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flexFS

Metadata Server Overview

The metadata server (meta.flexfs) is the central component that stores and manages all filesystem metadata — inodes, directory entries, file attributes, extended attributes, ACLs, and file locks. It serves as the coordination point for all mount clients accessing a volume.

Role

Section titled “Role”

The metadata server:

  • Stores filesystem structure — All inodes, directory entries (dentries), file attributes (size, timestamps, permissions), extended attributes, and access control lists are persisted in the metadata database.
  • Serves mount clients via RPC — Mount clients establish persistent RPC sessions with the metadata server. All filesystem operations (open, read, write, mkdir, unlink, rename, etc.) are coordinated through these sessions.
  • Serves utilities via REST — Utilities like analyze.flexfs, find.flexfs, and dedup.flexfs communicate with the metadata server over its REST API to query and analyze volume contents.
  • Manages block lifecycle — The metadata server tracks which blocks belong to which files, manages block retention for time-travel capabilities, and coordinates garbage collection of unreferenced blocks.
  • Reports to the admin server — The metadata server periodically reports volume statistics (size, inode count, session count) to the admin server.

Protocols

Section titled “Protocols”

RPC (mount clients)

Section titled “RPC (mount clients)”

Mount clients connect to the metadata server over a persistent RPC session. The RPC protocol supports:

  • Filesystem operations (lookup, getattr, setattr, create, mkdir, unlink, rename, link, symlink, readdir, etc.)
  • File locking (fcntl/flock)
  • Extended attributes (getxattr, setxattr, listxattr, removexattr)
  • ACL operations
  • Session management (connect, disconnect, heartbeat)

The RPC protocol version is negotiated at session establishment, allowing the metadata server to serve mount clients running different versions simultaneously (within compatibility bounds).

REST (utilities and admin)

Section titled “REST (utilities and admin)”

The metadata server exposes an HTTP REST API used by utilities (search, analysis, deduplication), monitoring (/metrics, /status), and administrative maintenance operations. See the Metadata REST API reference for the full list of endpoints and their parameters.

Relationship to volumes

Section titled “Relationship to volumes”

Each metadata server instance serves one or more volumes. The mapping between metadata servers and volumes is managed by the admin server. When a mount client presents a volume token, the admin server directs it to the appropriate metadata server.