Software RAID

Discussion in 'Mac Pro' started by goodcow, Oct 27, 2009.

  1. goodcow macrumors 6502a

    Aug 4, 2007
    I bought two Western Digital 320GB RE3 drives about a month ago to do a software RAID 0 to capture 720p uncompressed.

    Well, apparently they aren't fast enough to reliably handle the necessary data rate all the time. I probably should've gone with the Black line, but I figured "hey, Enterprise grade is more reliable!" At least they were only $50 each or so.

    So now I'm wondering if I should just buy two Blacks to replace these, if I should buy another RE3 or just add another Black to the mix.

    My question basically boils down to, can you mix and match drives (brands and size) using the OS X software RAID? And then secondary to that, what do you guys think I should do of the three options below? (add another RE3, replace with two Blacks, keep the two RE3's and add a Black)

  2. nanofrog macrumors G4

    May 6, 2008
    I hate to say it, but it's poor planning, not the difference between the RE3's and Caviar Blacks.

    Understand, that not all the drives in the same family will perform the same. In the case of the 320GB RE3's, it's a single platter drive. It won't deliver the performance of the 1TB units, as those have 4x platters running in parallel, which gives higher throughputs, and allows you greater capacity. Ideally, you want to keep the capacity to 50% full or less, as it keeps you off the inner tracks (the slowest on the drive).

    At this point, you'd either need to return the RE3's you have (if possible), or get more for parallelism in the array (increases throughput, and seems to be the issue here). Just not enough drives to provide the needed throughput.

    Usually, you want to work backwards. Figure out the throughput requirement for the use you intend, then divide by the avg. throughput of a given drive in the array type intended. A stripe set is easy, as it's x throughput * n drives for the total average throughput.

    If you can send the RE3's back, get larger capacity drives or more of the existing drives (or of any smaller capacity). With the RE3's, figure an individual performance of 85MB/s (seems to be avg. for the family, but the larger units will go a little faster, but not drastically so).

    I'm thinking you didn't get enough to run in parallel (assuming there's no other issues, as there's no information to derive any causality if that's the case).

    Technically, you can mix and match drives. But the array is based on the slowest drive, and in the case of enterprise + consumer model mixes, the reliability will be based on the least reliable unit as well. These two models happen to be close, but there are a few differences. Namely the sensors, and it results in a higher MTBF for the RE3 line vs. the consumer version (Blacks).

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