Is Mirrored RAID 1 Slow ?

Discussion in 'OS X' started by fab5freddy, Mar 9, 2008.

  1. macrumors 65816

    fab5freddy

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    #1
    Will Mirrored RAID 1 slow down my system alot....?
    compared to no RAID at all...?

    Mirrored RAID seems perfect for me,
    but i hope it isn't gonna slow down the system to much......?

    What if you take 2 Raptor 10 K drives and make a mirrored RAID 1,
    that would seem like a nice set up...?

    thanks!
     
  2. macrumors 601

    sammich

    Joined:
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    #2
    A RAID 1 setup is for redundancy, it's so that should one drive fail you don't lose ANY data.

    So that means that you effectively only have 1 single drive to work with (but absolute peace of mind). It will perform roughly the same speed (maybe a bit slower) of a single drive.

    Before you go out and buy a 10K Raptor drive, note that many of the larger 7200rpm drives are just as fast, offer much larger capacities and are far less noisy (and cheaper per gig).

    Typically, for ordinary use, drives rarely fail, a simple manual backup (to a second drive) is far more cost efficient for the home application.

    If you have a lot of money to waste, why not go for RAID0+1 (2 pairs in RAID 0, those pairs in RAID 1)
     
  3. thread starter macrumors 65816

    fab5freddy

    Joined:
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    #3
    ok, so if i did a mirrored RAID with 2 raptors, that would give me:

    10,000 RPM x 2 = 20,000 RPM

    or

    7200 RPM x 2 = 14,400 RPM

    speed is an relavant factor in professional tethered photo shooting.....
    so, i am looking for the quickest write speeds...

    any advice how to find the quickest write speed HDD's ?

    thanks!
     
  4. macrumors 601

    brand

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    #4
    Sorry RAID 1 does not work like that.
     
  5. thread starter macrumors 65816

    fab5freddy

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    #5
    ok, then how does it work ?
     
  6. macrumors 601

    sammich

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    #6
    Okay, you could think that it works like that EXCEPT you're missing one bit of the equation:

    ( 7200rpm x 2 ) / 2 = 7200rpm (- some overhead)

    Since you are mirroring two drives the two drives work in unison, when you write data to the RAID setup, the EXACT data is being written to both drives, meaning you don't get double the speed.

    The question now is: are you looking for insurance against data loss? or are you looking for plain performance? Or Both?
     
  7. thread starter macrumors 65816

    fab5freddy

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    #7
    Sorry, what i meant was doing a Striped RAID 0......

    Would that then be correct with those numbers i gave in the OP...?
     
  8. macrumors 601

    sammich

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    #8
    You would be logically correct but not technically.

    But never mind that, you want to Stripe Two drives together? Okay, so for your photo shoots, how much space would you be using, and what is your estimated maximum consistent writing speed would you need?

    The largest drives, from 7200rpm 750+gigs have a consistent write speed from at least 50mb/s, so double that would be 100mb/s.

    Check Toms Hardware site for a chart of speeds.

    That list also includes the 10K drives, so that should give you some insight to which drives would suit you.
     
  9. macrumors G5

    CanadaRAM

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    #9
    No, you don't get a doubling of performance, it doesn't work like that -- any more than if you have a car that can go 120 MPH, if you put two motors in it, it can go 240 MPH.

    In fact for most single user applications, the overhead of the RAID processing will be higher than the potential speed gains.

    RAID 0 (or other forms of RAID striping) is mainly useful in two situations:
    1) Multiuser servers
    2) Handling massive, sustained data streams, such as capturing HD video

    RAID 0 is also horribly risky, an error on one drive will kill the data on both drives.

    Raptors run hot and noisy. And because they are small drives (both in capacity, and the physical diameter of their platters), they drop in performance rapidly as they fill up, and start moving to the inner tracks of the drive. The outermost tracks of a platter are up to 2 x faster than the innermost tracks, because more data passes under the heads in a single revolution.

    This is also why the newer 7200 RPM drives nearly match the 10000 RPM Raptors in performance. They have larger platters, and with perpendicular recording, higher areal density. So even though they spin slower, they pass as much or more data under the heads every second. Plus, if you have 75 GB of data on a 150 Gb Raptor, you have already cut the performance by 25% or so 'cause the drive is 1/2 full. But a 750 Gb Barracuda or F1 will still be running on almost the outermost tracks, at near peak performance.

    A better strategy, then, is to get a couple of Perpendicular drives of 500 - 750 GB. Dedicate one drive to System and Applications, and the other drive to Data. This separates the two functions between 2 sets of heads, which reduces seek times as the software switches back and forth between data, application code, and scratch disk. If you really want to gild the lily, then get a third drive to dedicate to scratch disk / swap file space.
     
  10. thread starter macrumors 65816

    fab5freddy

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    #10
    thanks CANADA RAM, this is some very useful knowledge you have shared.

    Useful Knowledge these days is hard to come by ; )
     

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