Transparent heterogeneous backing store for file systems

We present Nswap2L-FS, a fast, adaptable, and heterogeneous storage system for backing file data in clusters. Nswap2L-FS particularly targets backing temporary files, such as those created by data-intensive applications for storing intermediate results. Our work addresses the problem of how to efficiently and effectively make use of heterogeneous storage devices that are increasingly common in clusters. Nswap2L-FS implements a two-layer device design. The top layer transpar- ently manages a set of bottom layer physical storage devices, which may include SSD, HDD, and its own implementation of network RAM. Nswap2L-FS appears to node operating systems as a single, fast backing storage device for file systems, hiding the complexity of heterogeneous storage management from OS subsystems. Internally, it implements adaptable and tunable policies that specify where data should be placed and whether data should be migrated from one underlying physical device to another based on resource usage and the characteristics of different devices. We present solutions to challenges that are specific to supporting backing filesystems, including how to efficiently support a wide range of I/O request sizes and balancing fast storage goals with expectations of persistence of stored file data. Nswap2L-FS defines relaxed persistence guarantees on individual file writes to achieve faster I/O accesses; less stringent persistence semantics allow it to make use of network RAM to store file data, resulting in faster file I/O to applications. Relaxed persistence guarantees are acceptable in many situations, particularly those involving short-lived data such as temporary files. Nswap2L-FS provides a persistence snapshot mechanism that can be used by applications or checkpointing systems to ensure that file data are persistent at certain points in their execution. Nswap2L-FS is implemented as a Linux block device driver that can be added as a file partition on individual cluster nodes. Experimental results show that file-intensive applications run faster when using Nswap2L-FS as backing store. Additionally, its adaptive data placement and migration policies, which make effective use of different underlying physical storage devices, result in performance exceeding that of any single device.