Add opt-in per-disk sync= and cache= attachment modes#42
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Expose VZDiskImageStorageDeviceAttachment's cachingMode and synchronizationMode (macOS 12+) via two new, optional storage options settable from both the JSON config and the --disk CLI: sync=<full|fsync|none> (default: full, unchanged) cache=<auto|cached|uncached> (default: auto, unchanged) When neither is set the behaviour is byte-for-byte identical to before (legacy initWithURL:readOnly:error: path), so existing configs are unaffected. The new initializer is taken only when an option is supplied and only under `if (@available(macOS 12.0, *))`, with a >=120000 SDK compile guard, preserving the macOS 11 build. Invalid values raise a clear VMConfigDiskStorageError. Motivation: ephemeral CI VMs pay a large per-fsync stall under the default full synchronization (every guest fsync blocks on a host-stable commit). Allowing sync=none,cache=cached removes that stall. Measured on a macOS 26.2 guest: 5000 fsync'd small-file writes dropped from 32.2s to 5.0s (6.4x), sequential write 984 -> 1593 MB/s. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Exposes
VZDiskImageStorageDeviceAttachmentcachingMode/synchronizationMode(macOS 12+) as two optional, opt-in storage options, settable from both the JSON config and the--diskCLI:Why
Ephemeral VMs pay a large per-
fsyncstall under the default full synchronization: every guestfsyncblocks on a host-stable commit. Allowingsync=none,cache=cachedremoves that stall.Measured (macOS 26.2 guest, same disk, only the sync mode changed)
Safety / compatibility
initWithURL:readOnly:error:path) — existing configs unaffected.if (@available(macOS 12.0, *))plus a__MAC_OS_X_VERSION_MAX_ALLOWED >= 120000compile guard, so the macOS 11 build is preserved.VMConfigDiskStorageError.Note:
sync=noneis intended for ephemeral VMs (no host-crash durability); not for golden/base images or during--restore.