2 Drive System: Mirror or Parity?


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14 hours ago, dev_guy said:

Sorry, by 3 drives I meant 2 or more data drives and 1 parity drive (a more common scenario than 2 parity drives and 1 data drive). In that situation unRaid has to do more work to calculate the parity data on writes and it shows in the write performance especially with a low power CPU/system. In my testing it's 90+ MB/sec for a single data drive vs ~50 MB/sec on sustained large writes for 2+ data drives. Even on my Xeon server I still don't get anywhere close to 90+ MB/sec on sustained large writes without using a cache drive.


The slowdown is due entirely to rotational nature of hard drives - nothing to do with processor speed.

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  • 1 year later...
On 10/17/2018 at 9:43 AM, johnnie.black said:

Yes, with only 2 drives Unraid uses them like a mirror automatically, same with 2 parity drives and a single data drive, it will work as a three way mirror.

@johnnie.black Hey there. I'm a noob with servers and unraid. I started with one 4TB HD 3 weeks ago and just added a second this new year. The plan is to use the second drive as a mirror for the first. However, after preclearing and adding the second HD to the array unraid has not started using it as a mirror automatically as stated in your post. I have been looking online but have not found anything information on how to actually accomplish the mirroring setup. If you could give me some pointers or direct me to links as to how to setup this mirror process I would greatly appreciate it. 

Thank you.

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  • 10 months later...
2 hours ago, madkatz said:

I have a question about this 1 data drive 1 parity drive mirror - is it a true raid1 mirror? If the data drives fails let's say, can I then take the parity drive and change it to a data drive and access all of the files?

Yes.  But you don't even have to reassign anything.  It will all be handled behind the scenes automatically (the same as if you had multiple drives and one failed.  You'd still have access to all of your files)

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