External RAID via "hidden" SATA Question...

Discussion in 'Mac Pro' started by dsa420, Jul 21, 2009.

  1. dsa420 macrumors 6502

    Joined:
    Feb 23, 2007
    #1
    So i just went through the cumbersome task of installing the Newer Tech SATA extender and connected to my OWC Mercury Elite-AL Pro Dual Bay enclosure. Inside the enclosure are a 1TB and a 1.5TB drive.

    When I want to set up the striped RAID arrangement via disc utility it shows the drives appropriatley as 1TB and 1.5TB respectively. However, when I set up the striped RAID arrangement it estimates the disc size as 2TB as opposed to 2.5TB as it should. Once the set up is complete is shows the RAID arrangement as 2TB as opposed to 2.5TB...

    Any ideas, am I doing something wrong here?
     
  2. gugucom macrumors 68020

    gugucom

    Joined:
    May 21, 2009
    Location:
    Munich, Germany
    #2
    You cannot stripe asymmetric drives. So effectively the RAID controller uses only 1 TB of the 1,5 TB drive. That cannot be changed. It is inherant to the technology.
     
  3. Tesselator macrumors 601

    Tesselator

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    Location:
    Japan
    #3
    That's normal. In most RAID levels (0, 1, 10, etc.) you only get the size of the smallest disk multiplied by the number of members (drives) (and in a RAID 1 or 10 it's also divided by 2). So that's right. :)

    If you want the extra space you can partition it first to 1TB and 500GB and then use that 1TB partition as the RAID member with the other 1TB drive - leaving you with a 500GB free standing volume. The 500GB partition will compete with the then 2TG RAID set if you try to access them both at the same time but other than that you'll be good.


    .
     
  4. dsa420 thread starter macrumors 6502

    Joined:
    Feb 23, 2007
    #4
    That makes sense, sorry for the stupid question. The 1.5TB drive is new and I can take it back and get a 1TB drive, if speed is my main goal, would I be better offer returning the 1.5TB for a 1TB drive?

    It sounds like if I partition than it will slow down my system and take away from my goal of attaining max speed.
     
  5. dsa420 thread starter macrumors 6502

    Joined:
    Feb 23, 2007
    #5
    Actually, to add a layer of complexity to the situation, I also have (3) 1TB drives that are internal striped together. Should I go ahead and stripe the (4) together and then leave the 1.5TB on its own?
     
  6. Tesselator macrumors 601

    Tesselator

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    Japan
    #6
    It won't mess with the speed unless you're reading and/or writing to the stripe and the 500GB partition at the same time.


    That sounds better. :)

    I wonder if the "Add New Member" feature of RaidEye will make this any easier? http://www.hanynet.com/raideye/index.html Give it a try and report back. :)


    EDIT:

    Ah, nevermind:
     
  7. JPamplin macrumors 6502

    JPamplin

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    Mar 12, 2009
    Location:
    Nashville, TN
    #7
    Hold on there, cowboy - there's a question that needs to be answered here. The marginal difference between a 3-drive RAID0 and a 4-drive RAID0 is pretty insignificant, especially if you're doing it in software via Disk Utility.

    I would ask you something, then I would recommend something...

    ASK: What are you using this for? Photoshop? If so, it will be best for you to assign your scratch disk on a different volume than your system/swap disk. If you want to get the most speed out of that, then you want a separate volume.

    RECOMMEND: SPAN your 4th 1TB and 1.5 TB drives together to create a 2.5 TB Time Machine / Photoshop scratch drive. It should be enough to handle your 3TB boot volume, as long as it's not full.

    UPDATE: Here's an interesting tidbit - RAID0 performance under 10.5.7 versus 10.6, at least in this current build (411), is absolutely the same. There doesn't seem to be any optimizations that XBench can pick up. I'll attach the XBench files for each HD test.

    JP
     

    Attached Files:

  8. teeck2000 macrumors regular

    Joined:
    Jun 20, 2009
    #8
    Try to stay with the same exact types of drives when doing a RAID, the performance is better. I saw a huge performance drop when I was mixing different sized disks, with the Software RAID 0.
     
  9. Tesselator macrumors 601

    Tesselator

    Joined:
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    Japan
    #9
    @JP,

    What you say is incorrect.

