Optimum partition size for new SSD volume?

Discussion in 'macOS' started by Spectrum, Jan 8, 2013.

  1. Spectrum macrumors 65816


    Mar 23, 2005
    Never quite sure
    I've recently installed a Crucial m4 512GB into a 2011 MBPro running 10.6.8

    I have cloned my old hard-disk. But: how large a partition should one format the new Crucial SSD?

    From various reading, I assumed that OSX would show the 512GB SSD as approx 480GB of useable space (due to 7% being left "spare" for the Crucial SSD firmware/defrag/garbage collection etc). However, OSX currently lists the drive as being a single 511.2GB partition. Please note that I have also installed the TRIM hack.

    Today I noticed that on my 2011 mac mini with Apple installed 256GB SSD, OSX displays the size as 251GB - i.e. smaller than 256GB, but proportionally much larger than is expected for a SSD with a spare area of 7% (expect approx 240GB).

    Is there in fact any reason/need to format the new 512GB drive to 480GB (or smaller)?

    What size (useable) do others see OSX reporting for SSDs (both installed by Apple and user installed)?
  2. benwiggy macrumors 68020

    Jun 15, 2012
    As much as there is.

    I don't know where you've read that you lose 7%.

    Just use the whole thing.

    Yes, Disk Utility will report the formatted size as slightly less than the 256 it is described as having -- but it does the same for a mechanical drive too. PC vendors always have "actual formatted capacity less" in small print. But it's certainly not 7%.

    And don't forget that you get differences between binary and decimal interpretations of "Mega" and "Giga".
  3. Spectrum thread starter macrumors 65816


    Mar 23, 2005
    Never quite sure
    Thanks. I thought all SSDs needed spare area so that they can run garbage collection routines efficiently?

    For example, many SSDs advertise only 240GB (when clearly the NAND will be packaged to make 256GB). For example all the OWC drives are multiples of 60GB (per 64GB). 60, 120, 240, 480, 960...

    Anandtech had an interesting article about how the amount of free space (up to 25%) can massively improve consistency of IO operations with heavy workloads. Now I'm not concerned about heavy workloads at all, but I just want to make sure I'm setting up the drive correctly for "normal" use.

    Does it only matter if one fills the entire volume with data? Hence formatting to 90% capacity ensure you never eat into the spare area? On a traditional HD I try to keep >10% free anyway.

    Secondly: I have another 2008 MBPro with new 512GB SSD that I cloned a 10.5 image onto. I plan to replace that with a cloned 10.6.8 image. Due to the way SSDs write data for wear-levelling, am I correct in thinking that I should first "erase" that SSD by writing one enormous contiguous file onto it - then deleting - basically to ensure that the new 10.6.8 image is written to contiguous blocks and not fragmented?

    Or does it not matter?
  4. benwiggy macrumors 68020

    Jun 15, 2012
    I have an OCZ SSD in my 2009 MacBook. It's 247Gb. I can't make it bigger than that: that's the maximum size. It may have 256Gb of chips, but I can't use that.

    In a traditional hard drive on OS X, you need some to keep some free space on the disk -- some say 10% -- but that's space WITHIN the partition.
    Even on an SSD, you still need to keep some free space on the system partition. Partitioning 10% off won't do you any good.

    I'm not sure you need to worry about wiping the disk first for fragmentation. OS X defrags on the fly anyway, and I'm not sure what the performance penalty would be on an SSD for fragmentation, if anything.
    I don't know much about SSD internals, but generally: I would just use the disk as normal, and not get too bogged down in the detail.

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