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This question clearly doesn't have one answer, so here are few assumptions:

  • I want to add an SSD to my Mac, let's say a MacBook [Pro], with the primary goal of speeding up my workflow.
  • The hard disk will stay in the machine, and be mounted as a separate volume.
  • the SSD will be too small to hold everything that I need, so I will have to put some stuff away on the secondary hard drive

So the question is: What will be on the SSD, what be be relegated on the hard drive, given this order of priority:

  1. Maximize typical system speed for common operations (boot, launching applications, opening/saving documents).

  2. Maximize space efficiency: use as much of the available SSD storage space for useful stuff (ie speed improvement) without risking running out of space when really needed.

  3. Minimize time spent setting up and tweaking the system

  4. Minimize SSD wear when it makes sense

I classify stuff in three categories:

  • A: Stuff that "obviously" need to be on the SSD
  • B: Stuff that "obviously" will be on the hard drive, because there is no speed benefit to put it on the SSD, and there is no to little cost to put it on the hard drive.
  • C: Grey area stuff: unclear to me, perhaps due to my ignorance/misconceptions.

This question is about the grey area stuff. But before, let's quickly list the other two categories:

A: Stuff that "obviously" need to be on the SSD:

  • Mac OS X itself. The OS will benefit a lot from the speed of the SSD. Moreover, most of it is seldom written. So little wear.
  • Commonly used applications. For me: mail, development tools, Keynote, a few others
  • Home directory: a lot of stuff there will benefit a lot from being on the SSD. Some will fall in the grey area as listed below.

B: Stuff that "obviously" will be on the hard drive:

  • iTunes music. More generally, all media files: they are big, and don't need more read speed than real time. This would be different if I edited those media files
  • photo database, such as iPhoto databases, unless I do heavy photo editing (which I don't).
  • Applications that I don't use very often.
  • old documents or projects that are only accessed once in a while
  • deep sleep file: while it would help the Mac wake up from deep sleep much faster, I seldom need deep sleep and deep sleep files are rather big

C: grey area:

  • OS X swap space: if I really need it a lot, it means I need more RAM. Moreover, thrashing might be less noticeable on a fast, silent SSD than a HD. So I would realize something is wrong later. Finally, should thrashing happen, it would accelerate SSD wear a lot. So put it on the hard disk? But app switching, the best use of swap space, would be much faster on an SSD.
  • home directory library folder cache files?
  • mail database?
  • Spotlight indexing?
  • noatime?
  • journalling?
  • other?

Thanks for any thoughts, especially about swap space.

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I recommend you using a hibrid disk, wich have SSD and HDD. Try a seach for Seagate Momentus XT. –  Diogo Jun 20 '11 at 11:47
I'm not sure how Mac OSX handles hibernation, but I bought my SSD specifically for swap space (in Linux), even though I have way more RAM than I ever use, because Linux uses swap space for hibernation. As a result, waking up from hibernation is now faster than my monitors can react--I can log in before my monitors display the log in prompt, if I'm quick enough. Anyway... you might research how Mac OSX handles that, because the answer may have a big impact on what you put on your SSD. –  Flimzy Jun 21 '11 at 1:33
Hibernation is called deep sleep. As I mentioned, I don't intend to put the deep sleep file on the SSD, as I almost never use deep sleep. I use regular sleep, where RAM power is maintained by the battery. By default, Mac OS X will only deep sleep when the battery runs out, which is almost never on my MacBook Pro and its 8 hour battery. –  jdmuys Jun 22 '11 at 0:00
I disagree with the recommendation for a hybrid disk. At least you haven't supported your recommendation. Tests I have seen show only moderate performance improvements with a hybrid disk relative to a conventional hard disk. This is probably because their Flash memory cache is not really large enough. Finally, this is off topic. –  jdmuys Jun 22 '11 at 0:03
How would you go about partitioning so that ~/Library is on one disk (SSD) and ~/Music and all the other media type directories are on a different disk? –  tladuke Jun 22 '11 at 19:20
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3 Answers

You will see the biggest performance gain from having the OS on the SSD. From there, any large applications you have will be directly affected by the increased throughput. I've found (although I am not a HUGE AV nut) that having media files (Mp3, Flac, video, etc) on the SSD provides little advantage. Your demands might be different.

As for swap, can it be disabled? I currently have 2 linux laptops running no swap, and I have run no swap in windows since I got my first SSD in ~08 and ran Windows XP. The only program I have ever had issues with the lack of swap space was Photoshop 7 (lol).

In short, whenever I set up a new machine with an SSD I put my OS and the big programs on the SSD, and everything else can sit on any other internal disks.

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I don't think you can disable swap space. You can relocate it to another disk though. The OS on the SSD is a matter of course. Next in line is whether to put the user home directory on the SSD or not. I think it must be done, as much performance can be gained thus. Except a few parts of the home directory are large and don't require speed (media files), and other parts are not so clear cut (browser history, archived mail messages...) –  jdmuys Jun 25 '11 at 12:49
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I'm gonna focus on the grey area, as I agree with the other areas you listed.

OS X swap space: I say on ssd. The performance gain from HD to SSD is not so much that you can't immediately tell you are swapping. The access rate between SSD and RAM is still a large gap.

home directory library folder cache files? mail database?

I say definitely. E.g. your firefox profile's web cache is stored in the library cache folder, and accessing that from SSD will make browsing seem faster. Mail DB on SSD I think is right as well for a similar reason.

Spotlight indexing? Only if you use spot light a lot. noatime? I don't think HFS supports noatime journalling? Do you mean like journaling the HFS filesystem on the SSD? I think this is the default on HFS, and why wouldn't you want it? other? I can't think of an example where I wouldn't want to move all disk-intensive tasks to the SSD if your FIRST priority is speeding up productivity. Things like swapping, caching, temp files, and such are all disk-heavy tasks that would show up as slow downs when using the system.

I say hardware is made to be used. Put anything that is disk intensive on the SSD. Your list is pretty good.

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Putting disk intensive tasks to the SSD is the goal. Identifying which task is disk intensive is the question. While it doesn't matter for small files, it can make a difference for larger/numerous files, especially in the context where the SSD is short on space. From this perspective, my mail database is not an obvious candidate for SSD: speed is network-bound, size is getting very large with about 20 years of archive that are seldom accessed. –  jdmuys Jun 25 '11 at 12:51
Yes I meant journalling as in HFS filesystem. It is now turned on by default but is not mandatory. It remains to be seen whether it brings any real benefit on an SSD, given its cost of additional wear on the SSD –  jdmuys Jun 25 '11 at 12:58
noatime is supported at the OS level but is not exposed in the UI. I don't believe the UI would be disturbed if turned on. I will probably try it. –  jdmuys Jun 25 '11 at 12:59
Moving write intensive tasks to an SSD is not a good idea, as the disks will quickly wear out, so I doubt that swapping to an SSD is a good idea. Rather add more RAM. –  Pierre Arnaud Jun 27 '11 at 4:51
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FWIW, I've done this. I've got a Mac Mini with a 100Gb OCZ Vertex2, and a Samsung external 1Tb 2.5" USB drive. Just as you suggest, I keep media on the external, and OS/swap and apps on the SSD. I find I've got plenty of space left on the SSD and it does absolutely fly. There is a tweak app to enable TRIM support on OS X for non-Apple SSDs, but on recent drives this can actually reduce performance quite a lot - garbage colletcon on new controllers is good enough not to need to worry about it any more.

Beyond that I think you might have to hold out for ZFS which will allow you to designate particular drives to use for faster access.

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