Performance w/external drive via Thunderbolt?

Discussion in 'MacBook Air' started by splitpea, Jan 25, 2013.

  1. macrumors 6502a

    Oct 21, 2009
    Among the starlings
    I'm thinking of going Air with my next laptop purchase but am nervous about storage options - I currently use 100GB just for system and software, and another 250GB for data, even before accounting for new files created in the future or the media library that's already on an external drive.

    Since 512GB internal SSD is prohibitively expensive, I was thinking of going with 128 or 256 and using an external HDD via Thunderbolt for all user files (possibly including mail, music, etc.). A few questions about that:

    1) does OS X work gracefully with the entire /Users directory on an external drive? Do most applications handle it well too?

    2) how does performance with large files on a thunderbolt external compare to performance with files on an internal drive?

    3) what happens if the external accidentally becomes disconnected while you're working? Do you have to leave it connected even when the computer is sleeping, if you have files (or just your mail client) open?

    4) Some of the work I do involves frequent writes to large files (developing and testing data migrations for large databases). Is this terrible for the health of the SSD?
  2. macrumors 68000


    Jul 11, 2008
  3. macrumors regular

    Feb 21, 2010
    This would be an exceedingly bad idea.

    It is near parity with the internal drive. It will be limited by the drive read/write speed, so you have the same drawbacks with spinning platters, but the bus speeds will quite literally go above and beyond the transfer rate of the drive.

    This is precisely why using it to store /Users is an awful idea. If anything, make a symlink to the drive under your home directory, but slapping the entire thing on the drive is a recipe for disaster. In either case, if you disconnect the drive, most applications will be OK with it as the contents of the file are already in RAM. Note, this does not apply to larger projects like videography/multi track audio where the source may be cached as needed. That would be a disaster.

    No. TRIM alleviates the majority of the risk here. By the time you reach the cycles per sector needed to harm or degrade an SSD, you and I will be laughing at how primitive the technology we're currently discussing is. Also, all modern SSDs are over-provisioned for just such a case where a sector fails. The drive will automatically remap to the reserve.
  4. macrumors 6502


    Jan 5, 2013
    I don't use a thunderbolt external, but I do have a USB 3.0 external and it runs HD movies and everything else on it just fine. I'd imagine Thunderbolt would be at the very least as good, but most likely better.
  5. thread starter macrumors 6502a

    Oct 21, 2009
    Among the starlings
    Thank you for the detailed explanations, Robyr!

    Is there any solution, then, for apps that insist on storing their data in places like ~/Library? (e.g. -- I have dozens of gigs of mail archives that I need access to.) I know ~/Music and such can be moved to an external reasonably safely, but I assume ~/Library would be one of the main culprits in any disaster related to unavailable user data.

    That's encouraging! :)

    I thought Apple hadn't enabled TRIM -- has that changed?
  6. macrumors regular

    Feb 21, 2010

    How many apps are you trying to install? Or, more relevant, which? The 128GB will hold the entire Creative Suite, a VM of Windows, and all sorts of stuff. By installing these apps on an external spinning disc, you are really evaporating the Air's whole performance advantage. Apps should be on the SSD.

    TRIM has worked on Apple SSD's since at least 10.6.8.
  7. thread starter macrumors 6502a

    Oct 21, 2009
    Among the starlings
    Oh, I'd want the apps themselves on the internal, but the data (possibly including that in ~/Library) on the external.

    Right now, /Applications, /Developer, /Library, /private, /System, and /usr account for about 50GB. I have 40GB in VMs, 20GB in databases, and 5GB of Python virtualenvs, all of which which are also best run off the SSD for performance reasons but could theoretically be moved to the external.

    That already nearly fills up a 128GB drive, and all of that data is growing. And that doesn't even account for leaving room for scratch space.

    ~/Library is another 30GB, including 20GB of mail and "mail downloads" and 5GB of savegames from a game that doesn't allow you to configure save locations; I also have several websites in development that I serve locally from ~/Sites -- that's another 30GB and growing quickly.

    Music, Photoshop files, and various work files (stuff that will load quicker from the SSD but could be easily moved to an external) are another 200GB, and I've got 250GB of less-frequently-used media that are already offloaded to a slower external drive.
  8. macrumors 68030

    Oct 15, 2008
    Just get a 256 and put stuff where it ought to go.
  9. macrumors regular

    Feb 21, 2010
    Bottom line, you need to go bigger.

    Realistically, I wouldn't offload anything like the /Library root to an external. I was in the same boat, but now I have learned to pare down my "carry weight" so to speak on the Air. I use a conventional desktop as a terminal server to do the heavier lifting for DB/Web/Systems stuff, and if I know I am going to a client's or working a particular project I check it out and do the work needed on the Air and check it back in (the server keeps this to a minimum because I almost always have access to an internet connection)

    I also use a Thinkpad x230 at the office, so a good amount of the truly redundant and less used data just sits on that.

Share This Page