How Drobo's BeyondRAID compares to traditional RAID technologies

The DroboElite uses BeyondRAID to provide a high level of protection to the array and to simplify overall storage administration. Find out how BeyondRAID compares to other RAID levels, and learn about the technology's features.

I recently published a Product Spotlight outlining my first impressions of the DroboElite, an iSCSI-based storage appliance aimed squarely at the small and medium business market. BeyondRAID is a technology intended to provide RAID-like data protection, while attempting to simplify it and remove some significant limitations. While Drobo's BeyondRAID is a very cool technology that simplifies storage configuration for small and medium business, it's not a panacea.

In this overview, I compare traditional RAID and BeyondRAID. I also look at RAID's high-level capabilities rather than the nitty-gritty like IOPS and such.

Comparing RAID and BeyondRAID

Traditional RAID has a couple of major limitations:

  • All drives in the RAID set must be the same capacity. If drives differ in size, space is wasted. For example, if you have four 300 GB drives and two 1 TB drives and you decide to create a five disk RAID set, your RAID set will consist of five drives each with 300 GB of usable capacity. Even if you have just one 300 GB drive and the rest are 1 TB drives, you're still limited to creating that RAID set with that smallest disk (the 300 GB unit), dictating the usable capacity on the remaining disks.
  • Desired protection level change requires a destroy-and-rebuild process. Let's say you create a RAID 5 disk set and then decide you need an additional protection so you want to move to RAID 6. With a traditional RAID system, your only option is to back the data up, destroy the RAID 5 set, build a RAID 6 set, and restore the data. There are time and potential data integrity issues with this process.

Drobo's BeyondRAID technology, which relies on similar data protection processes as traditional RAID systems, helps to minimize the limitations listed above.

  • Use drives of any size (BeyondRAID's Mixed Drive Size Utilization feature). BeyondRAID allows administrators to mix and match drives of different sizes. Through the creative use of this new technology, Drobo can make more effective use of drives of varying sizes than traditional RAID systems.
  • Forget worrying about drive order (BeyondRAID's Drive Reordering feature). In many traditional RAID systems, if you have to move a storage array, you need to be extremely careful to make sure that disks are removed and put back in the exact same order. With BeyondRAID, disk order makes no difference, so there is a bit less overall risk.
  • Use on-the-fly methods to change protection levels (BeyondRAID's Single Disk Redundancy and Dual Disk Redundancy features). With the click of a mouse, you can change your BeyondRAID protected system from single to dual-drive protection, assuming that you have enough free disk space to make the change.
  • Swap disks any time, any way (BeyondRAID's Instant Expansion feature). When a disk fails in a traditional RAID set, you replace it. If you need to expand the size of a traditional RAID set, you replace the disks one at a time, do a rebuild each time, and eventually expand the volume. With BeyondRAID, you can replace any disk at any time and even replace smaller disks with bigger ones. While this process will still initiate a rebuild, depending on what size disks you already have in your Drobo, you may immediately gain access to newly added storage. The difference is: An experienced storage administrator needs to work with the traditional RAID array, while any person with reasonable knowledge can handle the Drobo BeyondRAID technology.

There are, however, still some limitations with Drobo. The limitations include the following:

  • Dual-drive systems gain no storage benefit. As you add drives to your Drobo system, you start to gain the benefit of Drobo's ability to automatically resize and use new space. However, with just two drives, Drobo's BeyondRAID is simply a mirrored drive set, regardless of the size of the disks. So, if you have a BeyondRAID set with one 500 GB drive and one 2 TB drive, your resulting usable capacity will be 500 GB, just as it would be under a RAID 1 scenario.
  • Mixed-size drives are supported but still present real-world challenges. In order to gain the full protection offered by single or dual-drive redundancy, enough space needs to be available for protecting the array. In the case of systems with multiple drives of differing sizes, this means that the system has to be able to withstand the loss of the largest disk in the unit. So, if you have a fully populated DroboElite with seven 500 GB disks and one 2 TB disk, overall, the unit needs to hold back at least 2 TB in order to support the loss of that single 2 TB disk. Although drives of differing sizes are supported, you still have to face reality.
In Figure A, this particular DroboElite has three 2 TB disks and two 1 TB disks for a raw total of 8 TB of which 7.27 TB is actually available. Of that 7.27 TB, 1.86 TB is dedicated to data protection, leaving 5.41 TB available for data. (Note: Disk drive manufacturers continue to market disks with capacities that differ from the way that operating systems report available space, hence having only 7.27 TB of total available space as opposed to the advertised 8 TB of raw space. This same math is the reason that we require 1.86 TB of space for data protection overhead rather than 2 TB. A 2 TB disk actually formats to 1.86 TB of capacity, hence the discrepancy.) Figure A

