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Thread: Can single drive from a RAID 1 mirror be accessed as a normal drive?

  1. #1
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    Can single drive from a RAID 1 mirror be accessed as a normal drive?

    I'm planning to setup a new file server for my home office with a couple of SATA hard drives in a RAID 1 array, most likely connected to the motherboard's onboard RAID controller (I havn't chosen the hardware yet). If one hard drive goes down, the other one will keep my data available until I can get a replacement and I can easily pick up a replacement drive the same day from any local computer store... all good.

    However, if the server's motherboard was to die, obviously I can't access my data at all until the drives are setup on a new PC / motherboard so I need to be prepared for this scenario. A matching motherboard may not be something that I can replace without waiting for one to be delivered, so would I be able to plug my existing RAID array into any make/model of new motherboard that has a hardware RAID controller? Is a RAID array somehow tied to the model of RAID controller it was setup on?

    Also, would I be able to simply plug one of the RAID drives into my (non-raid) desktop PC, or laptop via USB caddy, and use it like any other ext3 partitioned drive in a non-raid way?

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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    I haven't tried this but since the data is not striped, I would think the answer would be yes. The odds of a mobo going bad in my experience is slim to none if it has been running for a week or two already and has been "burned in". You have a greater chance of the hard drive going. You should consider getting one more drive and using RAID5. You'll get better performance with fault tolerance, and with say three 500GB drives you get 1 TB usable space.

    The other thing you have to ask is do you really need RAID? Modern hard drives will go some 300,000 hours between failures. If you keep the drive cool it will probably outlast what you are doing. Just make sure you have good backups either way because RAID is NOT a substitute for backups.

    -Tim
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    Hard to find and obsolete PC and server parts. "If we can't find it, it probably doesn't exist"

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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Hi Tim, thanks for the reply.

    In the past, I wouldn't have considered motherboard issues but I have had the misfortune in losing two from desktop PCs over the last few years and it's made me a bit more paranoid than maybe I need to be, lol. Although chances of it happening again on any particular PC are pretty slim odds, I'd at least like some advance knowledge of how best to deal with it. It just worries me that if a RAID array is tied to a particular make/model of motherboard controller, it could take me more than a few hours, possibly days to get a replacement matching motherboard if possible at all.

    The main thing I'm looking to do is minimise down time in the event of pretty much any component failing, be it hard drive, motherboard, cpu, ram, etc. I do have automated nightly backups in place, but RAID could save me losing an afternoon's work if one drive did go down and this is pretty important to me as I'm self-employed and a day restoring data from backups is a day I can't do any other work, etc. For this reason, and because it's so cheap to do these days, I would really like RAID as an extra safety net.

    As for RAID 1 vs RAID 5, I have to admit I was initially planning on a 3 hard drive RAID 5 setup, but changed my mind really because I thought RAID 1 would be much simpler to manage in the long run, especially if I can just rip one of the drives out and plug it into any PC and have direct access to all the data in an emergency. I'm guessing that this definately wouldn't be possible on a RAID 5 setup, as no single drive would have a complete copy of all data.

    At the end of the day, I'm after long term simplicity, as close as I can get to the simplicity of running a non-raid server, but with the benefits of data being mirrored on the fly. Performance isn't too much of an issue as it's most often just going to be me accessing it and nothing too intensive at that. Finally, cost isn't a big factor either as hard drives are cheap (and huge!!) these days.

    If someone could just confirm that at least data from a RAID 1 drive can be accessed without problems by a simple non-RAID SATA controller if needed, then that would be nice to know for the future. I had previously considered a proprietary NAS box with RAID support, but that worries me even more as I know some of these use their own filesystems and would be even harder to recover from in the event of something going pop. Plus, I require server to run subversion which rules out any affordable NAS that I know of.
    Last edited by crystaline; September 18th, 2008 at 01:00 PM.

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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    If you use software based RAID1 it is no problem to use one of the drives directly in another computer. But hardware based RAID could use some extra partitions or blocks to store raid parameters, so I'm not sure if that works in all cases.

