Drobo - external USB RAID enclosure

[Tom Metro]

I saw Drobo:
http://www.drobo.com/

an external USB RAID enclosure for SATA drives, mentioned on the 
mythtv-users list.

Being USB, not very high performance (slower than a simple USB drive), 
and limited to using NTFS or HFS file systems, I didn't find it that 
interesting initially, but on further reading it has a few unique features.


One minor feature is the enclosure, as pictured here:
http://www.tomshardware.com/2007/06/27/meet_drobo/page3.html
  and here:
http://www.tomshardware.com/2007/06/27/meet_drobo/index.html

It's small for a 4-bay enclosure, with a sharp appearance, and uses 
trayless drive bays. I've hunted around for a similar enclosure for 
building a NAS and have yet to see anything nearly as nice on the 
commodity market. (I did however eventually find trayless SATA bays.)


But the big deal about Drobo is the automated array management. It 
doesn't require disks of matching size. It doesn't require a certain 
number of disk (though obviously more than one for redundancy). If you 
pull out a disk, it automatically reconfigures the array to work without 
it. If you add a disk or replace a disk with a disk of larger capacity, 
it automatically reconfigures the array to make use of the additional space.

This is discussed here:
http://www.tomshardware.com/2007/06/27/meet_drobo/page2.html#the_promise_automated_data_safety_without_configuration_hassles
   and here:
http://origin.arstechnica.com/reviews/hardware/storage-robot-at-your-service-a-review-of-the-drobo.ars/2


On top of that, this dynamic rebuilding of the array supposedly has 
minimal impact on the performance of the unit. Apparently there is a 
video on the manufacturer's site demonstrating how playback of a movie 
trailer is unaffected by pulling a drive and triggering a rebuild. This 
was confirmed by one of the review sites (bottom of the page):

http://origin.arstechnica.com/reviews/hardware/storage-robot-at-your-service-a-review-of-the-drobo.ars/3

The ability to stream a video doesn't really say that much. While that 
has a nice wow factor for a demo, a far more interesting test wold have 
been to repeat the benchmarks that review site collected while the 
rebuild was in progress.

It's also fairly easy to "cheat" at this by giving the rebuild really 
low priority, but according to the review site, the rebuilds seemed to 
happen in fairly reasonable time frames (70 minutes to recover from the 
removal of a 320 GB drive). Although the site makes no mention of what 
load was put on the array during the rebuild and how full the array was. 
(The other review mentions that briefly removing and replacing a 300 GB 
drive resulted in the software reporting that it would need 4 hours to 
rebuild, but they didn't report how long it actually took.)

(As a comparison, my just under 1 TB RAID5 array on my MythTV system 
used to be unusable for video playback during the monthly maintenance 
rebuild of the array, and the rebuild took 4 or 5 hours. Since tweaking 
the speed_limit_min parameter for the MD driver, which controls the 
minimum amount of array bandwidth given to a rebuild when the array is 
in use, it's now sluggish, but usable. I haven't timed how long the 
rebuilds now take. I assume even longer.)


In any case, I'm wondering what's under the hood of the Drobo (OS, CPU, 
etc.) and how it pulls off its automated array management.

According to:
http://www.tomshardware.com/2007/06/27/meet_drobo/page7.html
the OS is VXWorks. It also mentions how the storage is chopped up into 
logical drives, which are then allocated across the physical drives, as 
you'd expect.

And I'm wondering how the equivalent could be built with open source 
software.

I've been pondering the idea of an open source NAS project, where the 
emphasis of the project would be more on the hardware and integration, 
rather than the software, which would likely be FreeNAS or something 
similar, though probably tweaked. I'm waiting for Sun's ZFS, which could 
go a long way towards making some of the above features possible, to 
become available in FreeNAS. It hasn't trickled down from FreeBSD yet.

  -Tom

-- 
Tom Metro
Venture Logic, Newton, MA, USA
"Enterprise solutions through open source."
Professional Profile: http://tmetro.venturelogic.com/

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