SLOG
also: Separate Intent Log, ZFS Intent Log
SLOG is a ZFS intent log stored on a fast device (typically SSD) that caches synchronous write operations before they're written to the main pool, improving write performance and data safety.
SLOG stands for "Separate Intent Log" in ZFS (Zettabyte File System). When applications perform synchronous writes (writes that must be confirmed before the operation completes), ZFS writes to the SLOG first, then later writes to the main storage pool. This allows the application to resume quickly while the pool write happens asynchronously in the background.
A SLOG device should be fast and reliable—typically an SSD or battery-backed write cache—because it handles every synchronous write. Without a SLOG, synchronous writes must wait for the slow main pool storage, creating a bottleneck for databases and other performance-critical applications.
For example, a database performing fsync() calls will experience much lower latency if the system has an SSD SLOG: the write confirms to the application after hitting the fast SSD, not after hitting slow spinning disks in the pool.