    What kind of hardware RAID are you planning to use? Most motherboards with SATA RAID isn't really hardware raid, it is mostly some BIOS additions to support booting from both devices...

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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Ahhh, thanks for the reply. What you say about hardware RAID is kind of what I feared. I was ideally hoping each drive would appear just as a completely standard ext3 (or similar) filesystem, but what you say makes sense. I think maybe I should scrub any ideas I had that a hardware RAID 1 drive is going to be useable elsewhere without a lot of fiddling.

    If I forget about trying to be able to use a single drive from a RAID array in an emergency, and just treat RAID for exactly what it is, that again opens the doors to RAID 5... Hmmmm, is RAID 5 much fiddler to setup than RAID 1?

    As for type of hardware RAID controller, I'm not too sure yet to be honest. Part of my reason for posting this thread was to try and get a feel for best practices and maybe some advice on particular kit. I wasn't aware that some motherboards do it via some BIOS hacks so this is something I'll have to read up on more. I ruled out software RAID as I've read it is easier to corrupt data as compared to hardware RAID?

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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    One of the problems with hardware raid is that if the raid controller fails you must get a new compatible controller. If you use software raid this is not a problem. I don't see why it would be easier to get corrupted data with software raid, unless you compare with highend raid controllers with battery backed cache memories.

    If you go with software RAID5, remember that you cannot have RAID5 on the /boot partition (grub or lilo doesn't support reading from RAID5), so create a small RAID1 partition for /boot and a big RAID5 for the rest.

    And one more thing: I'm not sure if it is an issue to mount a disk from a hardware based raid1 separately, I just said that it *could* be a problem, and different raid hardware could behave differently

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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Thanks anderswestrup, this is starting to make a bit more sense to me now.

    Would you say software RAID is generally pretty reliable nowadays? I was scared off from it when I first considered it year or two back, but I've read of plenty of people using it without problems now and it's something I could even setup with my present hardware. I know it takes a bit of a CPU hit, but my fileserver's CPU rarely breaks into a sweat anyway. And as for speed, how would software RAID generally compare to hardware RAID read/write times? Or to a non-raid drive for that matter?

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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Quote Originally Posted by windependence View Post
    I haven't tried this but since the data is not striped, I would think the answer would be yes.
    For what it's worth... individual drives from a RAID 1 (mirror) array that were setup using MDADM *can* be accessed individually.

    I tried it - it works.

    Just don't WRITE anything to one drive or the array will have to re-sync if the drives are put back into RAID service.

    -- Roger
    Gentlemen may prefer Blonds, but Real Men prefer Redheads!

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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Quote Originally Posted by Krupski View Post
    For what it's worth... individual drives from a RAID 1 (mirror) array that were setup using MDADM *can* be accessed individually.

    I tried it - it works.

    Just don't WRITE anything to one drive or the array will have to re-sync if the drives are put back into RAID service.

    -- Roger
    Thanks for clarifying that Roger. As you're using MDADM already, can you vouch for it's general stability and reliability? And do you notice any significant load on the CPU doing it in software?

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    Re: Can single drive from a RAID 1 mirror be accessed as a normal drive?

    Quote Originally Posted by crystaline View Post
    Thanks for clarifying that Roger. As you're using MDADM already, can you vouch for it's general stability and reliability? And do you notice any significant load on the CPU doing it in software?
    Yes I am using MDADM for a home file server box. One box has two 1TB drives setup as RAID-1 and the other box has four 1TB drives setup as RAID 0+1. The OS is Ubuntu Server 64 bit v8.04.1

    MADAM seems to be very stable. I have had zero problems with it.

    Performance-wise, the bottleneck is always the hard drives. When I push the drives to the max (copying multiple files from multiple sources all at once), the CPU usage barely creeps up above idle.

    Now, RAID 5 (which I don't use) and other RAID levels that do parity and checksumming (i.e. more CPU intensive RAID algorithms) will probably consume more CPU power... I don't know how much though.

    Hope this answers your question.

    -- Roger
    Gentlemen may prefer Blonds, but Real Men prefer Redheads!

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