But if CoreStorage has been doing its job properly, around 80-90% of the writes should have been to the SSD rather than the hard disk in the Fusion Drive. Most hard drives don’t keep a reliable record of how much data has been written to them, so over the 33 months that I have been using this iMac, I don’t know the total data written to its Fusion Drive. In practice, an SSD is normally considered to be failing when it has reached half that total, so that 120 GB SSD should remain fine until 600 TB of data has been written to it. So the 120 GB SSD in this iMac’s 2 TB Fusion Drive would fail completely when (10,000 x 120) GB = 1,200 TB has been written to it.
Thus, any given SSD will fail totally when a total of 10,000 times its capacity has been written to it. That figure is arbitrary, and so long as it is uniform across the SSDs being compared is relatively unimportant at this stage. It is this strategy which allows Fusion Drives to deliver performance which is almost as good as an SSD, but which also makes them age faster than an SSD alone.įor the sake of this example, I’ll assume that the SSDs involved will take 10,000 writes to any given block of memory before that block fails. In this article, I will assume that wear levelling and TRIM both work perfectly, although in practice they won’t always be so good, which will shorten the life of an SSD.įusion Drives are managed by Apple’s CoreStorage software, which puts the files which are most frequently accessed onto the SSD, as far as possible, leaving those which are less active on the companion hard disk. This is combined with TRIM, which ensures that blocks of memory no longer needed for storage are returned for re-use. To prevent some blocks of memory from wearing out long before others, macOS and SSD firmware use wear levelling to ensure that each block of memory receives a roughly equal number of write operations, which evens out its ageing. The two main limitations on the life of an SSD are that all flash memory fails to retain its data after about ten years, and that it can only be written to a certain number of times. This article proposes another factor which you should take into account when making that choice: that an SSD should last longer than a similar-sized Fusion Drive. Fusion Drives would still save you money when configuring a new iMac, for example, but the difference between built-in 1 TB options is down to around $/€/£ 630, and if you could live with a 512 GB SSD instead of 1 TB Fusion Drive, that falls to less than $/€/£ 300.
Now that SSD prices have fallen, and look to continue to fall at least in the medium term, the cost of larger SSDs of 1 TB or more is no longer prohibitive.
#HOW FAST IS FUSION DRIVE COMPARED TO SSD MAC#
It makes so much sense that the data your Mac wants most should be kept on faster-access media, and it seems silly to pay the higher price for SSD capacity to store lots of files which are seldom even read. I’ve long been an enthusiastic user of Fusion Drives.
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