and match drive types and sizes, may not give you an optimal solution. Though if you plan to really keep the drive sizes similar across the Drobo, it may be for you….

  • Comments always welcome, please feel free to correct if you see this differently….
  • W. Curtis Preston

    You're comparing it's overhead to what? In every other RAID config your overhead would be much higher because you would be forced to use only the 500 GB drive (in your last example). So the real overhead with RAID 5 would be 3.14 – ((500 * 4) – 500), or 47% — still higher than Drobo.

    Second, while they make the storage “dumb” enough that you don't have to worry about sizes, based on what I know of HOW it works, you get far less overhead if you use matching drive types. For example, if you had used two 500s instead of a 500 and a 640, your overhead drops to 33%. If you use ALL matching drive types, your overhead typically drops to 25% on a four drive unit — as you would expect it to be. (Use the capacity calculator here )

    As I believe you know, they stripe what they can with RAID 5 (or 6 if it's offered in that model) and then take the excess capacity and mirror it. That means in your last example, they're creating a RAID 5 stripe with 500 GB “disks” and a mirror of the excess 500 GB on the two TB drives. So that's 1.5 TB of RAID 5 and 500 GB of RAID 10, for a total of 2 TB (roughly). The excess capacity on your 640 drive gets thrown away (for now) because there's nothing they can use to make it redundant.

    If you want to make max use of your drobo, buy matching drives. If you want to make maximum (while still protected) use of random drives laying around the house, put them in a drobo. It will do its best, but it can't work magic.

  • storagenerve

    Hi Curtis, totally agree with you, the point I am trying to make is even through the Drobo is sold with a marketing buzz of use different size drives, it creates this big overhead in terms of what space it can use. Sure the beyondraid technology is quite different than that with existing raid.

    Lets move a bit away from using disproportionate drive sizes to standard 4 x 1TB drives, the Drobo will still only give us 2.7TB of usable space with atleast 1.3TB overhead+protection. While a standard raid5 might offer you as much as 3TB of usable storage, which is still 300GB's more with a total size of 4TB's.

    I am totally with you on the interworkings on beyondraid technology, but it does create additional overhead possibly not found with raid technology.

    Even as a user creates a 4TB or 8TB volume, the total storage they will use will be limited to the type, size and overhead those drives will create.

  • W. Curtis Preston

    I think you're forgetting that a 1 TB drive doesn't give you 1 TB of capacity. 4 1 TB drives gives you 3.6 TB of actual capacity. So 2.7 TB is right on the money, as it's 75% of 3.6 TB.

  • storagenerve

    Curtis, i am taking this challenge on my ix4-200d, with 4 x 1TB drives. I just got one delivered home, will be installing Raid 5 on it and see how much storage we get out of an ix4. Game plan is to compare both the performance and usability of the Drobo to the ix4….more Drobo post coming out tomorrow…

  • MadMark

    Drives are base 10 size (1,000,000,000,000bytes = 1000^4 or 10^12 bytes) and computers generally use base 2 (1,099,511,627,776 bytes = 1024^4 = 2^40 bytes = 1 tebibyte (TiB)). That is why 4 “Terabyte” as a drive spec = 3.6 on a computer.

  • MadMark

    Drives are base 10 size (1,000,000,000,000bytes = 1000^4 or 10^12 bytes) and computers generally use base 2 (1,099,511,627,776 bytes = 1024^4 = 2^40 bytes = 1 tebibyte (TiB)). That is why 4 “Terabyte” as a drive spec = 3.6 on a computer.

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