The NOVA (''

non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data. Non-volatile memory typ ...

accelerated'') file system is an

open-source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use and view the source code, design documents, or content of the product. The open source model is a decentrali ...

log-structured file system A log-structured filesystem is a file system in which data and metadata are written sequentially to a circular buffer, called a log. The design was first proposed in 1988 by John K. Ousterhout and Fred Douglis and first implemented in 1992 by ...

for byte-addressable

persistent memory Persistent may refer to: * Persistent data * Persistent data structure * Persistent identifier * Persistent memory * Persistent organic pollutant * Persistent Systems, a technology company * USS ''Persistent'', three United States Navy ships ...

(for example non-volatile dual in-line memory module (NVDIMM) and 3D XPoint DIMMs) for

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

. NOVA is designed specifically for byte-addressable persistent memories and aims to provide high-performance, atomic file and metadata operations, and fault tolerance. To meet these goals NOVA combines several techniques found in other file systems. NOVA uses log structure,

copy-on-write Copy-on-write (COW), also called implicit sharing or shadowing, is a resource-management technique used in programming to manage shared data efficiently. Instead of copying data right away when multiple programs use it, the same data is shared ...

(COW), journaling, and log-structured metadata updates to provide strong atomicity guarantees, and it uses a combination replication, metadata checksums, and

RAID 4 In computer storage, the standard RAID levels comprise a basic set of RAID ("redundant array of independent disks" or "redundant array of inexpensive disks") configurations that employ the techniques of striping, mirroring, or parity to create la ...

parity to protect data and metadata from media errors and software bugs. It also supports checkpoints to facilitate backups.

Filesystem

NOVA was developed at the

University of California, San Diego The University of California, San Diego (UC San Diego in communications material, formerly and colloquially UCSD) is a public university, public Land-grant university, land-grant research university in San Diego, California, United States. Es ...

, in the Non-Volatile Systems Laboratory of the Computer Science and Engineering Department. Patches were initially made available for version 4.12 of the

Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

. it is limited to

x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit extension of the x86 instruction set architecture, instruction set. It was announced in 1999 and first available in the AMD Opteron family in 2003. It introduces two new ope ...

Linux, and not ready for merging with the upstream kernel.

Log structure

NOVA is primarily a log-structured file system, but it differs from other log-structured file systems in several respects. First, rather than using a single log for the entire file system, each

inode An inode (index node) is a data structure in a Unix-style file system that describes a file-system object such as a file or a directory. Each inode stores the attributes and disk block locations of the object's data. File-system object attribu ...

has its own, dedicated log that records the updates to the inode. This allows for increased concurrency in file operations, since different threads can operate on inodes in parallel. Second, the logs do not contain file data, but only metadata updates, resulting in smaller logs. Third, the logs are not stored in physically contiguous memory. Instead, NOVA stores the logs in a

linked list In computer science, a linked list is a linear collection of data elements whose order is not given by their physical placement in memory. Instead, each element points to the next. It is a data structure consisting of a collection of nodes whi ...

of 4 KB memory pages. NOVA uses the logs to provide atomicity for operations that affect a single file (e.g., writing to a file or modifying its metadata). To do this, NOVA writes a log entry to empty space past the end of the log and then atomically updates the inode's pointer to the log tail.

Copy-on-write

NOVA uses

(COW) to update file data. When a program writes data to a file, NOVA allocates some unused memory pages to hold the data and writes the data into them. Then, it appends a log entry to the inode's log that points to the new pages and describes their logical location in the file. Since appending the log entry is atomic, the write is also atomic.

Journaling

Some file operations (e.g., moving a file from one directory to another) require modifying multiple inodes. To make these operations atomic, NOVA uses a simple journaling mechanisms. First, it writes the new log entries to ends of the inodes that the operation will affect, then it uses the journal to record the necessary updates to the inodes' log tail pointers. Next, it marks the journal as committed and applies the updates to the tail pointers.

Metadata protection

NOVA uses replication and

checksum A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify dat ...

s to provide protection against metadata corruption due to media errors and

software bug A software bug is a design defect ( bug) in computer software. A computer program with many or serious bugs may be described as ''buggy''. The effects of a software bug range from minor (such as a misspelled word in the user interface) to sev ...

s. Every metadata structure (e.g., inodes, superblocks, and log entries) contains a

CRC32 Computation of a cyclic redundancy check is derived from the mathematics of polynomial division, modulo two. In practice, it resembles long division of the binary message string, with a fixed number of zeroes appended, by the "generator poly ...

checksum that allows NOVA to detect if structures contents have changed with its knowledge. NOVA also stores two copies of each data structure – the "primary" and the "replica" – and stores them far from one another in memory. Whenever NOVA accesses a metadata structure, it first recomputes the checksum on both the primary and the replica. If either check results in a mismatch, NOVA repairs the damage using the other copy. If neither checksum matches, then the structure is lost and NOVA returns an error.

Data protection

NOVA uses

to protect file data. It divides each 4 KB page into 512-byte strips and stores a parity strip in a dedicated region of persistent memory. It also computes (and stores a replica of) a

checksum for the eight data strips and the parity strip. When NOVA reads a page, it confirms the checksum on each strip. If one of the strips is corrupt, it tries to recover the strip using the parity bits. If no other strips have experienced data corruption, recovery will succeed. Otherwise, recovery fails, the contents of the page are lost, and NOVA returns an error.

References

External links

NOVA: A Log-structured File System for Hybrid Volatile/Non-volatile Main Memories

Hardening the NOVA File System UCSD-CSE Techreport CS2017-1018

NOVA: The Fastest File System for NVDIMMs
{{Linux Free special-purpose file systems Free software programmed in C Linux kernel features Unix file system-related software