Mac Pro w/ SSD system disk - need room for virtual memory?

Discussion in 'Mac Pro' started by samtasticra, Nov 11, 2011.

  1. samtasticra macrumors newbie

    Aug 22, 2010
    hi all,

    so I'm setting up a Mac Pro for my dad.

    And I've strongly recommended to him that we install a SSD for the system disk, and he can use the other HD bays (loaded with standard HDDs) for holding data.... so the remaining mystery is working out how large an SSD is large enough.

    my question is simple: if we're using an SSD for the system disk (in conjunction with other HDDs), do we need to keep a minimum amount of space free on it?

    in the past I've been recommended to keep at least 10% of my system disk free. And I know from experience that not doing this really slows the whole system down.

    So if we install a 60gb SSD, should my dad aim to keep at least 6gb free all the time?

    I know it's a simple question, but maybe this will help someone else also?

    kind regards,

  2. samtasticra thread starter macrumors newbie

    Aug 22, 2010
    actually i think i can paraphrase the question better:

    does the same rule of 'leave a minimum 10% free on your system disk' apply to hard-disks and solid-state disks equally?
  3. toxic macrumors 68000

    Nov 9, 2008
  4. Loa macrumors 68000


    May 5, 2003

    I would aim for significantly more than 10%, as SSDs need room for their wear levelling to work effectively. I couldn't specify a specific number as it depends on the age of the SSD; maybe others can.

  5. barkmonster, Nov 12, 2011
    Last edited: Nov 12, 2011

    barkmonster macrumors 68020


    Dec 3, 2001
    Wear levelling is a design feature and has nothing to do with leaving space on the drive.

    I have a 60Gb Vertex 2, it has 64Gb of NAND Flash Memory, therefore, 4Gb of space for wear levelling.

    The Vertex 2 EX is only 50Gb but still has 64Gb of Nand Flash Memory and offers 14Gb for wear levelling and uses a far more expensive form of Flash Memory too.

    If you have all your documents, movies and iTunes library on a second internal hard drive and all your Applications on the SSD, you'd be surprised how much space you actually have left for virtual memory.

    My mac only has 1.5Gb and yet if I open some large Applications all at once, it still has over 15Gb free.

    I've attached some screenshots of Activity Monitor with TenFourFox (PPC optimsed Firefox), Vuse, iTunes and a very complex Pro Tools LE session open at the same time.

    (15.63Gb Free on the SSD, 1.44Gb of RAM used, 104.58Mb of VM Swap used)
  6. deconstruct60 macrumors 604

    Mar 10, 2009
    Wear leveling, unless fundamentally flawed, uses all the "free space" available on the drive. The "over provision" memory you are referring to is used for three reasons. One, is the data management overhead (ECC, historical tracking, virtual block maps, "hold" a queue of partially scrubbed blocks. etc.). Some of that is the wear leveling management. Second, for "replacement" blocks for the ones that do fail from too much wear (the drive has a defined amount it will tolerate before kicking out "disk fail" SMART stats). Substituting dead cells is also part of wear management. Third a relatively small area to use if user does fill the drive all the way up and a cell is close to failure.

    It is much harder for the drive firmware to find "file blocks which almost never change" (predict what the user/system is going to do) then it is to find clean free blocks (which are constantly tracked after being explicitly deleted). Conceptually, the drive could snag a "young" cell from a file that doesn't change and put that data on a "older" cell that only has a couple more cycles left. Most of them don't.

    So "free space" and "leveling" are coupled.
  7. kevin2i macrumors member

    Apr 25, 2011
    If you already estimate 50gb on the system disk, go for a bigger drive. Your user account will grow, even if you keep iTunes, photos, music, documents, downloads on another disk. For example, Evernote stores its database in ~/Library/Application Support/Evernote. Garage band files are huge, and they disappear when moved (by alias or symbolic link).

    You could move your /Users/username directory to the HD, but this could cause issues if you try to boot with one disk.

    If you are setting it up for your dad, might be easier to spend a little up front rather than deal with headaches later.

    I have a 80gb SSD - and I have to cull or move files a few times a year (OS X gives disk approaching full warning). When cleaned up, I am using 51 GB (Lots of apps, cs5 master, etc)
  8. samtasticra thread starter macrumors newbie

    Aug 22, 2010
    thanks for the replies : )

    have decided to go for the bigger drive

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