Overall DroboElite storage capacity. (Click the image to enlarge.)
For completeness, I thought it was important to show the overall disk layout on the DroboElite that I've been using. In Figure B, you can see that my unit holds three 2 TB drives and two 1 TB drives. Figure B

The DroboElite's disk configuration. (Click the image to enlarge.)

DroboElite's BeyondRAID service also supports dual-disk redundancy so that the array can withstand the loss of multiple drives. As is the case with a single disk, you have to make sure there is adequate space in the array to withstand the loss of that second disk, so dual-disk redundancy means that you lose about the equivalent of the two largest disks in your array. In the case of the 3x2 TB disk and the 2x1 TB disk array, you'd lose enough capacity to withstand the loss of two of your 2 TB disks.

In Figure C, this DroboElite is in the process of converting to dual-disk redundancy, and only 3.61 TB of the original 7.27 TB will be available when the process is complete. It takes the unit a while to rebalance enough data in order to move to a dual-disk redundancy scheme. In order to make the change from single to dual-drive redundancy, you need to have enough disk space available on the Drobo to be able to lose that second disk's worth of available capacity. By the way, enabling dual redundancy is very, very easy -- simply open the DroboElite's Settings page and select the check box next to Dual Disk Redundancy (Figure D). Enabling Dual Disk Redundancy might take a rather long time on a Drobo with a lot of data; I enabled it on my unit, which has no data, and it took quite a while to complete the process. Figure C

With dual redundancy, the available capacity plummets. (Click the image to enlarge.)
Figure D

Enabling Dual Disk Redundancy. (Click the image to enlarge.)
Look at the bottom of this page to see a table on the Data Robotics Web site that shows a side-by-side comparison of how various features in BeyondRAID compare to other RAID technologies. I'll explain the features that I haven't already discussed in this column.
  • Smart Volumes. When volumes are created, the space necessary is pulled on demand rather than simply being allocated immediately to the volume. This is a sort of thin provisioning technique that allows volumes to use as little space as possible.
  • Automatic Healing. In order to provide a high level of protection, BeyondRAID alerts an administrator if a hard drive is going bad and then takes steps to attempt to work around potentially bad disk sectors. If a drive is starting to go bad, you want to replace it, but this interim repair step might just buy you some time.
  • Data Aware. BeyondRAID technology keeps track of where actual data (as opposed to unused space) is located on each drive. By doing so, the unit can rebuild and recover more quickly after a drive failure since unused portions of the disk won't be touched. If you've been managing storage for very long, you'll probably see this as a benefit; after all, a shorter rebuild time leaves less time during which you might suffer the loss of an additional disk.
  • Proportional Rebuild Times. The less data that is on a disk, the less time it takes to rebuild that data after a failure.
  • Virtualization Engine. Storage virtualization has become a major trend in the storage industry, and BeyondRAID takes advantage of this technology in order to abstract a data block from its physical location. This is a major method by which BeyondRAID is able to move information around drives pretty easily. While a user thinks they're accessing a file, they're actually accessing a pointer to a file. When that data is physically moved, BeyondRAID simply updates the pointer, which is a seemless process to the user.
  • Self-Managing. Once put in place and initially configured, BeyondRAID-based systems manage themselves for the most part. As you add and remove disks, the array does most of the work without asking an administrator to jump through hoops to get the job done.

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