Setting up RAID 1 across two separate drives.

Discussion in 'Apple TV and Home Theater' started by spice weasel, Oct 8, 2008.

  1. spice weasel macrumors 65816

    Jul 25, 2003
    After completely filling up a 500 GB external drive with iTunes media for my Apple TV, I've decided that it's high time I not only got a bigger drive, but also came up with a better way to back up all that data. So I recently purchased two 1TB Western Digital Studio II external drives.

    This may seem like a dumb question, but I have never used or set up a RAID array before so I'm not sure of the answer. Can I set up the two drives as a single RAID 1 drive? Or can that only be done for each drive individually (thereby creating a mirrored RAID across each 500 GB half of each 1TB drive)?

    I'm trying to think out the process for copying and backing up my files. My first instinct is to set up both drives (that is, all 4 halves) as a single RAID 1, thereby allowing me to copy a file to one 1TB drive and have it mirror to the other one automatically. But I'm not sure if that's possible. If not, then my other option is to split my iTunes library across both drives, setting each one up as a separate RAID 1 for redundancy. My current library would go on one 1TB drive, and as I add more media those files would go on the second. As another option, I can set each one up as a RAID 0 and then just manually copy the file onto each drive - one to run iTunes off of and the other keep as a backup.

    Any suggestions?
  2. d21mike macrumors 68040


    Jul 11, 2007
    Torrance, CA
    I have been doing this for a while (I am running Windows). I just lost one of my 1TB Drives this week. So all I had to do was get another 1TB Drive and let the RAID rebuild it. You of course need either a RAID Controller on the Mother Board or a separate card. Actually I think you might be able to do RAID with software but not sure. All of my new computers use INTEL Mother Boards with RAID Support. In the past I have used ASUS that also had RAID Support.

    In my case I have 2 TB Drives setup as RAID 1 which gives me a 1 TB C Drive and I also have 2 250MB Drives setup as RAID 1 which give me a 250MB D Drive.
  3. belvdr macrumors 603

    Aug 15, 2005
    You're jumbling your terms. A RAID, no matter what level, uses more than one drive, so you cannot put one drive in RAID 1 and another drive in RAID 0.

    Think of it this way:

    Drives -> Array -> RAID Level -> Partitioning -> Drive Letter/Volume Name

    RAID 1 consists of 2 drives, no more and no less. One drive is a mirror of the other.

    You can do this in software or with a RAID controller. Be advised that most of the onboard RAID controllers, as mentioned above, provide little benefit, as they don't truly offload the RAID overhead from the main CPU.

    Now, setup your RAID 1 with both drives and format the new volume. You will see one 1TB volume, if you only setup one partition. Copy your data and you're done.
  4. spice weasel thread starter macrumors 65816

    Jul 25, 2003
    Ok, so I can set up the two drives as a single RAID 1 drive then. Thanks.

    My question about setting up two separate RAIDs comes from the fact that each of the 1TB drives I ordered are comprised of two 500 GB drives inside. I know that each 1TB drive can be set up as a RAID 1 or Raid 0 because of this. I just didn't know if the two 1TB drives together could be set up as a single RAID 1 (for a total of 1TB of mirrored space).

    I also forgot to ask about software vs. hardware RAID controller, but it seems that I can do it with software. Does it put a strain on the system? I have a G5 iMac. Or would it be easier just to forgo RAID and manually back up one full TB drive to the other?
  5. belvdr macrumors 603

    Aug 15, 2005
    It seems these are external, so I would do a RAID 0 on those drives, then mirror (RAID 1) with your OS. This should improve write speeds.

    The overhead for a RAID 1 is very minor, as there are no calculations required to maintain the array. When you start talking RAID 5 or 6, that's where a dedicated and accelerated controller really helps.
  6. Alrescha macrumors 68020

    Jan 1, 2008
    This might improve write speeds, but not by much. It would be more reliable to have each drive mirrored and then stripe the two enclosures. With that configuration a drive could fail in each enclosure and the array would still function.

