Pegasus - RAID5 - Reads slower than write?

Discussion in 'Mac Pro' started by ogilloire, Mar 8, 2014.

  1. ogilloire macrumors member

    Joined:
    Feb 22, 2014
    #1
    Hi

    I was checking speed of my RAID5 array with blackmagic disk speed test, it roughly indicates 300MB/s write vs. 120-130MB/s read?

    I thought RAID5 was supposed to be faster in reading than writing!

    Does anyone have a clue? How do the numbers look? Normal or lower than you would expect?
     
  2. AidenShaw macrumors P6

    AidenShaw

    Joined:
    Feb 8, 2003
    Location:
    The Peninsula
    #2
    This effect is common, and can be the result of a poorly designed benchmark interacting with the cache on the RAID controller.

    The Pegasus2 has a 512 MiB cache in the box. You can write hundreds of megabytes into the cache before the controller actually has to touch the disk.

    Reading, however, has to move the disk heads and fetch the data. This means that (unless the read data is already in the cache) cold writes can be faster than cold reads if the amount of data isn't large enough to flush the cache.

    Do explain your setup, though. T-Bolt/USB/T-Bolt2 ???
     
  3. chfilm macrumors 65816

    chfilm

    Joined:
    Nov 15, 2012
    Location:
    Germany
    #3
    Those speeds seem to be slow, what Pegasus are u using?
     
  4. ogilloire thread starter macrumors member

    Joined:
    Feb 22, 2014
    #4
    Hi

    Thanks for your replies !

    Setup is Pegasus2 R4, and WD 2 x green + 1 x red disks over Thunderbolt 2.

    The 2x green are those extracted from my previous WD Duo, the red is a recent addition to expand capacity as i moved from RAID1 to RAID5.

    I don't think the disks are very fast themselves so that could be why. Also not sure if the WD red+green mix is very appropriate, i'd prefer to have all reds but i don't want to throw away good working disks either (or the money to buy the reds).
     
  5. Stephent macrumors member

    Joined:
    Jan 31, 2012
    #5
    I have heard of issues when you mix dim types with the Pegasus so that could be contributing to your issue as well.
     
  6. spaz8 macrumors 6502

    Joined:
    Mar 3, 2007
    #6
    WD greens are also not a good fit for a RAID/DAS.. they are all about saving power (and money) and go to sleep a lot. I had a Drobo full of greens and it killed the performance. Been replacing with Reds.

    The WD reds are a better choice.

    I have 4 Hitachi 7K4000's in the mail for my Pegaus 2 R4.
     
  7. theSeb, Mar 11, 2014
    Last edited: Mar 11, 2014

    theSeb macrumors 604

    theSeb

    Joined:
    Aug 10, 2010
    Location:
    Poole, England
    #7
    You only have 3 drives in there? Also, as mentioned, the WD Greens are not meant to be used in RAID and are pretty slow. In addition, make sure that you have checked for any firmware updates using the Promise app.

    I have 4 x 1 TB drives in my Pegasus 1 R4. I get the following with Blackmagic Disk Speed test (half of the array is full)

    516 MB/s read
    312 MB/s write
     
  8. BayouTiger macrumors regular

    Joined:
    Jul 24, 2008
    Location:
    New Orleans
    #8
    I was seeing the same thing on my R6. I had always had my 30" display with the MiniDisplayport adapter daisy chained off the R6. When I moved the display to another TB port the Pegasus sped up substantially. I would not have thought that would have saturated the bus, and maybe there is another answer, but that's what I saw.
     
  9. theSeb macrumors 604

    theSeb

    Joined:
    Aug 10, 2010
    Location:
    Poole, England
    #9
    I got similar results to the op when I bought my R4. The firmware update made a huge difference.
     
  10. ogilloire thread starter macrumors member

    Joined:
    Feb 22, 2014
    #10
    Thanks for that! Your benchmarks makes for a good starting point.

    I'll try to replace the greens with reds, and see wether i have the latest firmware indeed.

    Screen is on a different thunderbolt port/bus so that should not be a problem.

    ----------

    [/COLOR]
    Yes I use the 4th one as pass-thru to be my time machine drive.

    What kind of drives are yours?

    ----------


    There does not seem to be a firmware update for pegasus2 though on their website.
     
  11. BayouTiger macrumors regular

    Joined:
    Jul 24, 2008
    Location:
    New Orleans
    #11
    I should mention that I meant writes faster than reads. My R6 is much faster than that. More like 500MB each way or more. Also makes a big difference the size chosen for the test.
     
  12. ogilloire thread starter macrumors member

    Joined:
    Feb 22, 2014
    #12
    May I ask also what are your stripe sizes on your RAID arrays?

    Pegasus utility chose a very large 1MB size by default, i'm not against it as most of my usage has to do with 40megs photos, just wondering how the stripe size could affect performance.
     
  13. BayouTiger macrumors regular

    Joined:
    Jul 24, 2008
    Location:
    New Orleans
    #13
    I am not sure. I went with the defaults. Mine has been running since they first came out.
     
  14. ogilloire thread starter macrumors member

    Joined:
    Feb 22, 2014
    #14
    I ended up changing my setup to have 2x3TB WD red in RAID0 configuration on the pegasus 2, deactivated write back cache and managed solid 250MB/s read and write speed, which is perfectly good for my needs.

    The other two slots of the pegasus are WD greens for my time machine backups
     

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