NOVA's goal is to provide a high-performance, full-featured, production-ready file system tailored for byte-addressable non-volatile memories (e.g., NVDIMMs and Intel's soon-to-be-released 3DXpoint DIMMs). It combines design elements from many other file systems to provide a combination of high-performance, strong consistency guarantees, and comprehensive data protection. NOVA support DAX-style mmap and making DAX performs well is a first-order priority in NOVA's design. NOVA was developed by the Non-Volatile Systems Laboratory in the Computer Science and Engineering Department at the University of California, San Diego.
NOVA is primarily a log-structured file system, but rather than maintain a single global log for the entire file system, it maintains separate logs for each file (inode). NOVA breaks the logs into 4KB pages, they need not be contiguous in memory. The logs only contain metadata.
File data pages reside outside the log, and log entries for write operations point to data pages they modify. File modification uses copy-on-write (COW) to provide atomic file updates.
For file operations that involve multiple inodes, NOVA use small, fixed-sized redo logs to atomically append log entries to the logs of the inodes involned.
This structure keeps logs small and make garbage collection very fast. It also enables enormous parallelism during recovery from an unclean unmount, since threads can scan logs in parallel.
NOVA replicates and checksums all metadata structures and protects file data with RAID-4-style parity. It supports checkpoints to facilitate backups.
This repository contains a version of the mainline kernel with NOVA added. You can check the current version by looking at the first lines of the Makefile.
A more thorough discussion of NOVA's design is avaialable in these two papers:
NOVA: A Log-structured File system for Hybrid Volatile/Non-volatile Main Memories
PDF
Jian Xu and Steven Swanson
Published in FAST 2016
Hardening the NOVA File System
PDF
UCSD-CSE Techreport CS2017-1018
Jian Xu, Lu Zhang, Amirsaman Memaripour, Akshatha Gangadharaiah, Amit Borase, Tamires Brito Da Silva, Andy Rudoff, Steven Swanson
Read on for further details about NOVA's overall design and its current status
NOVA aims to be compatible with other Linux file systems. To help verify that it achieves this we run several test suites against NOVA each night.
- The latest version of XFSTests. (Current failures)
- The (Linux testing project)(https://linux-test-project.github.io/) file system tests.
- The (fstest POSIX test suite)POSIXtest.
Currently, nearly all of these tests pass for the master branch, and we have
run complex programs on NOVA. There are, of course, many bugs left to fix.
NOVA uses the standard PMEM kernel interfaces for accessing and managing persistent memory.
By default, NOVA makes all metadata and file data operations atomic.
Strong atomicity guarantees make it easier to build reliable applications on NOVA, and NOVA can provide these guarantees with sacrificing much performance because NVDIMMs support very fast random access.
NOVA also supports "unsafe data" and "unsafe metadata" modes that improve performance in some cases and allows for non-atomic updates of file data and metadata, respectively.
NOVA aims to protect data against both misdirected writes in the kernel (which can easily "scribble" over the contents of an NVDIMM) as well as media errors.
NOVA protects all of its metadata data structures with a combination of replication and checksums. It protects file data using RAID-5 style parity.
NOVA can detects data corruption by verifying checksums on each access and by catching and handling machine check exceptions (MCEs) that arise when the system's memory controller detects at uncorrectable media error.
We use a fault injection tool that allows testing of these recovery mechanisms.
To facilitate backups, NOVA can take snapshots of the current filesystem state that can be mounted read-only while the current file system is mounted read-write.
The tech report list above describes the design of NOVA's data protection system in detail.
Supporting DAX efficiently is a core feature of NOVA and one of the challenges in designing NOVA is reconciling DAX support which aims to avoid file system intervention when file data changes, and other features that require such intervention.
NOVA's philosophy with respect to DAX is that when a program uses DAX mmap to to modify a file, the program must take full responsibility for that data and NOVA must ensure that the memory will behave as expected. At other times, the file system provides protection. This approach has several implications:
-
Implementing
msync()in user space works fine. -
While a file is mmap'd, it is not protected by NOVA's RAID-style parity mechanism, because protecting it would be too expensive. When the file is unmapped and/or during file system recovery, protection is restored.
-
The snapshot mechanism must be careful about the order in which in adds pages to the file's snapshot image.
The research paper and technical report referenced above compare NOVA's performance to other file systems. In almost all cases, NOVA outperforms other DAX-enabled file systems. A notable exception is sub-page updates which incur COW overheads for the entire page.
The technical report also illustrates the trade-offs between our protection mechanisms and performance.
Although NOVA is a fully-functional file system, there is still much work left to be done. In particular, (at least) the following items are currently missing:
- There is no mkfs or fsk utility (
mounttakes-o initto create a NOVA file system) - NOVA doesn't scrub data to prevent corruption from accumulating in infrequently accessed data.
- NOVA doesn't read bad block information on mount and attempt recovery of the effected data.
- NOVA only works on x86-64 kernels.
- NOVA does not currently support extended attributes or ACL.
- NOVA does not currently prevent writes to mounted snapshots.
- Using
write()to modify pages that are mmap'd is not supported. - NOVA deoesn't provide quota support.
- Moving NOVA file systems between machines with different numbers of CPUs does not work.
- Remounting a NOVA file system with different mount options may fail.
None of these are fundamental limitations of NOVA's design. Additional bugs and issues are here [here][https://github.com/NVSL/linux-nova/issues].
NOVA is complete and robust enough to run a range of complex applications, but it is not yet ready for production use. Our current focus is on adding a few missing features list above and finding/fixing bugs.
This repo contains a version of the Linux with NOVA included. You should be able to build and install it just as you would the mainline Linux source.
To build NOVA, build the kernel with PMEM (CONFIG_BLK_DEV_PMEM), DAX (CONFIG_FS_DAX) and NOVA (CONFIG_NOVA_FS) support. Install as usual.
Documentation/filesystems/nova.txt provides more detailed instructions on building and using NOVA.
The NOVA source code is almost completely contains in the fs/nova directory.
The execptions are some small changes in the kernel's memory management system
to support checkpointing.
Documentation/filesystems/nova.txt describes the internals of Nova in more detail.
If you find bugs, please report them.
If you have other questions or suggestions you can contact the NOVA developers at cse-nova-hackers@eng.ucsd.edu.