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MitchA

Raid for Backup?

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I've been doing some reading, and I've seen that you can easily setup a redundant raid setup in OS X. If I were to purchase an external hard drive that was equal or greater in capacity to my internal on the iMac, would this be a good option for me? What are the advantages or disadvantages involved with RAID at this level?

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It seems like having a RAID array with one internal drive and one external drive might actually slow performance.

 

I'd expect any external drive to have higher latency and delay because of protocol conversions getting on and off a FireWire or USB bus.

 

RAID works best with exactly identical drives connected to identical hardware controllers.

 

I think you could get RAID setup the way you describe, but it might require high levels of maintenance.

 

If the goal is a backup strategy, I'm sure you're better off using the typical CarbonCopyCloner approach. If the goal is higher mounted disk capacity, I'd just connect an external drive in the typical way.

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RAID is a great option for backup of a desktop computer of any kind. It's not really suited to a portable computer because it needs to be a perminent connection really.

 

A great source for information is, as always, Wikipedia: http://en.wikipedia.org/wiki/Redundant_arr..._configurations

 

Atomic Write Failure: also known by various terms such as torn writes, torn pages, incomplete writes, interrupted writes, etc. This is a little understood and rarely mentioned failure mode for redundant storage systems. Database researcher Jim Gray wrote "Update in Place is a Poison Apple" during the early days of relational database commercialization. However, this warning largely went unheeded and fell by the wayside upon the advent of RAID, which many software engineers mistook as solving all data storage integrity and reliability problems. Many software programs update a storage object "in-place"; that is, they write a new version of the object on to the same disk addresses as the old version of the object. While the software may also log some delta information elsewhere, it expects the storage to present "atomic write semantics", meaning that the write of the data either occurred in its entirety or did not occur at all. However, very few storage systems provide support for atomic writes, and even fewer specify their rate of failure in providing this semantic. Note that during the act of writing an object, a RAID storage device will usually be writing all redundant copies of the object in parallel. Hence an error that occurs during the process of writing may leave the redundant copies in different states, and furthermore may leave the copies in neither the old nor the new state. The little known failure mode is that delta logging relies on the original data being either in the old or the new state so as to enable backing out the logical change, yet few storage systems provide an atomic write semantic.

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So it wouldn't be a good idea to do it on my iMac with an external firewire drive? Is that the general consensus?

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It would work, but I think you would find it to be slow and buggy because it wouldn't be an internal (preferably SATA) HDD and it wouldn't be the same model. I wouldn't bother personally, I would do as the Professor says and use something like CCC or Backup or the like.

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Alright. Thank you much. I think I'll go with Carbon Copy Cloner, once I get another drive.

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The other reason not to use redundent RAID for backup is that if you have a file become corrupt it will instantly mirror that to the backup therefor making both copies unusable. The same thing will happen if you accidentally delete the wrong file. It is better to have a little bit of a lag before it gets backed up. That way you can have some amount of time to recover a lost file before it gets overwritten.

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