Should I use software RAID?

Discussion in 'Mac Pro' started by Demon Hunter, Dec 21, 2006.

  1. Demon Hunter macrumors 68020

    Joined:
    Mar 30, 2004
    #1
    I'm totally new to RAID so I appreciate the help. I learned a little from reading previous threads.

    I have 3 x 750GB Seagate drives that are just begging for some tinkering. I've heard a lot about RAID, but I have no clue what I'm doing (and that scares me).

    What should I do? I can't decide which type I need, if any. I'm mostly concerned about Leopard. Will Time Machine render my mirrored drives useless? What about striping? Concatenated? Should I use my boot drive in the array, or is this a bad idea? What's the difference between striping 2 drives as opposed to 3 drives?

    Thanks!
     
  2. Cromulent macrumors 603

    Cromulent

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    Oct 2, 2006
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    The Land of Hope and Glory
    #2
    Software RAID = Slower and cheaper.
    Hardware RAID = Faster and more expensive.

    The software option is probably the easiest as there is no need to go out and buy a RAID PCI-E card. But personally if I had just shelled out the cash for some hard drives to put in a RAID array I would go the hardware route.

    I believe the software needed to do software RAID arrays is built into Mac OS X, although someone else will need to confirm this for you.

    Which raid level are you planning on using? 0? 1? 5? 10? It might make a difference.
     
  3. Apple Corps macrumors 68020

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    #3
    What exactly are you trying to accomplish? That is an important question. For performance, hardware RAID solution is the faster. For data backup - the software RAID may meet your needs. Again, it will help if you outline your end goals, applications, file size, etc.
     
  4. Fearless Leader macrumors 68020

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    #4
    raid software is built into osx through the disk utility app.
     
  5. WildPalms macrumors 6502a

    WildPalms

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    Jan 4, 2006
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    #5
    Actually Software RAID does not equal slower. The reason for going with hardware raid is to gain OS independance. Depending on the OS used with software RAID it can actually outperform hardware RAID.
     
  6. Harpo macrumors member

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    Jun 23, 2006
    #6
    But can one's boot partition be part of a striped software RAID?
     
  7. Demon Hunter thread starter macrumors 68020

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    Mar 30, 2004
    #7
    From what I've read, using the boot drive in any RAID configuration (especially stripe) is a bad idea because a drive failure could make your system unusable.
     
  8. topgunn macrumors 65816

    topgunn

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    Nov 5, 2004
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    Texas
    #8
    Software RAID DOES equal slower. With hardware RAID, the hardware does the processing. With software RAID, the CPU does the processing. With today's CPUs, you may not notice the slowdown but it is there. A reason for hardware RAID is indeed OS independence but another is not having to use the CPU to handle RAID processes.

    To the OP, I would not use RAID 0 via software RAID because the performance increase is not considerable and you double your chances of data loss to HW failure. For something like a scratch drive, you would be much better off using the two drives seperately than one RAID 0 array.

    I also would not use software RAID level 1 because of the performance hit and because I prefer not to use RAID 1 as my backup method for my desktop machines. For example, if I delete or modify a file on accident, having a mirrored drive won't help me. But, having a second drive that gets updated every night will help me because it will have yesterdays version on it. But that is more a matter of personal preference.
     
  9. topgunn macrumors 65816

    topgunn

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    #9
    A drive failure when using RAID 0 will do more than make your system unstable. It will make it unusable until you split up the RAID array. That and you will have lost whatever data is on the two drives. One misconception with RAID 0 (stripping) is that each drive has half of your data (ie if one drive goes out you only lose half of your stuff). While technically that is true, you typically do not have one whole file on one drive and another whole file on the other drive. You have half a file on one drive and half on the other. Picture having one drive and losing every other group of 64-bits. Thats what happens when you have a drive fail in a two drive RAID 0 array.

    Now, if you have a drive failure in RAID 1, you shouldn't notice any instability, however until you replace the failed drive, you are running on borrowed time because if the good drive fails, you are sunk.
     
  10. Cromulent macrumors 603

    Cromulent

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    #10
    Uhm, software solutions will always be slower than hardware solutions as they have to use the CPU to do the processing instead of dedicated hardware.
     
  11. THX1139 macrumors 68000

    THX1139

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    Mar 4, 2006
    #11
    I couldn't disagree more. And since you didn't explain why, neither will I. I just popped in to throw some cold water on the spread of bad information. If your going to make statements like that, you really need to back it up with facts... else it's just crap.
     
  12. topgunn macrumors 65816

    topgunn

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    #12
    Well, I wasn't writing a thesis so I thought I could get away with not citing my sources but apparently I was wrong. I am not sure exactly which part of my post that you quoted is in question so I guess I will go over the whole thing. I hereby call BareFeats to the stand to testify on my behalf (specifically this article).

    Basically, the article is testing the stock PM G5 HD vs a Raptor and DiamondMax HD in both single and RAID format. You will notice that for random read/writes, the RAID setups are roughly twice as fast (in MB/s). This fails to mention that the average read/writes are only a few KB so the difference would be in ms. You will also notice that launch times and boot times are roughly the same. These tests more adequately depicts what your computer "feels" like. The RAID doesn't "feel" any faster. I suppose if your computers primary purpose is to duplicate 1GB files or to see who can get the highest transfer rates, then a software RAID may be for you. Also note the comments at the end specifically this paragraph:
    If you have two drives and need a scratch drive/partition, it is just as fast to have it on a dedicated drive as it is to have it on a RAID array which is shared with the OS and whatnot. This is because Photoshop (or whatever other application) is having to read and write from the same drive (ie mucho head movement which equals slower transfer times) versus two separate drives which can read and write simultaneously. So, back to my original post that you quoted, I said "I would not use RAID 0 via software RAID because the performance increase is not considerable and you double your chances of data loss to HW failure." I believe everything in that sentence is factual, at least for what I do (and a number of other people on these forums I do believe). As shown, the increase is not substantial and you do indeed double your chances of a HW failure (ie there are two drives to fail vs one).

    Now, things would be different if I had three or more drives.

    OK, I defended my side. Lets hear yours.
     
  13. CanadaRAM macrumors G5

    CanadaRAM

    Joined:
    Oct 11, 2004
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    On the Left Coast - Victoria BC Canada
    #13
    A range of independent tests have shown that for typical single user desktop use, a RAID (0, 3 or 5) does not improve real world performance and may actually be slower (www.storagereview.com).
    Real-world performance is not about maximum throughput numbers or XBench scores!

    Striped RAIDs come into their own when there is intense multiuser traffic (as in a file or database server) or in specialized cases where the streaming of continuous high volume data (such as HD video) can saturate an individual drive system. Even then, the advantage only really plays when there are multiple drive busses to support the RAID -- so for example a software RAID with two drives on a single Firewire controller (and despite there being 2 physical Firewire ports, they are still handles by one controller) is of marginal use, because the controller becomes the chokepoint.

    As has been mentioned RAID 0 has (for me) unacceptable risks. RAID 1 offers no performance benefit (and a slight write speed / CPU penalty) but does offer redundancy in case of a single drive failure. Note that this does not equal a backup, because if data is lost through deletion, overwriting, file corruption from the software, malware, etc, the damage is done to the mirror at the same time as the master. RAID 3 or 5 or 10/1+0/0+1 is expensive, and, as above, of dubious value for single user applications.

    My opinion is that for maximum performance, a multi-drive system should be deployed as individual disks, ideally placing each of the System, Scratch, Application and Data functions on a separate SATA drive, then have periodic online backup to massive external Firewire or eSATA drives that are portable/dockable.
     
  14. tyr2 macrumors 6502a

    tyr2

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    Leeds, UK
    #14
    Personally I'd recommend RAID 1 (mirroring) for your boot disks. I just had one of my mirrored pair fail and it was invaluable to just keep working rather than restoring from backup.

    RAID 1 isn't a replacement for a good backup system but if uptime is important to you then it can save a whole load of hasstle when a disk goes bad.
     
  15. THX1139 macrumors 68000

    THX1139

    Joined:
    Mar 4, 2006
    #15
    Topgunn,

    Okay, nice rebuttal. I agree with you in that situation where you have everything coming off a two disk raid. However, you will see some benefit over a single disk if you set up your raid as scratch and run the os and applications off a separate drive. Those people who setup raid as their startup and scratch are not going to notice a difference over having two separate drives.... one for os and the other for scratch.

    On my computer, I'm running a single disk just for OS and apps and a two disk RAID0 that is optimized for video and photoshop scratch; and I have another 500GB disk setup to store all of my files that are in my user folder. That user folder is backed up to firewire and DVD.
     

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