Add virtio-blk cachingMode and synchronizationMode options #383
Description
virtio-blk devices have cachingMode and synchronziationMode options that are critical for correctness and performance, but the user of vfkit cannot control them.
Disk image caching modes
- VZDiskImageCachingModeAutomatic
Allows the virtualization framework to automatically determine whether to enable data caching. - VZDiskImageCachingModeCached
Enables data caching. - VZDiskImageCachingModeUncached
Disables data caching.
The default mode is not documented, but VZDiskImageCachingModeAutomatic is the only mode that make sense as default.
Disk image synchronization modes
- VZDiskImageSynchronizationModeFull
Synchronizes data to the permanent storage holding the disk image. - VZDiskImageSynchronizationModeFsync
Synchronizes data to the drive using the system’s best-effort synchronization mode. - VZDiskImageSynchronizationModeNone
Disables data synchronization with the permanent storage.
The default is not documented, Apple probably want to keep the option to change the default.
Add new virtio-blk options
We cannot select the right options for the user, since every user may have different needs. vfkit should not dictate the policy but pass the options the virtualization framework gave us to the user.
When we give the user the options, the user is responsible for issues and we don't need to spend time on debugging and support. If we document the options correctly, and pass them to Apple virtualization framework correctly, now any issue is on the user or Apple side.
User can contribute performance testing results using the new options.
cache:auto: Allows the virtualization framework to automatically determine whether to enable data caching.cached: Enables data caching.uncached: Disables data caching.
sync:full: Synchronizes data to the permanent storage holding the disk image.fsync: Synchronizes data to the drive using the system’s best-effort synchronization mode.none: Disables data synchronization with the permanent storage.
Use cases
We can document typical use case and add fio performance results to show the effect of the configuration. The following are my guess for typical usage - I did not do any preforamnce testing since we don't have the option yet.
General purpose VM
vfkit should keep the virtualization framework defaults, unless we know that the defaults are broken. In this case we need a vfkit issue and open an Apple bug.
--device virtio-blk,...
Ephemeral VM
In this case you don't care about the data so you can use:
--device virtio-blk,cache=cached,sync=none,...
Database VM
In this case you care about the data so you can use:
--device virtio-blk,cache=uncached,sync=full,...
Testing
User can any combination for testing vfkit performance or robustness.
Similar options in other systems
qemu
See -drive cache=cache section
https://www.qemu.org/docs/master/system/invocation.html#hxtool-1
krunkit
Currently does not provide any options, we need the same options for compatibility with vfkit.