In this work, we develop the Orchestrated File System (OrcFS) for Flash storage. OrcFS vertically integrates the log-structured file system and the Flash-based storage device to eliminate the redundancies across the layers. A few modern file systems adopt sophisticated append-only data structures in an effort to optimize the behavior of the file system with respect to the append-only nature of the Flash memory. While the benefit of adopting an append-only data structure seems fairly promising, it makes the stack of software layers full of unnecessary redundancies, leaving substantial room for improvement. The redundancies include (i) redundant levels of indirection (address translation), (ii) duplicate efforts to reclaim the invalid blocks (i.e., segment cleaning in the file system and garbage collection in the storage device), and (iii) excessive over-provisioning (i.e., separate over-provisioning areas in each layer). OrcFS eliminates these redundancies via distributing the address translation, segment cleaning (or garbage collection), bad block management, and wear-leveling across the layers. Existing solutions suffer from high segment cleaning overhead and cause significant write amplification due to mismatch between the file system block size and the Flash page size. To optimize the I/O stack while avoiding these problems, OrcFS adopts three key technical elements. First, OrcFS uses disaggregate mapping , whereby it partitions the Flash storage into two areas, managed by a file system and Flash storage, respectively, with different granularity. In OrcFS, the metadata area and data area are maintained by 4Kbyte page granularity and 256Mbyte superblock granularity. The superblock-based storage management aligns the file system section size, which is a unit of segment cleaning, with the superblock size of the underlying Flash storage. It can fully exploit the internal parallelism of the underlying Flash storage, exploiting the sequential workload characteristics of the log-structured file system. Second, OrcFS adopts quasi-preemptive segment cleaning to prohibit the foreground I/O operation from being interfered with by segment cleaning. The latency to reclaim the free space can be prohibitive in OrcFS due to its large file system section size, 256Mbyte. OrcFS effectively addresses this issue via adopting a polling-based segment cleaning scheme. Third, the OrcFS introduces block patching to avoid unnecessary write amplification in the partial page program. OrcFS is the enhancement of the F2FS file system. We develop a prototype OrcFS based on F2FS and server class SSD with modified firmware (Samsung 843TN). OrcFS reduces the device mapping table requirement to 1/465 and 1/4 compared with the page mapping and the smallest mapping scheme known to the public, respectively. Via eliminating the redundancy in the segment cleaning and garbage collection, the OrcFS reduces 1/3 of the write volume under heavy random write workload. OrcFS achieves 56% performance gain against EXT4 in varmail workload.

In most file systems, performance is usually sacrificed in exchange for crash consistency, which ensures that data and metadata are restored consistently in the event of a system crash. To escape this trade-off between performance and crash consistency, recent researchers designed and implemented the transactional functionality inside Solid State Drives (SSDs). However, in order to investigate its benefit in a more realistic and standard fashion, this scheme should be re-evaluated in enterprise storage with standard interface. This paper explores the challenges and implications of a transactional SSD with extensive experiments. To evaluate the potential benefit of transactional SSD, we design and implement the transaction functionality in Samsung enterprise-class and SATA-based SSD (i.e., SM843TN) and name it TxSSD. We then modify the existing file systems (i.e., ext4 and btrfs) on topof TxSSD, making both file systems crash-consistent without redundant writes. We perform performance evaluation of two filesystems by using file I/O and OLTP benchmarks with a database. We also disclose and analyze the overhead of transactional functionality inside SSD. The experimental results show that TxSSD-aware file systems exhibit better performance compared to crash-consistent modes (i.e., data journaling mode of ext4 and cow mode of btrfs) but worse performance compared to weak consistent modes (i.e., ordered mode of ext4 and no datacow mode of btrfs).