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I just upgraded the CPU of my 2015 27" Retina iMac and while I was in there I took out the 1TB HDD and replaced it with a 512GB Samsung 850 Pro SSD. Now I'm ready to set it all up, but I'm not sure how to best proceed.

  • I know the SSD will be on a relatively slow SATA III bus
  • I know the blade SSD is 24GB and on a faster bus
  • I know I could use external Thunderbolt for a faster experience (should have considered this but too late)
  • I've read mixed opinion on merging the two SSDs into a fusion drive, but most people suggest against it, since:
    • the speed of both drives will be reduced to the slower of the two
    • if one drive fails I will lose everything

I currently think I should keep them separate and use the 24GB SSD for the OS and use the other SSD for the home folder. Does this seem like a good way to proceed?

I am not sure which file system to use (APFS?)

I am not sure if I should be doing something with TRIM to extend the lifespan of the drives

If anyone can help guide me I would appreciate it. Thanks.

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    1) Yes SATA III is a bottleneck for PCIe SSDs but you won't notice it in normal use. Your R/W speeds will be very impressive. 2) Use APFS. That's the current FS protocol Apple has transitioned to. 3) 24GB is tight for the OS but it will work so long as it is bare bones OS and you're not installing other Apps on it. All other non-OS apps need to be installed on the Samsung drive. 4) Don't merge them into a fusion drive. Use the 24GB as your boot volume. 5)Do not do s*** with TRIM, not needed for what you're running (you aren't making a hackintosh, eg). – njboot Sep 2 '18 at 4:15
  • I formatted them both APFS and went ahead with the Samsung as the main drive (before that I tried using the 24GB as the main drive, but after the OS was installed there was only 8GB or so left, which somehow quickly went down to 5, not enough to install High Sierra, so I aborted that plan). Now I have a bit over 20gb available on the Apple blade SSD. Maybe that would be a good place to move the applications to, if that's something I can do. I have not found a good answer on whether or not it's ok to move the Applications folder. – libbynotzoey Sep 3 '18 at 6:42
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Learnings:

  • There doesn't seem to be any benefit to merging two SSDs, other than having one contiguous space.
  • The disadvantages of merging are that 1) both drives will have the slower of the two bus speeds, and 2) if one drive fails everything is lost
  • The small 24GB SSD that Apple includes in the 1TB Fusion Drive setup is too small to act as the startup drive. After installing Sierra, there was about 8GB remaining, and that quickly dropped down to around 5 once the update to High Sierra downloaded, leaving insufficient space to actually install High Sierra. Maybe I'll use that space for some applications.
  • It doesn't seem that the relatively slow 6GB/s speed of SATA III will have any noticeable impact
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    About this idea to merge two SSDs as a single Fusion drive: I don't get the claim "speed of both drives will be reduced to the slower of the two" that we can read everywhere on this subject. In a traditional Fusion drive (small & fast SSD + big & slow HDD) the speed of the fast SSD is not "reduced to the speed of the slower HDD". Not at all! It is quite the opposite, everything feels snappier as most important and most used files are always in the fast portion. Please clarify this point. – flux_capacitor Nov 17 '18 at 0:13
  • My understanding is that, for any fusion drive, the bus speed of the entire drive is equal to the slower of the two bus speeds that are used to make it. It appears that late-2015 iMac has has a Serial ATA (6 Gb/s) connector for a 3.5" hard drive and a PCIe connector (PCIe 2.0 x4 NVMexpress interface) for the SSD. Of course even 6 GB/s is much faster than the "big & slow HDD", and the fusion drive with the SSD can take full advantage of this, where the HDD alone cannot. – libbynotzoey Nov 17 '18 at 4:07
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    Thank you for this answer. Unless you have a custom SSD controller, you are correct that concatenation game or applying RAID to consumer drives has far more drawbacks than benefits. – bmike Jan 5 '19 at 21:12
  • @sshanky: Fusion Drives don't care about bus speeds, when reading/writing to/from the SSD, the reads/writes are the full speed of the SSD. The caveat with Fusion Drives is that data not on the SSD will be slower, that and there no zero redundancy (if either drive fails, the whole volume is lost). There is no fusion drive controller, it's all handled by the file-system driver (software). – Haravikk Nov 24 '19 at 18:45
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I would recommend to go for the Fusion drive. I am using a Fusion drive with a SSD (128 GB) and a HDD (3 TB) in my iMac for six years now, not a single problem and definitely much faster that a plain HDD. With that experience, I recently bought a new iMac with a 1 TB SSD and added an internal 4 TB Samsung 860 EVO. I combined them to a single Fusion drive (APFS on Mojave) and speed tests show the speed of the NVMExpress bus, which is about four times faster than SATA (with these drives). So far there haven't been any problems.

24 GB is not that much nowadays, and as a separate startup dive, for example, you will run out of space soon. But if you use it as a fast cache for the SDD on SATA, you have 24 GB more space and you can let the OS decide what to put on the faster drive. This will give you some speed advantage.

I would opt for APFS as file system if you use Mojave (10.14) or higher.

Apple has a support document on how to combine two drives to a Fusion drive. According to that document, the simple Terminal command diskutil resetFusion (after booting in recovery mode, of course) should do the trick on Mojave. Unfortunately, this only works if one drive is a HDD. If you want to combine two SSDs, you have to use the following commands (which destroy all the data on your disks!), and it is important to state which drive shall be the main (faster) drive.

Boot into recovery mode (cmd-R) and open Terminal. Look for the names of your drive partitions:

diskutil list

Let's assume the biggest partition on your blade disk is disk0s2, the biggest partition on the SATA SSD is disk1s2:

diskutil apfs createContainer -main disk0s2 -secondary disk1s2

Find out the name of the Fusion container:

diskutil apfs list

Let's assume the name is disk3. So finally create a volume named 'MacSSD' (or whatever) in that container:

diskutil apfs addVolume disk3 APFS MacSSD

Afterwards, you can install macOS again or restore your drive from the backup to the Fusion drive.

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