RAID0 as Boot Drive - Block size?

Discussion in 'Mac Pro' started by justit, Nov 10, 2009.

  1. justit macrumors 6502a

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    Dec 1, 2007
    #1
    I've got a 2 drive 640GB Black Caviar RAID that I want to set up as just an OS and apps boot drive.
    I realize larger block sizes have more through put but waste disk space, and smaller sizes save space but are less efficient.

    My user folder (mail, media, itunes, video, docs) will be on a separate drive.

    (1) So how do apps and OS files load at boot time and runtime?

    (2) If apps like CS4 are large apps does it not make sense to set 256K as the block size?

    (3) Or does OSX use many small files thus needing a smaller block size?

    Drive space is not an issue. I realize the default 32K was designed as a mixed bag for the all around average user. Does anyone have experience running a RAID0 as just a boot drive?
     
  2. alphaod macrumors Core

    alphaod

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    #2
    I use 64K, but that's purely personal preference.
     
  3. gugucom macrumors 68020

    gugucom

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    #3
    I run two 80 GB Intel SSDs as a RAID0 boot drive. I've set the stripe to 32 or 64, can't remember. It seems to work well.
     
  4. justit thread starter macrumors 6502a

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    #4
    I think SSDs can block at whatever size and stil be efficient. Next few months will be SSD and I'll try 16K
     
  5. alphaod macrumors Core

    alphaod

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    #5
    Well you don't want the stripe size to be smaller than the size of the cells.
     
  6. justit thread starter macrumors 6502a

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    #6
    By cells you're referring to what, the data size?
     
  7. mfka macrumors regular

    mfka

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    Oct 20, 2009
    #7
    I do not want to steal the thread from you justit :D, but i find it as
    a good opportunity to ask what should be ideal block size for lots
    of illustrator and indesign files plus a tons of photos? :rolleyes:

    So it is basically a work disk.

    I will run those on 2 WD Black Caviars in RAID 0.
     
  8. justit thread starter macrumors 6502a

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    #8
    That was my next question, I'd add PS files as well ;)

    From my understanding, you're using it as a general media drive which means larger files, which means larger boot block perhaps 128K . Hopefully it won't yield much waste.
     
  9. mfka macrumors regular

    mfka

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    Oct 20, 2009
    #9
    Correct. Heavy InDesign files.


    What do you mean by that?


    Thanks for your response. ;)
     
  10. justit thread starter macrumors 6502a

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    #10
    If a file is only 3K (which I doubt in your media situation) the entire 128K block is used, so that's 125K that goes unused wasted space. The higher the block size the more disk space is used.
     
  11. VirtualRain macrumors 603

    VirtualRain

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    #11
    The correct term for configuring RAID0 arrays is "stripe size"... not block size. Block sizes or sector sizes determine the smallest amount of data that is committed to each disk transaction. Stripe sizes, determine how much data is committed to each disk in the array before interleaving to the next.

    Assuming the OP is really asking about RAID0 array stripe sizes... (and not file system block sizes which are determined when formatting a drive - I'm not sure OSX HSF+ even offers an option for block size... does it? EDIT: Wikipedia says HFS+ uses 512 byte block sizes)

    With magnetic drives...

    A small stripe size (eg. 16K) provides added parallelism for reading/writing small files with the trade off of more seeks required for large fragmented or multi file transactions.

    A larger stripe size (eg. 128K) trades superior large file performance for less parallelism (performance) for small file operations.

    A generally recommended balance of 64K stripes provides a good compromise between extremes.

    For SSD's, which have very low latency and access times, logic would lead you to believe that small stripes are ideal as there's no penalty for seeking. However, there is a different penalty to be paid with SSD's... that is the write erase block penalty. All SSD NAND flash writes data in large blocks... in Indilinx drives as large as 512K... and Intel I believe is 128K. That means, even if the OS is writing a 4K chunk of data to disk, if the drive has any used cells in the block, the whole block of 512K or 128K must be first read to the drives DRAM, the new data allocated, and then the whole block flashed back to NAND. Thus small stripe sizes on moderately used SSD's can be a performance killer as writing just 16K of data with 4K stripes will incur a write erase block penalty of over 1024KB on each physical disk! Thus it's ideal if your stripe size matches the SSD's write erase block... or at least the biggest stripe size you can define which is typically 128K.
     
  12. Badger^2 macrumors 68000

    Badger^2

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    Sacramento
    #12
    what are you hoping to achieve?

    if you arent saving files to the disk, then you are losing all the benefits of a RAID 0? or is this going to be a RAID 1?

    how many times a day do you reboot and restart your applications?
    I dont think I have rebooted in months and months, and applications never get shut down?

    I think you are overthinking the whole block/stripe size thing by a factor of 10...
     
  13. justit thread starter macrumors 6502a

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    Dec 1, 2007
    #13
    Apple's Disk Utility uses the label 'Block Size' but it's good to know real world terminology outside of just OSX :)

    That's a compromise for mix file types all cobbled together (OS, Apps, media, documents)

    I'm trying to find relevance of block..err...stripe size to just OS/App file types, which excludes user folders, media, documents etc in a RAID0.
    Meaning what types of blocks does the OS read/write. What sizes are used to load larger apps like PS.

    I've set my RAID0 disk to 128K stripe size. When I get an SSD, I hope it's as simple as cloning the system drive onto the SSD.
     
  14. justit thread starter macrumors 6502a

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    Dec 1, 2007
    #14
    It would be RAID0, and more than doubling drive performance are the benefits of RAID0

    Not sure why that matters, I'm just trying to get a simple optimized software RAID0 going, but you can ask all those that have SSD boot drives what they think about using a separate drive for boot volumes.

    Yes Egor say too much think bad :rolleyes:
     

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