Choosing your new iMac: picking the best storage compromise

Yeterday’s quiet release of updated iMacs must have put them back on the wishlist for many Mac users. First indications, though, are that those may be a stopgap, pending a definitive replacement later this year or early next.

One important clue here is the absence of any T2 chip, which has been a feature in all Apple’s redesigned Macs since it first appeared in the iMac Pro at the end of 2017, in MacBook Pro models since July 2018, and most recently in the more modest MacBook Air and Mac mini. Incorporating a T2 chip in a logic board revision for the iMac would have required more investment, and a longer intended lifespan for these new models. This may in part have been dictated by the availability of Intel CPUs and chipsets.

It’s no disaster if these iMacs will relatively soon be replaced by even better iMacs. The natural history of Macs, like all computers, is that today’s hot new release will sooner or later become yesterday’s model. If you need a new iMac but can’t step up to an iMac Pro (which will also be replaced sooner or later), then these new models could be ideal.

The one feature about them which concerns me is their continuing use of rotating hard disks, alone in the cheaper configurations, or as part of a Fusion Drive if you need more and faster storage. There are two issues which you need to consider: using the APFS file system on a hard disk, and the Fusion Drive itself.

These new iMacs will only run macOS Mojave, and although some users have managed to get that to boot off HFS+, Apple only supports it booted from an APFS-format drive. To all practical intents and purposes, your new iMac will therefore boot Mojave from its internal storage, which will be in APFS format.

Hard disk?

APFS works on hard disks, and has done so in release form for around a year now, but it hasn’t been designed to get the best out of them, in contrast with HFS+. Although I haven’t been able to find good performance comparisons, all those who I know who use APFS on hard disks consider its performance to be relatively poor.

It is also prone to fragmentation of files and free space, a long-recognised cause of even worse performance on hard disks. Mojave includes a background defragmenter which attempts to address that, and I have again been unable to find figures to assess how effective it is. APFS isn’t necessarily bad news on hard disks, it’s just so much better on SSDs.

Fusion Drive?

APFS has only been supported on Fusion Drives since the release of Mojave last September, so there is limited experience of it still. Apple hasn’t provided any detailed description of how APFS works on Fusion Drives, but states that all the file system metadata for the Fusion Drive is kept on its SSD component, which also appears to be used as a large read and write buffer for the hard disk. That is different to the way in which CoreStorage manages physical storage in a Fusion Drive.

In practice, the overall performance of a Fusion Drive should be expected to fall somewhere between that of a pure SSD, and that of a straight hard disk. Exactly where your iMac’s performance would be depends heavily on how you use your Mac, and isn’t easily predictable.

What is even less clear is how long the SSD in a Fusion Drive can be expected to last when operating under APFS. If almost all data read from and written to the hard disk in the Fusion Drive passes through the SSD as a temporary buffer, the SSD will see a great deal more write activity than would be expected for its size. A 120 GB SSD acting as buffer to a 2 TB hard disk could therefore see more than fifteen times the write activity than a plain 2 TB SSD would. Because the memory used in SSD only works for a fixed number of write cycles, that additional load on the SSD within a Fusion Drive could lead to it failing before it is ten years old.

Evidence from some with older Fusion Drives is that this can lead to worryingly early failure in SSDs, which appear to have aged much more quickly than would be expected. This may be the result of bugs in wear levelling, which aims to even out the number of writes across the whole of an SSD. Without information from a large number of Fusion Drives, it’s impossible to say whether this is a problem specific to certain Fusion Drives, but the smaller the SSD component, the more likely it should be.

Consider SSD

When you price up your new iMac, look at another option as well as a hard disk or Fusion Drive: a smaller internal SSD coupled with a larger external drive. For example, if you were thinking of getting a 2 TB Fusion Drive, as is standard with some new iMacs, a 512 GB SSD would only cost an extra £/$/€ 90, but should perform very much better using APFS, and last longer than the rest of the components. Although a 1 TB SSD is a large additional expense, an extra £/$/€ 450, it should comfortably accommodate your whole macOS system and apps, and most of your Home folder too.

It may be a tad messier to set up, but I’d far sooner have a smaller internal boot SSD and bulk external storage than a large internal hard disk or a Fusion Drive. Indeed, that is exactly what I have done: I have a basic 1 TB internal SSD and 1 TB external, plus backups on an external RAID of hard disks. It is a small fraction of the cost of a single SSD, and works very well indeed.

(Many thanks to @boletrone for spotting the date that was twenty years out!)