Everything you ever wanted to know about Software RAID, but were afraid to ask!

Discussion in 'Mac Pro' started by timb, Jan 4, 2007.

  1. timb macrumors regular

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    Jun 6, 2003
    #1
    So I just got in four Seagate 7200.10 Perpendicular Recording drives. I've set them up in various configurations on my mac pro and benchmarked them.

    Specs:
    2.66 Ghz (x2) Mac Pro with 2 GB of RAM (512 MB x 4)
    Seagate 7200.10 250 GB 16 MB Cache (x4)

    Clean install of 10.4.7 booted from a 2nd Gen iPod.

    We're using the original Bonnie to do the tests with 2 GB files.

    Something else of note; The CPU percentages listed below are out of a maximum 400%, due to having four processors. I'm also basing my read and write speeds off the block transfers and not sequential, as that's a bit more realistic for day to day operations.

    We tested 4 different RAID modes, and did a single drive test as a baseline.
     
  2. timb thread starter macrumors regular

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    #2
    Single Drive

    Okay, just one drive. We're getting about 48 MB/s write and 58 MB/s read. This is a bit higher than the stock Mac Pro 250 GB drives. This is about average and what I expected.
     

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  3. timb thread starter macrumors regular

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    #3
    RAID 0 (Two Drives)

    Okay, two of the drives in a striped configuration.

    156 MB/s write, 110 MB/s read. That's actually a little higher than I expected, a good speed overall. The CPU usage was a bit higher than I liked, but not a deal breaker since we have 4 CPUs. This mode is also riskier to run, as you're twice as likely to lose data due to drive failure.
     

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  4. timb thread starter macrumors regular

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    #4
    RAID 10 (4 Drives)

    Okay, so next up is RAID 10. This is a stripe of mirrors. That is to say we broke the 4 drives up in pairs:

    Mirrored Pair A = (Drive 1) (Drive 2)
    Mirrored Pair B = (Drive 3) (Drive 4)

    Then we took those two mirrored arrays and striped them:

    Stripe = (Mirrored Pair A) (Mirrored Pair B)

    155 MB/s write, 116 MB/s read.

    This is on par with the two drive RAID 0 speed wise, and only one or two percent higher CPU usage. This is as expected, and makes a great choice for both speed and data redundancy. The array can always tolerate the failure of one drive, and sometimes two.
     

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  5. timb thread starter macrumors regular

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    #5
    RAID 01 (4 Drives)

    Alright, now down to RAID 01... This is a mirror of stripes.

    Stripe A = (Drive 1) (Drive 2)
    Stripe B = (Drive 3) (Drive 4)

    Mirror = (Stripe A) (Stripe B)

    We're getting 155 MB/s write, 125 MB/s read from this setup.

    This is not as expected. Because we're doing mirroring on the front end here, we theoretically should be getting around 250 MB/s read speed (though the write is correct). The problem seems to lie in the fact that Apple's Software RAID cannot stripe reads from a mirror. Very disappointing, as this could be a very impressive RAID level. You'd be able to lose at least one drive from the array, but you'd have darn impressive read speeds. Maybe Apple will fix this in 10.5, but as it stands right now I do not recommend this mode.
     

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  6. timb thread starter macrumors regular

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    #6
    RAID 0 (Four Drives)

    Alright, 4 drives in RAID 0. All I can say is wow!

    310 MB/s write, 235 MB/s read. But look at those CPU numbers, 75% on the write, ouch! This is an all in all risky RAID mode. You have a four times higher chance of a drive failure here, and a high CPU overhead. In fairness however, you do get 4 x Drive Capacity worth of storage, and if you're doing propper backups anyway (which you should be, as mirroring RAID is not a replacement for backsup), then you could be fine.
     

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  7. timb thread starter macrumors regular

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    #7
    Installation Errata

    One thing I should mention, you cannot setup RAID 01 or RAID 10 via Disk Utility on the Mac Pro Installation Disc. It just doesn't work. I talked to a friend at Apple and he checked and confirmed that it is a known bug. It's a issues with the pasteboard they use on the CD, not allowing drag and drop to work properly (which is required for multi-array RAID setups). What you'll need to do is install 10.4 to a firewire drive, boot from that, setup your array that way then do a normal install.
     
  8. erkanasu macrumors 6502

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    Jan 11, 2006
    #8
    lol im using mactheripper to rip Everything you ever wanted to know about Sex, but were afraid to ask! right now and stumbled on this thread! hah though i don't know what to ask about raid unfortunately. BTW this is a fantastic woody allen film for those who havnt seen it
     
  9. aneks macrumors regular

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    Aug 29, 2006
    #9
    pretty much the same as I am finding with my disks. I am using drive 1 as OS and drives 2 and 3 as RAID 0 and getting in the realm of 140mb/s read and 110mb/s write.
     
