Does RAID-1 slow down the Mac Pro?

Discussion in 'Mac Pro' started by pprior, Jun 30, 2008.

  1. pprior macrumors 65816

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    Aug 1, 2007
    #1
    Wondering if I use RAID-1 for my primary drive to help safeguard my data, then i could have a third disk for scratch and a 4th for time machine, all internal.

    However I don't want to slow down performance.

    I do have external hot swap drives that I back up critical data and put in fireproof storage, please no comments on that whole topic, just insterested in the speed of software raid-1 native to mac or other options.
     
  2. matt321 macrumors member

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    Oct 27, 2007
    #2
    I can't answer just a mirror (Raid 1) but if you look at this thread http://forums.macrumors.com/showthread.php?t=266328&highlight=raid you'll find that Raid 0 and Raid 10 add, at most, 20% CPU usage. Since you have 4 cores, unless you max all 4 out, you will not notice a difference.

    What do you normally do with your Mac Pro?
     
  3. pprior thread starter macrumors 65816

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    Aug 1, 2007
    #3
    The only thing I do that routinely maxes out all 4 cores is handbrake encodes.

    I guess I'm not so worried about overloading the CPU as I'm worried about slowed disk writes and reads - that creates a much bigger bottleneck I would think.

    Generally I spend quite a bit of time in Aperture and Photoshop cs/3, also do a fair amount of Final Cut Express and then encoding with Handbrake as noted.

    Limited gaming and of course the usual Safari and mail use
     
  4. Phil A. Moderator

    Phil A.

    Staff Member

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    Apr 2, 2006
    Location:
    Shropshire, UK
    #4
    Software Raid (mirroring in this instance) will inevitably slow down disk writes as the system will have to write things twice. How much it will affect you is something I can't answer because I've never done it myself but it shouldn't have too much of an impact on a SATA system. It also depends on how the disk caching works - I suspect that mirrored setups may have write caching disabled for safety, which could have a big impact. If you do have write caching enabled on a mirrored setup, you really should have a UPS in too to protect against power outages.
    Disk reads shouldn't be affected at all and may even be speeded up depending on how intelligent the software raid implementation is on OS X (e.g. interleaving large reads from the two drives).
    Personally, if I was going to implement RAID in a machine, I'd be considering a hardware solution, although they're not cheap for the Mac Pro ($800 I think!) but do bring a lot of benefits. You can see it in all it's glory here
     
  5. Frozonecold macrumors 6502

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    Mar 23, 2005
    #5
    Why do you need a scratch disk? That is only for when you run out of memory. I only ask this, because many are buying high performance HDDs as scratch disks, when they could spend that money on more ram and get better performance.
     
  6. CanadaRAM macrumors G5

    CanadaRAM

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    On the Left Coast - Victoria BC Canada
    #6
    Applications such as Photoshop and many of the video production apps manage their own scratch space independently of the OS, and heavy duty work can be sped up by dedicating a scratch drive to them.

    The key to splitting the OS/App/Data/Scratch between separate (physical) hard drives is that you avoid a bunch of latency. On a single drive, the heads have to continually transit between data, scratch and application areas, since the nature of most programs is to alternate between reading and writing these rapidly. No work gets done while the machine waits for the heads to seek the next thing.

    By putting the functions onto separate drives, the different hard drives' heads can each remain 'on station' for faster response.
     
  7. Frozonecold macrumors 6502

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    Mar 23, 2005
    #7
    I understand that, but thats not what I was asking. I was wondering why you would want to get a fast scratch disk, if it's never going to be used due to ample amounts of ram. OR rather why spend $300 on a Velociraptor as a scratch disk when you can buy 8gigs of ram for that price. With 8gigs of additional memory I can't imagine ever using the scratch space at all. Plus memory can be accessed a lot faster than a HDD.
     
  8. pprior thread starter macrumors 65816

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    Aug 1, 2007
    #8
    LOL, I've got 8GB of ram and am frequently out.

