1. A computer-implemented method, comprising: configuring, by a first host system, storage of the first host system to serve as a primary cache for a virtual machine running on the first host system; configuring, by a second host system, storage of the second host system to serve as a secondary cache; booting, by the second host system, a placeholder virtual machine on the second host system to reserve processing, memory, or storage resources within the second host system to account for operation of the secondary cache or when the secondary cache is promoted to become a new primary cache; transmitting, in response to write operations from the virtual machine directed to the primary cache, the write operations from the first host system to the second host system to create mirrored copies on the secondary cache, wherein the first host system acknowledges success of each write operation from the virtual machine when the write operation is committed to both the primary cache and the secondary cache; restarting or resuming the virtual machine on the second host system in response to a failure or migration event; and promoting the secondary cache to serve as the new primary cache for the virtual machine in response to the restarting or resuming of the virtual machine on the second host system.