vphone
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      • Disk caching mode

      vphone-vm attaches Disk.img through VPhoneDiskAttachment.make, which spells out caching .cached and synchronization .full. Before, it used VZDiskImageStorageDeviceAttachment(url:readOnly:), whose defaults are caching automatic and synchronization full (read back on macOS 27.0.1 with a probe; the header does not state them).

      Why .cached

      Measured on 2026-10-21 (Mac fleet investigation W7) on two idle Mac mini M4 hosts (16 GB, macOS 27.0.1, SIP as shipped, no other VMs), with Linux guests rather than vphone guests, since SIP as shipped cannot run a vphone. 2 GiB guests checked known patterns in 1.3 GiB of RAM, through a virtio-blk disk with O_DIRECT and through the page cache, and in O_DIRECT read buffers left idle, while a host program pushed memory pressure to warning and then critical. Each round ran 900 seconds.

      DiskGuest-runsLost data
      Sparse image, automatic or uncached, host compressing2626
      Sparse image, automatic or uncached, host idle20
      Fully written (not sparse) image, automatic or uncached80
      cached (sparse or written, all three synchronization modes)390
      NBD90
      virtiofs60
      • The loss is always zeros, never stale data, in runs that are whole multiples of 2 MiB of guest RAM. It hits anonymous memory the guest never did I/O into, the guest kernel itself (one guest oopsed on a NULL pointer), page-cache pages and idle O_DIRECT buffers. A zeroed dirty page written back leaves a zero block in the image.
      • What gates it is the host compressor working on guest memory, not the pressure level: one round lost data at level normal, and every round where the compressor stayed idle was clean. A memory gate or a 1.0 overcommit ratio does not protect a non-cached guest.
      • 36 of the 39 cached guest-runs saw the host at critical, some for five minutes.

      So the loss needs a non-cached attachment on a sparse image while the host compresses guest memory, and .cached removed it on these hosts. A vphone guest itself was not tested; it uses the same attachment.

      Effect on existing machines

      Nothing on disk or in config.plist changes, and no manifest key is added. The mode is chosen by vphone-vm at each launch, so a machine takes it at its first launch with a bundle carrying this change; a running machine keeps the old mode until it restarts. Going back to an older bundle goes back to automatic with nothing to undo. .cached lets the host page cache hold more of the image; the memory and speed effect on a running vphone is not measured.