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I'm going to get an SSD, making that my boot drive and moving the existing hard drive to the optical drive bay. I will keep my data on the original hard drive.

MacPerformanceGuide.com says "If the Boot drive is a fast SSD, moving the home directory can be self-defeating for performance."

Let me be clear. I'm talking about the home directory specifically. MacPerformanceGuide talks about the goodness of moving the Documents, iTunes library, etc. folders, but not so the home folder.

Why is that?

We are not talking about wearing out the SSD with excessive write cycles; we're talking performance.

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Some more info on making the most out if your SSD: apple.stackexchange.com/a/25237/10355 –  cksum Mar 21 '12 at 16:49
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3 Answers 3

OS X stores a considerable amount of data, from program caches to a multitude of preferences and other vital tmp files, in your home folder (Library to be precise). Shifting these off the SSD is detrimental. The system would still pull files from your speedy SSD but any cache files from your apps (both third party and default) would be pulled from traditional and cumbersome HDD.

Offloading large, static files, like music, movies, books, and other documents that benefit little from the speed of an SSD are recommended. But moving small core files that benefit greatly from solid state disks isn't. It's crucial to keep as many integral system components on your SSD as possible. A 2 GB movie or 1,000 songs won't benefit because movies don't need to be played faster and neither do songs. They may load a little quicker, and in the future, when SSDs will have TB of space, this won't be a problem anymore, but for now, it's all about triage.

I suppose you could try to micromanage this aspect but to be 100% safe, I would just move the big ticket items. An 120 GB SSD with 5 GB on it won't be faster than one with 20 GB on it.

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Shifting the tmp files off the SSD onto something slower is only detrimental for those tmp files. For files outside the home folder performance is improved by moving the home folder to another drive. Also, any hard drive (including SSD's) will be significantly faster if it hasn't been used much and is mostly empty. All drive usage "dirties" the internal data structure and slows it down. Basically, it depends how @radarbob uses his system. –  Abhi Beckert Mar 17 '12 at 3:32
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Because you will be constantly on the Desktop and Library when you open/close programs..

If those are on the regular HD, they will read/write at the regular HD speed.

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In general, having your data split among multiple drives will always improve performance. In an ideal world, every single file on your system would be on it's own hard drive with it's own drive cache/etc.

But only if the other drive is at least as fast as the one you remove it from. If you replace an internal drive with a USB or even thunderbolt drive, then performance will probably be reduced.

If your internal boot drive is a high end SSD and you move your home directory to a $3 USB memory stick, then performance is going to be crap. On the other hand, if you move your home directory to a second internal SSD, then you will improve performance in many situations.

For something in between those two, you'll have to use your own judgement and/or test it out. The answer is going to depend on exactly what you use your hard drive for. Without knowing exactly how fast your SSD and existing hard drives are, or how you use your system, it's hard to predict what will be the best solution.

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-1 for "In an ideal world, every file...on it's own hard drive.." Not helpful. Also "...move..to a second internal SSD...will improve performance..." - does not negate the premise that the home folder on a non-boot drive is not performance-optimal. –  radarbob Mar 17 '12 at 22:20
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