In qemu-system-ARCH versions older than v1.2 (e.g. SLES11 SP2), not specifying a cache mode meant that writethrough would be used as the default. Since that version, the various qemu-system-ARCH guest storage interfaces have been fixed to handle writeback or writethrough semantics more correctly, allowing for the default caching mode to be switched to writeback. The guest driver for each of ide, scsi, and virtio have within their power to disable the write back cache, causing the caching mode used to revert to writethrough. The typical guest's storage drivers will maintain the default caching mode as writeback, however.

This mode causes qemu-system-ARCH to interact with the disk image file or block device with O_DSYNC semantics, where writes are reported as completed only when the data has been committed to the storage device. The host page cache is used in what can be termed a writethrough caching mode. The guest's virtual storage adapter is informed that there is no writeback cache, so the guest would not need to send down flush commands to manage data integrity. The storage behaves as if there is a writethrough cache.

This mode causes qemu-system-ARCH to interact with the disk image file or block device with neither O_DSYNC nor O_DIRECT semantics, so the host page cache is used and writes are reported to the guest as completed when placed in the host page cache, and the normal page cache management will handle commitment to the storage device. Additionally, the guest's virtual storage adapter is informed of the writeback cache, so the guest would be expected to send down flush commands as needed to manage data integrity. Analogous to a raid controller with RAM cache.

This mode causes qemu-system-ARCH to interact with the disk image file or block device with O_DIRECT semantics, so the host page cache is bypassed and I/O happens directly between the qemu-system-ARCH userspace buffers and the storage device. Because the actual storage device may report a write as completed when placed in its write queue only, the guest's virtual storage adapter is informed that there is a writeback cache, so the guest would be expected to send down flush commands as needed to manage data integrity. Equivalent to direct access to your hosts' disk, performance wise.

This mode is similar to the cache=writeback mode discussed above. The key aspect of this “unsafe” mode, is that all flush commands from the guests are ignored. Using this mode implies that the user has accepted the trade-off of performance over risk of data loss in the event of a host failure. Useful, for example, during guest install, but not for production workloads.

This mode causes qemu-system-ARCH to interact with the disk image file or block device with both O_DSYNC and O_DIRECT semantics, where writes are reported as completed only when the data has been committed to the storage device, and when it is also desirable to bypass the host page cache. Like cache = writethrough, it is helpful to guests that do not send flushes when needed. It was the last cache mode added, completing the possible combinations of caching and direct access semantics.

These are the safest modes, and considered equally safe, given that the guest operating system is “modern and well behaved”, which means that it uses flushes as needed. If you have a suspect guest, use writethough, or directsync. Note that some file systems are not compatible with cache=none or cache=directsync, as they do not support O_DIRECT, which these cache modes relies on.

This mode informs the guest of the presence of a write cache, and relies on the guest to send flush commands as needed to maintain data integrity within its disk image. This is a common storage design which is completely accounted for within modern file systems. But it should be noted that because there is a window of time between the time a write is reported as completed, and that write being committed to the storage device, this mode exposes the guest to data loss in the unlikely event of a host failure.

This mode is similar to writeback caching except the guest flush commands are ignored, nullifying the data integrity control of these flush commands, and resulting in a higher risk of data loss due to host failure. The name “unsafe” should serve as a warning that there is a much higher potential for data loss due to a host failure than with the other modes. Note that as the guest terminates, the cached data is flushed at that time.