Read-write and VM disk images are created and maintained differently, but they can both be APFS sparse files. Explanation of their creation and maintenance.
APFS
From early code pages based on language-specific variants of Extended ASCII, through 2-byte support in WorldScript II, to Unicode and the annual emoji update.
A brief start with MFS for 400 KB floppies, followed by HFS intended for the first hard disks, upgraded to HFS+ in 1988, and followed in 2017 by APFS for all the OSes.
How read-write disk images and those used in Apple silicon virtual machines use sparse file format to save space on disk.
How do clone files and sparse files cope with being backed up and restored? Can they save space in iCloud Drive? Some of these answers may surprise.
Deleting clone files saves no space, but converting copies into clones could free up plenty of storage. Sparse files can also be highly efficient, and squeeze 285 GB into just 16.5 GB.
Systematic and thorough account of the structure and function of bootable external disks and dual-boot systems from High Sierra to Sequoia, and how to diagnose their problems.
Speed up your hard disk by partitioning it so that its innermost 20% remains unused. Reserve space for the SLC write cache on an SSD by limiting the size of volumes.
When it started up after its macOS update, available space on its internal SSD had shot from about 160 GB to nearly 400 GB. Where had all my files gone?
How FileVault and APFS Encrypted are enabled and managed differently, and details of how they work internally. Concentrates on T2 and Apple silicon Macs, but also covers older Intel models.
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