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I have a Mid 2011 Mac mini with 2x 500G HDD configured as RAID 1. Recently one of the HDD died.

I planned to

  1. replace the dead HDD with a 500G SSD, and
  2. let the RAID 1 rebuild

Is this a viable option?

If viable, should I

  1. Leave the setup as is, or
  2. Buy another 500G SSD and replace the other HDD

OS is Yosemite.


Update:

Since the question and the answers, I had went ahead to buy 1 SSD, replaced the disk in the Mini by following videos on Youtube. The RAID rebuilt without a problem.

The system "appears" to be more responsive than before.

  • IMO If you're going to maintain a RAID 1 Array, then the drives should be of the same type and size (and make and model) when reasonably possible. The read/write speed and latency between the HDD and SSD are probably considerable and as such I'd rebuild the mirror on the new SSD and then swap out the HDD for another SSD and rebuild the mirror again. – user3439894 Jun 21 '16 at 13:05
  • @user3439894 Those days are long gone. You don't need "identical" drives anymore for RAID - just drives of the same capacity. Having managed a datacenter with 1000s of drives, I can assure you that having "identical" drives available at all times was next to impossible. – Allan Jun 21 '16 at 13:08
  • @Allan, In a data center that's understandable however I have several RAID Arrays with different numbers of disks. I maintain and extra disk, still in the box, of the same make, model and size as a replacement. I do this because if/when one goes out I don't what to have to go get a new drive, I want to take cake of it immediately. I agree with most of what you said however I will never mix HDD's with SSD's in an RAID as the read/write speed and latency between the HDD and SSD is considerably different. – user3439894 Jun 21 '16 at 13:16
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Is it a viable option?

Yes.

You could do either of your options given that you are using RAID 1.

That said, I am not a fan of software based RAID. It consumes CPU cycles that could otherwise be used somewhere else. Given that SSDs are far more reliable than TSM (traditional spinning media), the chances of a drive failure go down significantly.

What I would do is use the SSD as my primary drive and the second hard disk as an internal Time Machine drive. You will get increased performance, more reliability and much quicker backups

  • I suppose this could a separate question: How to change the SSD to primary drive without having to reinstalling? Currently the entire RAID is the startup. – kenchew Jun 22 '16 at 1:51
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    Thanks. I just saw your additional question...since you are using RAID 1, if you get an SSD of the same size, you could replace one TSM drive with the SSD, have it rebuild then remove the TSM drive once finished. Or....if you get a different size (most likely) just image the RAID onto the SSD with dd then repartition using diskutil. Or...backup everything to Time Machine from RAID then restore to SSD – Allan Jul 15 '16 at 22:11
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With RAID1 your disk drives write operations will be directed to both drives: your write operations will proceed at the speed of the slower drive. That is to say your system will always be waiting for the slower drive, and you will lose all the speed benefit of the SSD.

Read operations will likely be directed to one drive only: you might or might not find a speed increase depending on configuration. The comments by @Allan and @user3439894 on the original question are appropriate.

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