Performance w/external drive via Thunderbolt?

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?

Thank you for the detailed explanations, Robyr!

Robyr said:

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

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This would be an exceedingly bad idea.

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Is there any solution, then, for apps that insist on storing their data in places like ~/Library? (e.g. Mail.app -- 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.

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.

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JoshMKB24 said:

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.

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That's encouraging!

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.

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I thought Apple hadn't enabled TRIM -- has that changed?

Robyr said:

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.

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Robyr said:

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.

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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.