Explainer: Cloud storage

There are three basic forms of storage: local, which is directly attached to and accessed by your Mac; local network, which is connected to your Mac’s local network; and cloud, which your Mac can only access over its internet connection. Cloud storage includes that provided free of charge, or at additional cost for larger amounts, in iCloud, and that offered by services such as Dropbox and Microsoft OneDrive.

Just as important as how your Mac connects to that storage is how it’s managed. Your Mac, under your direct control, manages all its local storage. Local network storage is managed by another computer, typically that built into a Network-Attached Storage system, or NAS; as you normally manage that NAS device, it again remains under your control. Cloud storage is different in that its servers are remote, and their service is managed and operated by someone else. Although you’re given options, you have essentially no say in how that storage is managed for you.

Local and local network storage is based on familiar file systems, like APFS. These depend on file system metadata to manage file data. When accessed over your network, network file system protocols like SMB and AFP are used, so that the file system on the server can be quite different from any used on your Mac. Cloud systems are different again, in that most don’t use anything resembling a local file system, but operate using huge distributed file systems in which your files may be replicated across multiple servers as chunks of data.

Many cloud services provide a range of different products in addition to storage, and Apple’s iCloud+ includes Private Relay, email enhancements such as Hide My Email, and Find My, for example. Non-storage services usually operate using different protocols on different servers, as illustrated by Apple’s ever-growing system status page.

Even when you have an extremely fast and capable internet connection, accessing cloud storage for the great majority of users is slower and less reliable than accessing a NAS or an external disk. Accordingly, files stored remotely are usually presented in either of two ways: each file can keep a local copy synchronised with that on the cloud servers; or a local ‘stub’ file represents a file whose data are only stored remotely. Cloud storage services refer to these using different terms. In iCloud Drive, the latter is referred to as having been evicted from local storage; in OneDrive, the former is said to be pinned to local storage.

When no copy of a file is stored locally – in iCloud when it has been evicted, in OneDrive the file isn’t pinned – it has important consequences for local services which might want access to that file. Making local backups is an obvious problem, as a file which isn’t stored locally can’t be backed up directly. But this extends beyond that to include features such as Spotlight search. While some types of search, such as those based on file names, could still run normally without a local copy of the file, others including searches for file content rely on instant access to the file’s contents.

Integrating cloud and local storage is another difficult area. We’re all familiar with the Finder’s convenient illusion for the contents of iCloud Drive, but for apps which need to access the contents of cloud storage the underlying arrangement is more complex. This can cause problems of its own, and when apps depend on accessing files stored in the cloud, any degradation of its performance can make those apps unusable. Users usually have little or no ability to address problems with cloud storage.

As internet access becomes faster for many, some products may offer the ability to back up or search files which don’t have local copies available. However, that comes at the cost of having to download the file contents repeatedly, and has to handle occasions when cloud access is degraded or lost altogether. Their future with any given cloud storage product must also remain uncertain, because of the increased load they impose on the cloud service. As the most popular cloud storage products are either free or offered at low consumer pricing, changing terms and conditions can be used to control that. Some users are already discovering that consumer services usually limit the number of files that can be downloaded in a single session, or over a period of time, something few services make clear.

Unless you pay extra for a cloud storage product which guarantees high levels of availability, you should always plan for delays in access to files which are only stored remotely. Relying on immediate access to anything which isn’t stored within your local network puts you at the mercy of too many external factors over which you have no control: your internet connection and ISP, external network connectivity, location and availability of remote servers hosting the service, and the health of the service itself.

Cloud storage is a major advance for many Mac users, but as ever it’s no free lunch. Working within its limits is essential if you’re to avoid losing data. Although cloud storage can be a valuable means of keeping important files off-site, safety still requires users to keep multiple copies of files locally in more traditional backups. Cloud storage is a valuable adjunct, not a substitute for thorough local backups, and it’s essential that anything you store in the cloud is also backed up locally.