Last Week on My Mac: Time Machine travels

There’s a school of thought among Mac users that Apple invests as little as possible in macOS, having lost interest in it and Macs more generally. Although at times it doesn’t get the attention which Mac users think it deserves, digging a little deeper into Catalina provides several strong counter-arguments.

The introduction of process lifecycle and resource management in RunningBoard, which I described here a week ago, is an important sign that, far from it slowly decaying, Apple is bringing new subsystems to enrich and improve macOS. But even more compelling is the continuing investment made in one part of macOS which has no equivalent in iOS or iPadOS: Time Machine. Last week, I turned more attention to look at how this widely-used feature works in Catalina, and was pleasantly surprised to see how much it has changed.

The most radical change for which many of us are waiting expectantly, to support APFS as a destination file system for its backups, still hasn’t been realised. As I have pointed out, this isn’t entirely surprising given the limited range and high cost of suitable SSD-based backup storage.

What we have got is a significant improvement on Time Machine in Mojave. Since it was last remodelled in High Sierra, Time Machine has made snapshots when backing up APFS volumes and compared them to determine which files and folders need to be copied to the backup, a technique known as snapshot diffing. When I first investigated this in Mojave, I noted that the process of comparing snapshots hadn’t brought any performance improvement, and seemed to result in an increased number of slower deep traversal scans.

Catalina restores the earlier and highly efficient method reliant on the hidden database of file system events, FSEvents, maintained on each volume. Although at times, FSEvents can be of little or no use, it had been the backbone of Time Machine since its first introduction with Leopard back in 2007. When it was replaced by snapshots in High Sierra, Apple’s engineers must have thought that this new mechanism would eventually prove superior.

The reintroduction of FSEvents is acknowledgement that in many cases, even when backing up APFS volumes, it may prove superior in speed, and just as reliable, as snapshot diffing. This is interesting, as Jon Becker’s section in session 710 at WWDC back in June praised the use of snapshot deltas in restoring APFS volumes, something that Time Machine in Catalina seems to have retreated from. Maybe Apple Software Restore (ASR) remains best for APFS volume replication, but isn’t the direction in which Time Machine is heading.

The other and more obvious challenge to Time Machine in Catalina is coping with the new Volume Group formed by the Data and read-only System volumes. It’s good that Apple has offered the user the choice of not backing up the System volume, and will be interesting to see how that option works out with full system restores. Not only has Time Machine had to cope with that structural change, but Apple has added backup of the Recovery volume, a welcome addition which should also make full restores more comprehensive.

This new version of Time Machine isn’t without its problems, though. Worst affected are those who use it to back up to networked storage (NAS), where its undocumented changes can make backups fail completely. Instead of documenting its changes properly so that users and third-party vendors can address them, Apple has once again walked away and left it to unpaid volunteers to support its own products, and effectively bolster its profits.

This is a particular problem here, as Apple accepts no responsibility for what it considers to be a third-party problem, leaving users dependent on those third-parties, who are as ignorant of what has changed as we are. In the NAS market, Mac users are also a small minority; so long as vendors can keep marketing their hardware as being compatible with Time Machine, some don’t seem interested in supporting those users either.

In the long run, Apple is the loser, as its Mac products acquire the reputation of being incompatible and difficult to use, and users brand Catalina as being riddled with bugs. This must infuriate the engineers who have put so much effort into improving macOS, and are caught up in what becomes a blame game driven entirely by Apple’s persistent failure to invest in documentation. Simply stating that Time Machine in Catalina makes changes to backups which are not compatible with previous versions of macOS is not only insulting to users, but accepts that Apple isn’t discharging its obligations to document those changes.

None of us outside Apple knows where Time Machine is heading. Maybe next year we’ll see Apple tempted to re-enter the peripherals market with a neat little solid-state backup store which makes backing up macOS, iOS and iPadOS systems far simpler. Wherever Time Machine is travelling, one of its major destinations has got to be APFS running on SSDs. Has anyone got an ETA?