Promise Pegasus R4 Performance Tests

Discussion in 'Mac Accessories' started by joema2, Jan 1, 2014.

  1. joema2 macrumors 65816

    joema2

    Joined:
    Sep 3, 2013
    #1
    I've done extensive performance tests on the Promise Pegasus R4 Thunderbolt disk subsystem. Config = 4 x 2GB Toshiba 7200 rpm SATA drives, unformatted capacity = 8TB, RAID 5 formatted capacity = 6TB. Computer = 2013 iMac 27, i7@3.5Ghz, 32GB RAM, OS = Mavericks 10.9.1

    My main priority was isolating the huge variations in RAID synchronization time reported by different users. However I also did regular performance tests.

    Problem: before it can be fully used, the Pegasus R4 in RAID 5 must undergo one-time "synchronization", whereby RAID data is written to all drives in the array. Supposedly this takes 10-15 hr, but some users report 60, 70, or 80+ hours.

    Finding: RAID sync time is directly related to stripe size. The default stripe size is 128k, which produces a sync time of 90+ hours. Using the provided Promise Utility to select a 256k or larger stripe size reduces sync time considerably. For 512k stripe size, sync time is about 10 hours. See attached graph. This is not documented in any Promise articles or manuals.

    Three different R4 chassis were tested on two different iMacs, and two different sets of 4 x 2TB drives, with consistent results.

    Stripe size is the amount of data "striped" in a single I/O across all drives in the array. Smaller stripe sizes are more efficient for small I/O sizes, with less wasted "slack space". Larger strips are more efficient for large I/O sizes. In general a larger stripe size is better for images and video.

    You can very roughly approximate the I/O size of a given workload by using Activity Monitor, and dividing bytes per sec by IO per sec. For several Adobe Lightroom tests, I/O size varied from 256k to 512k, indicating either of those stripe sizes would be better than the default of 128k.

    I didn't test the R6, but I wouldn't be surprised to see similar behavior on that chassis.

    Besides the problem of RAID 5 sync performance, regular Pegasus R4 performance in RAID 5 was excellent. See attached graphs.
     

    Attached Files:

  2. maflynn Moderator

    maflynn

    Staff Member

    Joined:
    May 3, 2009
    Location:
    Boston
    #2
    Nice, given the expense, its good see it having good throughput
     
  3. drmacnut macrumors member

    drmacnut

    Joined:
    Mar 7, 2005
    Location:
    Hawaii
    #3
    Thank you Joema2. This information is still valuable (5 years later!) for those of us finally upgrading our Pegasus RAID arrays to larger capacity yet still finding ourselves having to still sit through dozens of hours of syncronization.

    I am bumping two Pegasus R4 units from the original total size 4TB (using 1TB drives) to total 16TB (using 4TB WD Reds). In RAID 5, this means 12 happy TB of space. Yipee. Downside of course is the sync time.

    BTW, appreciated your other posting here on MacRumors where you mentioned updating the Pegasus firmware.

    Thanks again.
     

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