Host-side flash caching emerges as an effective solution for improving the performance of virtual machines (VMs) in cloud computing environment. However, for VMs with the commonly used Copy-on-Write (COW) virtual disks, flash caching in fact has negative impacts since it brings lots of unnecessary cache writes, hurting both the VM performance and the flash endurance. This paper presents COWCache, a COW-aware caching solution that addresses this problem by co-designing flash caching with COW-based virtual disks. First, COWCache designs a new architecture that bridges the semantic gap between flash caching and virtual disk management for realizing the VMs' real data locality. Second, it separately manages COW metadata with fine-grained caching and journaling to improve the metadata caching efficiency. Third, it provides a novel decoupled Copy-on-Write mechanism, which decouples the amplified data requests from the critical I/O path and only admits the data with real VM locality into flash cache. COWCache also introduces a new data structure, the virtual cache map, to mitigate the memory footprint overhead for indexing the cached data in flash. Evaluations show that COWCache improves the application performance by up to 122.7% and reduces the flash cache writes by up to 78.5% compared to traditional flash caching solutions.

Virtual machines (VMs) have become the predominant paradigm for deploying applications, and flash-based caches are widely deployed for VMs to improve their performance. However, due to the semantics of journaling in the VMs' file system are transparent to flash caches, traditional flash caching with journaling induces inefficient logging I/O traffic to the shared storage server and duplicated caching in flash, which not only underutilizes flash caches but also aggravates their wearout. This paper presents JCache, a journaling-aware flash caching solution to address these problems. First, JCache proposes the virtual journal device design to receive and deliver the journaling semantics in the VMs to the cache manager so as to optimize the VM's journaling. Second, JCache proposes a cache-only logging mechanism, which transparently uses the persistent flash caches as the journal area and does cache-only logging to safely eliminate the logging I/O traffic to the shared storage server. Third, JCache proposes the logical caching mechanism, which eliminates the duplicated flash caching induced by logging I/Os and in-place updates to mitigate the wearout of flash caches. Evaluations show that JCache improves the application performance by up to 11.4× and reduces the flash cache writes by up to 42% compared to traditional flash caching solutions. INDEX TERMS Cloud computing, virtual machines, journaling file system, flash caching, caching with journaling.

We present GluStack, a virtual machine(VM) cluster system based on OpenStack using the KVM hypervisor and GlusterFS distributed file system. Software assembly is one of GluStack's most important features that different software chains can be based on the same base image to start different types of VM instance so as to satisfy variable working scenarios for different users easily. We also developed a new VM image format for QEMU called LBCS that implements the feature of image local block caching and sharing to achieve high I/O performance and enable instant VM startup in the GluStack architecture.

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