    I went ahead and ordered a RAID-5 SATA external box. 5 x 1TB drives with 1 being a hot spare. Got sick and tired of being out of drive space.
     
  9. disconap macrumors 68000

    disconap

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    Oct 29, 2005
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    Portland, OR
    #9
    First of, as usual, CanadaRAM is 100% right. As for why you want a faster disk, scratch is written in the background during other operations, and the way it is managed in Adobe apps, if you are swapping between them (say Illustrator and Photoshop), the applications are spending a lot of time writing data to disc (which leads to slow app switching, long load times, etc). On the system disk, the heads have to work double time, which is why a dedicated drive is better. The faster the drive (provided it isn't bottlenecked by the bus speed), the faster the writes, and the faster the reads--anyone who has hit undo in photoshop and has seen a beachball can relate there, a fast dedicated drive would resolve that, or at least limit the length of time of beachballing. If you have 6gb of RAM (like I do) and you reach a point where Photoshop needs to write to scratch, you can be sure it's a ton of info, which is why I often use a RAID as my dedicated scratch drive, and it really does increase performance significantly. Trust me, when you are working with files that are more than 2gb in size, and you have history turned on in Photoshop, and there are 2 or 3 of them open...well, RAM gets filled pretty damned fast. ;)

    Of course, if you have enough RAM, work with small enough files, and don't swap between apps, it's not even an issue...
     
  10. Frozonecold macrumors 6502

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    Mar 23, 2005
    #10
    I wasn't doubting CanadaRAM, I was just wondering how often the scratch is needed. I personally have never had a 2Gb Photoshop file.
     
  11. Tbarr macrumors member

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    Mar 27, 2008
    #11
    Please explain where the 20% CPU usage comes from.

    From your own link;
    Single drive - NO RAID - CPU usage 3.6%

    Two drives - RAID 0 - CPU usage 4.6%

    Four drives - RAID 10 - CPU usage 4.5%

    From your own link, the RAID adds ONE PERCENT (01) CPU Usage.

    How did this grow to 20%?
     
  12. dchao macrumors 6502a

    Joined:
    May 20, 2008
    #12
    When using OS X RAID-1 option, for every DMA transfer to the hard drive, OS X has to set up two DMA transfers now. Even the DMA transfer is performed in the background in dependent of the CPU, it still takes up processor time to setup two transfers instead of one. Since the disk IO's are all sequential, the next block has to be delayed.

    The only way is to use a hardware RAID controller, so the CPU is doing the same number of disk IO's, the RAID controller is then in charge of the mirroring.
     
  13. NRose8989 macrumors 6502a

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    Feb 6, 2008
    #13
    If I'm not mistaken, Final Cut Express 4 can only utilizes a little over 2.5 GB of RAM (at least thats the most it allows me to specify in the system preferences of final cut). Everything else gets written to the scratch disk, which can get huge very quickly (6GB+ for a 5 min. project).
     
  14. disconap macrumors 68000

    disconap

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    Oct 29, 2005
    Location:
    Portland, OR
    #14
    Where are you getting these numbers? The screen shots clearly say:

    RAID0 (2 drives only) -- Block o/p: 35% Block i/p: 11.4%
    RAID10 -- Block o/p: 37.7% Block i/p: 13.4%
    RAID01 (a RAID1 built of 2 RAID 0s) -- Block o/p: 39.7% Block i/p: 14.5%
    RAID0 (4 drives) -- Block o/p: 74.7% Block i/p: 25.5%
     
  15. matt321 macrumors member

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    Oct 27, 2007
    #15
    @Tbarr

    Those numbers are only for seeks.

    I was looking at the "Sequential Input" "Block" section, which would be a useful number to look at since we were talking about Raid-1 (Output would stay the same since it doesn't matter which drive we read from AND OS X does not do sequential reads from a mirror.)


    Those numbers:

    Single Drive - 7.7%

    Raid 0 - 11.4%

    Raid 10 - 13.4%


    I have personally seen, on my own setup, it go as high as ~20%.
     

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