    RAID should not be considered as a replacement for backup. It keeps you from losing data in the event a drive failure, but that's all. If you accidentally erase something, or some program goes nuts and mangles some of your data, the RAID process will happily erase the backup copy and mangle the data on all the drives before you can blink.

    If you want a proper backup, configure your external drives as RAID0 (for maximum space) and put your data on one of them. Then use Time Machine or your favorite backup program to copy that data to the other external drive.

  7. belvdr macrumors 603

    Aug 15, 2005
    Depends on what you are comparing it to. Compare against RAID 5 and it is a huge increase.

    RAID 10 and RAID 01 are equal in terms of reliability. Saying they are unequal is not even close to being accurate. You can lose two drives, and still work, in either RAID 10 or RAID 01, but they can only be certain drives in either config. For example, in RAID 10, which you are referring to, you can lose a drive in each RAID 1 and still function. However, if you lose both drives in one enclosure your data is inaccessible, as you have lost an array which is part of a larger RAID 0 array.

    For RAID 01, it is just the reverse; you can lose 2 drives in one enclosure, but not a drive in each enclosure. So, either way you look at it, you can still lose up to 2 drives and still function; it just depends on which drives fail. It's a coin toss.

    I agree RAID is not a substitute for backups, however, recommending RAID 0 as part of a backup solution really isn't accurate. If the data is important, even if you have a backup, I would not recommend RAID 0. The reasoning is that I don't have to restore the data from backup in case of failure; I simply replace the drive. That's more of a preference of mine than good backup strategies. I would always prefer to just swap a drive and rebuild the array versus restoring a backup.

    In the end, you can do whatever you want with the array; the backup solution is getting the data off the original medium, and ideally, off site.
  8. Alrescha macrumors 68020

    Jan 1, 2008
    I'm sorry, but I don't believe this is true for any situation except for the four-drive configuration. If that's all you are referring to, then fine. Otherwise I disagree.


    "Mathematically, the difference is that the chance of system failure with two drive failures in a RAID 0+1 system with two sets of drives is (n/2)/(n - 1) where n is the total number of drives in the system. The chance of system failure in a RAID 1+0 system with two drives per mirror is 1/(n - 1). So, using the 8 drive systems shown in the diagrams, the chance that losing a second drive would bring down the RAID system is 4/7 with a RAID 0+1 system and 1/7 with a RAID 1+0 system."

    From Wikipedia:

    RAID 0+1: striped sets in a mirrored set (minimum four disks; even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if drives fail on both sides of the mirror the data on the RAID system is lost.

    RAID 1+0: mirrored sets in a striped set (minimum four disks; even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. In a failed disk situation, RAID 1+0 performs better because all the remaining disks continue to be used. The array can sustain multiple drive losses so long as no mirror loses all its drives."

    Apologies to the original poster for getting silly in his thread.

  9. spice weasel thread starter macrumors 65816

    Jul 25, 2003
    No apologies necessary. I've learned a lot about RAIDs!

    In the end, I decided to forgo setting up a RAID and just mount the two drives normally. Anytime I add a movie file to one drive, I also copy it to the backup. Should the first fail. I have a complete and ready-to-go backup drive. I use a third drive as a dedicated Time Machine drive for my non-iTunes files.

    Thanks for all the input.
  10. belvdr macrumors 603

    Aug 15, 2005
    I was only referring to 4 drive configurations, as that's what the poster had. That said, the mathematics don't seem to take into account the likelihood of a drive failure in any scenario (RAID or not). Hard drives do not fail as much as they once did. I'm still trying to figure out how he came up with that formula. I'll look at it more when I get time.

    For example, it is saying there's a > 50% chance of losing data due to a second drive failure in RAID 10. I have just not seen this in real life over the past 20 years. Of course, mathematically, it could be theoretically possible, but it doesn't mean it is practical in real life experience.

    As for performance, I have noticed any performance degradation due to drive failure, whether 10 or 01, is so slight that nobody ever notices. Drive failures in RAID 5 always seem to stink, and the old EMC Clariion units were horrendous. They would disable their cache even after the array was rebuilt with a hot spare.

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