Gone are the days when one of your first questions about a new Mac was its expandability. To obtain best performance and engineering optimisation, all Apple Silicon Macs come with memory and internal storage which are fixed and can’t be upgraded later. This makes it even more important that, when choosing your new Mac, you get its specification right. If you’re one of the lucky few who simply buys the top of the range, then this article isn’t for you, but for the rest it should help you strike the right balance between performance and cost, whether you’re looking at an M1 MacBook Air, Mac mini, MacBook Pro, iMac, or Mac Studio.

Memory

As Apple Silicon Macs use memory mounted in the chip carrier, what your Mac comes with is all it’s ever going to get. Neither are there any external options, unlike storage. This is made more complicated by the fact that Apple Silicon Macs don’t have dedicated video or graphics memory: instead, they come with Unified Memory, which means that their main memory is shared by the GPU, Neural Engine, and everything else as well as the CPU.

The good thing about Unified Memory is that it’s used far more efficiently than in Intel Macs. The graphics card in my iMac Pro has 8 GB of dedicated memory, which remains about 90% in use fairly constantly. A naive approach might be to add that to the main memory total to arrive at the amount of memory needed for your new Apple Silicon Mac, which in my case would then be 40 GB.

In fact, comparing similar loads on that Intel model against my M1 Pro suggests that you don’t need to make any additional allowance for GPU use. You can see tentative figures in this article.

What you do want to avoid is using swap space on disk, as that will slow your Mac down, and persistent use of swap increases workload on the internal SSD, in turn shortening its working life. This became apparent to some of those who bought the first M1 models with only 8 GB of memory. Even light users found that those required VM much of the time. It’s probably wisest to make 16 GB your starting point, if you can afford it.

Internal SSD

Choosing the size of internal storage is far more flexible, as you can always relegate much of the contents of your Home folder to an external SSD. The difficulty is deciding how much to move there. While it’s possible to set up the whole of your Home folder to be stored externally, that makes your Mac completely dependent on that external disk being attached, and ties its performance to that disk as well.

Be very cautious of over-optimistic claims of external SSD performance, which could persuade you that Brand X will be just as fast as the internal SSD. Unless that SSD has its own active cooling system, it’s almost certain to run into problems with thermal throttling if you do start using it heavily. Unless your budget is being stretched to its limit, plan to keep all the working contents of your Home folder on internal storage.

There are two other benefits of large internal SSDs you should consider carefully. Larger SSDs perform better than smaller ones, and if you read the fine print on benchmarks quoted by Apple and others, you’ll see performance figures are invariably obtained on their highest capacity models. The other important benefit is slower wear, particularly if you can keep several hundred GB free at all times.

This is because, for the same quantity of data written to an SSD, the larger the SSD is, the fewer the number of times each of its storage units has to be erased and re-written. As SSDs are expected to fail from age once they’ve been erased and re-written a certain number of times, the slower it takes to reach that number, the longer the SSD should last before failure.

To take a simple example, let’s assume that a Mac writes 100 TB to internal storage every year, and that its SSDs are good for 1,000 writes before they fail. With a 500 GB internal SSD, the total life capacity of that storage is 500,000 GB, in other words the SDD is likely to fail after 500 TB has been written to it, which would take five years. A 2 TB SSD has four times the total life capacity, and would be expected to run out of write capacity after 20 years. Investing an extra £/$/€ 600 in a larger SSD may thus give it four times the working life.

My final thought on working out the capacity of internal storage is growth. Every Mac that I have used in production has ended its life with more in its internal storage than it started with. It’s never easy to guess how much more, but that’s essential when you know you can’t increase internal storage.

What did I buy?

Despite great temptation to buy a Mac Studio in the M1 Ultra configuration, there’s no way that I would ever use all that performance. So I settled for an M1 Max version with 32 GB of memory and a 2 TB internal SSD, which will replace this iMac Pro with 32 GB of memory and a 1 TB internal SSD. It should arrive in a couple of weeks, although I’m going to have to wait a little longer for the Studio Display with tilt and height adjustment. I’ll let you know what they’re like in due course.