An Inexpensive Backup Solution
(Last updated: 12/07/2013)
The purpose of this article is to build the fastest, largest capacity, backup storage solution for the least cost per TB. The motivation for this study is the need for an inexpensive, external, RAID implementation suitable for backup purposes. We expect new/improved storage/bus technology and/or the reduced cost of existing SSD’s to make the solution we pick today obsolete within about 3 years. Let’s keep the cost as low as possible knowing we will be buying again in a few years no matter how much we spend now. Commercial products targeting the 12 TB + backup market include OWC, Buffalo, Promise, Drobo and LaCie.
To perform this analysis the following equipment was used:
- 15 inch MacBook Pro with Retina Display, although any of these will work, just make sure your computer has USB 3.0.
- 4ea – USB 3.0 drives. For this article we selected the Toshiba Canvio, 3 TB, USB 3.0, 5700rpm drives with an average seek time of 14ms and a 32MB buffer. This is the least expensive drive per TB sold by Adorama. We could have gone with 4 TB drives, or we could have picked that 7200rpm drive for $60/drive more, but that was not the objective. We are trying to make a point here.
- 1 or 2 – USB 3.0 hubs with at least 4 ports. Any of these hubs should work. (Disclosure: clicking links to Adorama from within this article, and buying something, helps fund this site. Thanks!)
- You will also need one of these:
- The disk array was tested for read and write speed with Blackmagicdesign Disk Speed Test set to stress the system at 5 GByte reads and writes. 20 read and 20 write tests were conducted for each configuration.
- The disk array was tested for robustness by unplugging the USB port from the computer while active, then reinserting it, and unplugging a single drive while active, then reinserting it.
- The number of disks in the RAID 0 striped array was increased until saturation (no further increase in read or write speed could be obtained).
- The disks were then tested in RAID 10 for speed and robustness.
- The array was then used as the “backup disk” for Apple’s Time Machine with the portable USB 3.0 hub RAID attached with 2 TB of RAW files and Aperture libraries:
12TB RAID 0 Under $400 Review:
- No! There are too many power and USB connections to be considered portable. You can not use this in an airplane seat, even with an AC power outlet, unless you purchased the seat next to you also.
- It should be noted here that the entire 12 TB system including both hubs and a 4 headed extension cord fit in a 13” laptop bag
- It should also be pointed out that setting up the array for each backup may deter a user from doing backups more frequently. There are a lot of wires.
- The disks allow for easy storage in a Bank safe deposit box, even the small cheap ones, whereas the competing solutions will require the largest (and most expensive) box.
- Some suggest storing your offsite backup at a friend or family location. With the “USB 3.0 Hub RAID” solution you could divide the array up among 2 or more family or friends ensuring no one family member or friend has access to your data.
2TB drives will yield an 8TB RAID 0, 3TB drives will yield a 12TB RAID 0 and 4TB drives yield a 16TB RAID 0. The best bang for your buck at the moment is 12TB. We recommend no more than 4 drives because:
- Most portable 3.0 hubs provide 4 ports.
- There is no speed enhancement above 4 drives.
- With each added disk the chance of RAID 0 failure increases dramatically.
The chart was generated by first testing one drive attached to the USB port on the computer.
RAID 0 Striped:
We next attached two drives to an AC powered USB 3.0 hub and created a RAID 0 striped array using the “Disk Utility” program which comes with Mac OS X. We then repeated the test, striping three, then four drives into a RAID 0 array. The chart shows a marginal improvement in write speed as more drives are striped together. The results demonstrates saturation of the USB 3.0 hub attached to a single USB 3.0 port on the computer. Although little was gained at the top end, adding a third drive reduced the standard deviation of the results from 24.2 to 11.3. Adding the forth drive took the standard deviation down to 3.7. In other words the more drives we added the more reliably the read speed was measured above 300MBytes/sec.
Distributed Array (2 Hubs):
We then rearranged the array using a second hub. Now with 2 drives attached to one port on the computer and the other 2 drives attached to the other port on the computer we see a dramatic increase in speed more closely aligned with 4 times the single disk speed that we would expect with no bus limitations. This demonstrates the flexibility of the array in that any arrangement of the drives is acceptable with no disk utility reconfiguration necessary. Simply unplug the drives from one hub and distribute to another (after ejecting the striped set of course) as your needs require. If you get the dreaded “the disk you inserted is not readable” error please see the bottom of this article.
Next the four drives were repartitioned into two RAID 1 mirrors that were then striped together into one RAID 10 array yielding 6 TB total capacity and a reduced speed, but with the added security of being able to remove a drive from each mirrored pair without interrupting any ongoing processes. In testing, the array worked flawlessly when we removed a drive from the array. When we reattached the drive, the system immediately started to rebuild the drive. Unfortunately, it takes hours.
For some reason fully encased external drives with a USB3.0 interface, its own AC power supply and a cable cost less than raw drives with only a bare SATA connector and no cable. When you add to that the fact that you will have to spend at least a few hundred dollars to encase those raw drives in a RAID enclosure the expense of a USB 3.0 Hub RAID Array seems pretty small.
An artifact of using these drives is the following error message:
This usually occurs when you change the order of the drives in the hub. You can receive 1 to 3 of these messages with a 4 drive array although we have only seen two at one time. This is apparently due to a timing issue on initially installing the array. Simply respond “Eject” to each of the error messages. It does not hurt to have disk utility up to help you resolve the issue, but it is not necessary. Simply unplug and replug each drive into the hub. Remarkably, unplugging the hub from the computer and replugging has no effect, you must unplug and replug the drives that were determined to be unreadable. As you unplug each drive it would be advantageous to note and mark on the drive, which slice of the array each drive occupies. The next time you build the array you will only have to cycle the drives that were not recognized.
RAID 0: We detached the hub from the computer during testing and received the “Disk Not Ejected Properly” alert, as you would expect. Once reattached the system recovered normally. Next, we detached a single drive of the array from the hub while testing was ongoing with the same results. Once reattached the system recovered normally. There was no loss of pre-existing data and no need to run “Disk Utility”.
RAID 10: We detached the hub from the computer port during testing and received the “Disk Not Ejected Properly” alert, as you would expect. Once reattached the system recovered normally. Next, we detached a single drive of the array from the hub while testing was ongoing. This time there was no alert. The test continued without interruption. Once the drive was reattached all operations with the array continued apace, but a background process was initiated that started rebuilding the drive. The screen shot below demonstrates the rebuild of 2 drives, 1 from each mirror. The RAID 10 is completely functional while rebuilding 2 drives: