RAID 0 write speeds?

Discussion in 'Mac Pro' started by Doc69, Jan 12, 2013.

  1. macrumors 6502

    Joined:
    Dec 21, 2005
    #1
    If I create a RAID 0 volume out of two external drives, is the maximum write speed determined by the slowest drive, i.e. if one drive is USB 2.0 and the other eSATA, will max write speed be same as if I had two USB 2.0 drives, or will the eSATA drive in fact speed things up?
     
  2. derbothaus, Jan 12, 2013
    Last edited: Jan 12, 2013

    macrumors 601

    derbothaus

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    #2
    The maximum is the sum of both drives together. So if you stripe 2x HDD's capable of 120MB/s write's you could get 240MB/s out of your RAID set. They are working together to expand. RAID1 is different. Other RAID levels require more than 2 drives.

    http://en.wikipedia.org/wiki/RAID

    Read the RAID level guide.

    Edit: Just re-read. Sorry. Just try it and see. Not too many people would do that as it is completely shakey.
     
  3. macrumors 68030

    wonderspark

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    #3
    I have not tried such unorthodox RAID volumes, but if it did in fact work at all, my guess would be that the speed would be that of the slowest drive x2 in a 2-disk RAID 0.
     
  4. hfg, Jan 12, 2013
    Last edited: Jan 12, 2013

    hfg
    macrumors 68030

    hfg

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    #4
    Personally, I would never build striped RAID arrays out of mis-matched drives!

    I really wouldn't put them on different interfaces! ! !



    I always use drives (SSD and HD) of the same brand, model, and revision and have never had a problem over the years I have had them in operation.
     
  5. macrumors 68030

    wonderspark

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    #5
    I agree, and I believe the attempt is destined to fail, but then again I'm curious to see what happens. Am I a jerk for that? :p
     
  6. macrumors 6502

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    Nov 10, 2006
    #6

    +1

    FWIW, I just set up a RAID 0 in my Mac Pro using two 1TB WD Caviar Black 7200RPM 64MB Cache drives, and they're running great! Now I have a 2TB boot drive, getting ~260MB/s reads and writes according to Black Magic Disk Test, and it only cost me $180.
     
  7. hfg
    macrumors 68030

    hfg

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    #7
    Now all you need is a RAID-0 SSD pair for boot/apps to go along with your RAID-0 Hard Disk data array and your Mac Pro will really scream! :D

    My old 2008 Mac Pro has been running that setup for quite some time and it really brought new life to that old computer. :)


    -howard
     
  8. thread starter macrumors 6502

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    Dec 21, 2005
    #8
    Thanks guys for your replies. I've done software RAID 0 for my backup drives for years and never had any problems (2 or 3 drive configurations). The reason for doing RAID 0, is that I have a lot of data, over 15TB or so, and with two full backups and three of some important stuff, that's almost 50TB of hard drives. As hard drives have become bigger, I have had to RAID smaller drives to match the volume size of the larger ones, like 2x1.5TB or 3x1TB to match one 3TB drive.

    I have now noticed that some of my internal drives can not be mounted simultaneously on the same interface (FW 800) without automatically unmount (perhaps because I already have 6 internal drives in my Mac Pro). However, if I mount one via USB, then it works. So for that reason, it makes sense for me to make a RAID 0 volume from one bare HD connected via FW800 and one regular external USB 2.0 drive. But if the volume gets slow as if it was made of two USB 2.0 drives, then I may not do it.

    If no one knows for sure, then I will just have to test it myself. But since all my drives are already full, I would have to trash two full 3TB sets to test it. And I had hoped not to have to do that unnecessarily.
     
  9. macrumors G3

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    #9
    If by RAID 0 you mean striping, each disk would write at its own speed.

    Now as for using RAID 0, you do realize one disk dying wipes out all the information on the raidset, right?
     
  10. thread starter macrumors 6502

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    Dec 21, 2005
    #10
    If each disk writes at it's own speed, I imagine that the backup would not be ready until the slowest disk has finished writing? Which would take the same time as two slow drives?

    That the whole RAID set is lost if one disk dies is not a problem for me. I always have other backups. But it is very convenient to be able to make one 3TB volume out of two or three disks to match the original drive size. I then use Intego Backup Express to duplicate the volumes.

    Also, I expect all my disks to fail at some point, and when they do, I just whip out a new one and redo the backup. Simple stuff I've learned the hard way years ago. In my view, you don't have a backup unless you have two or three.
     
  11. hfg
    macrumors 68030

    hfg

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    #11
    RAID-0 is for SPEED! I would not recommend having your backup array in a RAID-0 as there is no fault tolerance ... 1 disk dies and all your data is gone. :eek:

    If you are just trying to make more disk space, you should span (concatenate) your disks (sometimes referred to as JBOD). This still has no redundancy, but at least you don't lose it all. Your total disk will be the sum of all the disks included, and they can be different disks (unmatched).

    Best for your backup is 2 disk RAID-1 mirroring or a multi disk RAID-5 or 6 for large arrays. :)

    RAID-0 is great for your working disk however ... just make sure you have a backup strategy. :) :cool: :)


    -howard
     
  12. macrumors 68030

    wonderspark

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    #12
    Sounds like Doc69 isn't worried about his backup strategy. I just want to know the speed result of putting two external drives with different interfaces into RAID 0, or if it even works.
     
  13. thread starter macrumors 6502

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    #13
    As I'm not worried about redundancy, RAID 0 gives me both a large volume AND speed. When writing 3TB volumes to external drives, the extra speed from RAID 0 is very welcome.
     
  14. macrumors G3

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    #14
    If you understand how RAID 0 works, then you would understand it's alternating between disks, with each disk reading/writing at its own speed means that the slowest disk only affects its own speeds.
     
  15. Loa
    macrumors 65816

    Loa

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    #15
    Hello,

    Simple to calculate, even very precisely if you want.

    Normally, each drive in a 2 drive RAID0 set only writes 50% of the data. As they both write at the same time, the time needed to write is reduced to 50%. (Rough approximation.)

    On your set-up, one of your drives will be slower than it could, let's say it will write its data in 75% of the time (Instead of 50%). In this case, **IF** the RAID0 survives this strange dual speed set-up, the slower drive (or connection) will dictate how fast the entire array will work.

    I you want to know how fast the RAID0 will be, just compare the speeds on your two individual drives using the actual connections. If your USB drive is 50% slower than your esata drive, then using a RAID0 won't help you at all.

    My guess is that your usb drive will be even slower than that, defeating the entire purpose of a RAID0.

    My advice: set your drives as a JBOD: it won't be significantly slower than a RAID0 would be with your planned set-up, and it will be safer for your data. Also, the JBOD will have a lot more chances of surviving the strange dual speed set-up than the RAID0.

    Loa
     
  16. macrumors 601

    GermanyChris

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    #16
    why would you stripe BU..

    Do you need to BU really fast? just do a soft JBOD you'll get the full use of all the disk's space and a least a little data retention.
     
  17. thread starter macrumors 6502

    Joined:
    Dec 21, 2005
    #17
    Thank you all for your replies. I think I'll skip the idea of mixing interfaces then and just stick to striped disks with the same speed and interface. This is what I have always done.

    The reason for wanting to blend interfaces is that since I already have so many internal drives connected to my Mac Pro, sometimes when I add more external ones via FW800, the external automatically unmount. This does not happen if the drives have different interfaces, like FW800 and USB.

    To GermanyChris:
    I like to stripe BU drives because I get twice the speed. Makes a big difference when you do 3TB at a time. But I would not stripe if I had enough 3TB drives. But since I have many 1.5TB and 1TB, I use them striped for backup of my main 3TB drives.
     
  18. macrumors 601

    GermanyChris

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    #18
    I have 15TB to back up the first was horrendous the rest were nothing..I raid 5 you still should JBOD
     
  19. thread starter macrumors 6502

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    Dec 21, 2005
    #19
    Sure, after the first backup, subsequent incremental backups takes less time. However, I frequently like to redo the full backup. In my experience, I can not trust that a file is OK until it has actually been read from the disk. And when doing incremental backups, it doesn't seem like the file is actually read, only the attributes are compared. But when copying, the file has to be read.

    So when doing these full backups, which can take many hours, sometimes days depending on the interface used, any speed boost you can get is welcome. So I'm sticking with RAID 0.

    Also, over the last few days I have done a lot of testing. I have combined various different drives sizes and makes with different interfaces to create striped disks. I have not had any trouble with any combination, i.e. FW800 with USB 2.0, 1.6TB with 1.5TB, quad-disk or dual disk enclosures with single bare drives etc.
     
  20. macrumors 601

    GermanyChris

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    #20
    I've said my peace..
     
  21. macrumors 68030

    wonderspark

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    #21
    This is interesting... thanks for testing that! Is this all software RAID via OSX Disk Utility?
     
  22. thread starter macrumors 6502

    Joined:
    Dec 21, 2005
    #22
    Yes, the RAID volumes were created in Disk Utility as "Striped RAID Sets".

    And by the way, I have had a few hardware raid systems in the past (from Promise and a couple of others). But they have been nothing but trouble. With software RAID through Disk Utility, I've never had even one problem over the years.
     

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