  10. PmattF macrumors member

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    Dec 28, 2006
    #10
    How much impact do these faster test numbers have on real world tasks? I.e. what sorts of things will go how much faster under RAID?
     
  11. Pressure macrumors 68040

    Pressure

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    #11
    Video-editing :D
     
  12. timb thread starter macrumors regular

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    #12
    Anything that uses heavy disk access; Video editing, audio editing, photo editing. It should also help you a great deal if you constantly load and unload applications. It will also help level and general load times in games.
     
  13. PmattF macrumors member

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    #13
    Does it make sense to buy a Mac Pro with its standard smallest disk, and use that as the system disk, then add two more disks in RAID 0 as data/scratch?

    Or would also replacing the system disk with a RAID 0 make a really big impact too? If so, what do you do with the original disk?
     
  14. timb thread starter macrumors regular

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    #14
    It was a real toss up for me between RAID 10 and RAID 0 (4 Drive). I'm going to burn these drives in for a month under RAID 10, and if they're still going strong then, I might switch to RAID 0 and pick up a 1 TB external firewire drive to perform backups on.
     
  15. timb thread starter macrumors regular

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    #15
    Actually, that's what a lot of people do. They go for a moderate sized base disk, and then get two fast drives and put them in RAID 0 for data / scratch.

    If you went that route I'd recomend getting a decent sized primary drive, and then perhaps two really fast 10k RPM Raptor drives, put them in RAID 0. That should give you really great performance for your scratch.
     
  16. livingfortoday macrumors 68030

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    #16
    I don't think OS X can do this through software, but do you have the means to test out RAID 5 with your system? I'd be interested in seeing how that stacks up.
     
  17. timb thread starter macrumors regular

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    #17
    Sadly, no support for software RAID 5. It would tax the CPUs a decent amount anyway, plus writes would be pretty atrocious without a cache (though, I suppose they could be clever and cache writes to RAM, but then if you have a power failure you're screwed).

    I have a "hunch" that Apple will be releasing a PCI-e RAID card that will be Xserve and Mac Pro compatible sometime in the near future though. When that happens I will pick it up and post the results here.
     
  18. livingfortoday macrumors 68030

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    #18
    Well, I'm fairly sure that OS X server supports software RAID 5, but not the regular OS X. I guess it doesn't matter much, though, as I can't afford that!
     
  19. timb thread starter macrumors regular

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    #19
    I've been told that, but I don't have an option for it on my PowerMac G4 or Xserve G4 (both running 10.4 Server). I'll do some more research on it and see what I can find.
     
  20. Aniej macrumors 68000

    Aniej

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    Oct 17, 2006
    #20
    I am surprised that I have never had to ask this before, but how do you propose or make a thread sticky? I think if this one continues to develop we might want to do make it sticky since there are a decent number of questions about this topic, but often they are very long or people are completely clueless about this. Personally, I fall into the first problem, but in terms of raid, I am clueless.
     
  21. livingfortoday macrumors 68030

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    #21
    Well, on the XServe page, they talk a lot about RAID 5

    But I don't know if that's software based, or just hardware that comes with the new XServes.
     
  22. timb thread starter macrumors regular

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    #22
    It's hardware based. The new Intel Xserves have PCI-X / PCI-e slots on them. Apple does have a PCI-X RAID 5 card that's fully hardware, it works in the G5 Xserves, G5 Towers and Intel Xserves. They have yet to release a PCI-e solution, but I've been given the impression it's in the pipe.
     
  23. timb thread starter macrumors regular

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    #23
    It would be great if we could get this stickied. That was one of the main reasons for writing this all up, so we could have a comprehensive overview of the RAID solutions.
     
  24. tyr2 macrumors 6502a

    tyr2

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    Leeds, UK
    #24
    Interesting write up thanks. Nice to see the 7200.10's working properly, I've seen some threads with people having issues. Could you post the exact model numbers of the drives?

    Just one comment.

    RAID01 and RAID10 are expeceted to give roughly the same performance. I certainly wouln't expect a doubling in read speed.
     
  25. timb thread starter macrumors regular

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    Jun 6, 2003
    #25
    Under ideal conditions RAID 01 will give higher reads, here's why:

    You've got two striped arrays that are being mirrored. Under a true RAID 1 mirror, read speeds are increased, because it can stripe the reads from all the mirrored drives. So, in the case of RAID 01 read speeds could be increased. I know, I know, you're thinking, "Wait! RAID 10 has mirrors as well, it's just backwards, but you should still get a read speed boost!" The short answer, is no. When the mirrors are on the other end, the overheard is to high to see a boost. (At least that's my understanding of why it didn't work in 10... It seems a little fishy to me, I'll research it a bit more and update accordingly.)

    This has been my experience with a 3ware card that supports 01 and 10, at least.
     

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