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Users who purchased prior to the recent 5.2.0 update will almost certainly be aware that the latest update removed two algorithms, namely “Quick (on-line)” and “Selected Files (on-line)” because of problems with online defragmentation (which we think are caused by kernel bugs in the macOS itself).
First, a bit of terminology. When we say “on-line”, what we mean is that the algorithm can run while the disk is still mounted and in use by the system. The converse, “off-line”, means that the disk must be unmounted and not in use. The “on-line” algorithms can only work by using ordinary system APIs — they don’t access the disk directly, and they don’t rely on our filesystem code; in theory you’d think that would make them safer and less likely to break, but of course that hasn’t turned out to be the case in this instance. The “off-line” algorithms, on the other hand, use our filesystem code (not the Apple code in the system kernel), and access the disk directly.
Now, a number of customers have been using the “Quick (on-line)” and “Selected Files (on-line)” algorithms for reasons other than being able to continue to use the disk at the same time. For those customers, the 5.2.0 release is potentially a bit of a blow, because we’ve removed a feature they very much liked and appreciated. We hear this.
So, in addition to continuing to work on getting the on-line algorithms to work reliably on future versions of the macOS, we’re adding off-line versions of the two on-line algorithms to iDefrag. These function in the same way you’re used to, but they do require that the disk be unmounted (iDefrag will do that itself). The other difference you might spot is that they will function in reboot-and-defragment mode, where the on-line algorithms cannot for technical reasons.
Because new algorithms are a riskier change, we’re releasing them for now in beta form for customers to test and comment on. You can grab the beta from the iDefrag page.
N.B. If you are running without backups, or are not prepared to restore your files from backups in case of failures, we do not recommend using beta versions of our software.
Posted by alastair at 2017-Aug-01 14:08:42 UTC. Last updated 2017-Aug-01 14:08:19 UTC.