    There is a very significant difference between a 3-Drive RAID0 and and a 4-Drive RAID0 both in terms of throughput AND seek access speeds.

    If your 3-Drives RAID0 averages 280 MB/s for example then the 4-Drive RAID0 will probably average around 360 MB/s. If your 3-Drive RAID0 averages 360 MB/s then the 4-drive RAID0 would average something close to 450 MB/s.

    Spanning offers no speed increase at all. So not better than single volumes.

    The only time spanning is to your advantage is when you have a file to store that is larger than the individual drive volumes, if you have many tiny drives and you want to make one larger one, or if you just prefer to have everything in a single logical volume. Spanning can be bad in that seek speeds can become unbearably slow under many conditions. Disks are merely concatenated together, end to beginning, so they appear to be a single large disk. This is sometimes also called JBOD, or "Just a Bunch Of Disks".

    Additionally "software" RAID on the Mac through Disk Utility is as good or better in terms of speed, than any RAID controller in the sub-$500 range I'm familiar with.
     
  10. JPamplin macrumors 6502

    JPamplin

    Joined:
    Mar 12, 2009
    Location:
    Nashville, TN
    #10
    Ah, thanks for the clarification - I was probably thinking about Windows.

    But don't you think separating a Photoshop scratch disk onto a separate drive than swap would be best, regardless of the increase in RAID speed?

    JP
     
  11. Tesselator macrumors 601

    Tesselator

    Joined:
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    #11
    If I understand your sentence correctly then I used to think that - yes. But I do no longer. Unless maybe you're going to be editing video, recording multiple audio streams, and forcing PS to use it's scratch by editing MASSIVE (or many!) images - all at the same time. Then maybe. A few points should clarify:

    • The Photoshop application itself is the speed bottleneck for scratch. I just tested this not long ago for a post here at MR and I don't remember exactly right now but it was something quite ridiculous like 1 MB/s or something.

    • If you have a generous amount of RAM installed and available, PS may never actually use the scratch disk in a significant way:
      http://kb.adobe.com/selfservice/viewContent.do?externalId=kb404440
      Some bits from the link:
      "Photoshop can directly access 3.5 GB of RAM when run on Mac OS (10.4.11 or later). If you have additional RAM on your computer, the RAM above 4 GB is used by the operating system as a cache for the Photoshop scratch disk data. Data that previously was written directly to the hard disk by Photoshop is now cached in this high RAM before being written to the hard disk by the operating system."
      And judging from observable behaviors OS X doesn't write this out until you execute another application which asks to use the high memory - and this system swapping is fast! About a tenth of a second lag occurs for a 24 MPx image with 10 or 15 layers when PS becomes the focus again under those conditions - after the 2nd application relinquishes the memory to took from PS.

    • If you're getting very many page-outs when using PS then it means you need to add more RAM to your system or change your workflow such that fewer applications are open during PS sessions. Page-outs are slow. But OS X page-out and Photoshop's proprietary virtual memory technology (AKA Scratch Disks) together are not enough to tax a modern drive - even a single drive. Back when HDDs were around 1GB or less and the IDE interfaces were ultra slow, there might have been some contention. In a Mac Pro this just isn't going to be noticeable unless you were silly enough to put a scratch partition on the inside of a drive which is almost full and is being used by OS X for heavy swapping - then the raking seeks may impose a further penalty.

    • With 12 GB of RAM I can typically have open and be working with five to ten 24 MPx 16-Bit images while running Safari with 20+ tabs, iTunes with 50,000 songs loaded in, my bit torrent with ~1,000 finished files - 50 seeding and 5 or 10 DLing, plus a buttload of other small apps and BG apps (about 150 processes): and at the end of the several days of working like that, end up with over 10,000,000 page-ins and less than 500 page-outs. This is everything all on one RAID0. And I perform fairly extreme edits on those images usually. This would indicate to me that PS in extreme but proper use, living in 12 GB of memory, never meets conditions which would require the scratch disk to be on a separate drive from OS X's swap.

    Now, the term "proper" used in that last point is a subject that can be expanded to a fairly large book but given that it's already known (and most of it is common sense plus a little math), this summarizes the case. That said the OP didn't mention anything about using Photoshop. :D
     

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