FSYNC - File Synchronization

Enhanced Definition

`fsync` is a POSIX system call available within z/OS UNIX System Services (USS) that forces all modified in-memory data and metadata (e.g., file size, timestamps) for a file associated with a given file descriptor to be written to the underlying storage device. Its primary purpose is to ensure data durability and integrity, guaranteeing that changes are physically committed to disk and will survive system crashes or power failures.

Key Characteristics

- POSIX Standard Compliance: fsync is part of the POSIX.1 standard, making it a common interface for ensuring data persistence in Unix-like environments, including z/OS USS.
- Data Durability Guarantee: It provides a strong guarantee that all buffered changes for a file are flushed from the operating system's cache and written to non-volatile storage.
- Operates on File Descriptors: The fsync function takes an open file descriptor as an argument, applying the synchronization operation to that specific file.
- Performance Impact: Invoking fsync can be a relatively slow operation as it forces physical I/O, potentially blocking the calling process until the write completes.
- Error Handling: It returns 0 on success and -1 on failure, setting the errno variable to indicate the specific error (e.g., EIO for I/O errors, EBADF for an invalid file descriptor).
- Scope on z/OS: fsync is exclusively applicable to files residing within z/OS UNIX file systems, such as Hierarchical File System (HFS) or z/OS File System (zFS), and does not apply to traditional MVS datasets.

Use Cases

- Database Transaction Logging: Critical for database systems (e.g., DB2 for z/OS using USS for logs, or third-party databases) to ensure transaction log records are persistently written before a transaction is committed, guaranteeing ACID properties.
- Journaling File Systems: Used by file system implementations (like zFS itself) to ensure the integrity of file system metadata and user data during crash recovery.
- Application Crash Recovery: Applications that maintain critical state or data in files can use fsync to ensure that data is recoverable even if the application or system crashes unexpectedly.
- Configuration File Updates: When updating critical configuration files, fsync can be used after writing to ensure the new configuration is durable and will be present after a system restart.
- Batch Processing in USS: For batch jobs running in the USS environment that produce critical output files, fsync can be called before job completion to ensure all output is safely written to disk.

Related Concepts

fsync is fundamentally tied to z/OS UNIX System Services (USS), as it is a core POSIX API implemented within this environment. It operates on files stored in HFS or zFS file systems, which are the native file systems for USS. It directly contributes to data integrity and atomicity by ensuring that data is not merely buffered but physically committed. While similar in intent, fsync is distinct from traditional MVS dataset I/O mechanisms and does not apply to VSAM, sequential datasets, or PDS/PDSE; those datasets have their own integrity and synchronization methods managed by DFSMS and access methods. The open() system call's O_SYNC or O_DSYNC flags provide an alternative to fsync, forcing synchronous writes for every write() operation on a file.

Best Practices:

Use Judiciously: Due to its performance overhead, apply fsync only when data durability is absolutely critical and the cost of data loss outweighs the performance impact.
Always Check Return Codes: Implement robust error handling by checking the return value of fsync and inspecting errno to diagnose and handle potential I/O errors.
Batch Writes: Where possible, aggregate multiple writes to a file before issuing a single fsync call to reduce the number of costly disk synchronization operations.
Understand fsync vs. fdatasync: If only data (and not metadata) needs to be synchronized, consider using fdatasync (if available and appropriate for the specific z/OS USS version), which can sometimes be faster by avoiding metadata updates.
Consider O_SYNC for Specific Files: For files where every write must be synchronous, opening the file with the O_SYNC flag can simplify application logic, though it applies to all subsequent writes.