Glossary

MinIO has accumulated its own vocabulary over a decade. This page collects the terms a new contributor will hit in the first hour of reading the code.

Term	What it means	Where to look
Server pool	A homogeneous group of nodes and drives that share the same erasure parameters. A deployment may have several pools to grow capacity over time.	cmd/erasure-server-pool.go
Erasure set	The actual erasure-coding domain inside a pool — by default 16 drives. Reads and writes for a given object always hit one set.	cmd/erasure-sets.go
Drive / xl-storage	One mounted filesystem that a MinIO server is responsible for. The local on-disk implementation is xl-storage.	cmd/xl-storage.go
xl.meta	MessagePack-encoded sidecar that lives next to every object's shards. It carries versions, parts, encryption metadata, free versions, and inline tiny data.	cmd/xl-storage-format-v2.go
Free version	A logical "deleted" object version still on disk pending lifecycle cleanup. Used to make versioning operations idempotent and recoverable.	cmd/xl-storage-free-version.go
Format file	.minio.sys/format.json on every drive describes which set/pool the drive belongs to. Drives without a format file are considered fresh and get healed in.	cmd/format-erasure.go
Heal	Repair an inconsistency: missing shard, bitrot, missing version, or whole drive. Two flavours: foreground (per-object) and background (full scan).	cmd/erasure-healing.go, cmd/global-heal.go
MRF	"Most Recently Failed". A per-server queue of operations that failed against a peer. The MRF worker re-tries them once the peer recovers.	cmd/mrf.go
Bitrot	Silent on-disk data corruption. MinIO computes a HighwayHash per shard and checks it on read.	cmd/bitrot.go, cmd/bitrot-streaming.go, cmd/bitrot-whole.go
Decommission (decom)	Draining a pool's contents to other pools so the pool can be removed.	cmd/erasure-server-pool-decom.go
Rebalance	Spreading objects more evenly across pools after expansion.	cmd/erasure-server-pool-rebalance.go
Bucket replication	Per-bucket async copy of objects to one or more S3-compatible targets.	cmd/bucket-replication.go, cmd/bucket-targets.go
Site replication	Cluster-level mirror: every bucket, IAM user/policy/group, and policy is kept in sync across N MinIO sites.	cmd/site-replication.go
Tier / warm-backend	A remote S3/Azure/GCS/MinIO endpoint that lifecycle rules can move cold objects to.	cmd/tier.go, cmd/warm-backend-*.go
ILM	Information Lifecycle Management — bucket lifecycle rules for expiration, non-current cleanup, and transitions to tiers.	cmd/bucket-lifecycle.go, internal/bucket/lifecycle/
Object Lambda	S3 Object Lambda style transforms: a request is forwarded to an external function before the object reaches the client.	cmd/object-lambda-handlers.go, internal/config/lambda/
STS	Security Token Service. AWS-style temporary credential issuer (AssumeRole, AssumeRoleWithLDAPIdentity, AssumeRoleWithWebIdentity, AssumeRoleWithCertificate, AssumeRoleWithCustomToken).	cmd/sts-handlers.go
IDP	Identity Provider. MinIO can plug LDAP and OpenID Connect providers as external IDPs for STS and console login.	cmd/admin-handlers-idp-ldap.go, cmd/admin-handlers-idp-openid.go
Madmin	The madmin-go SDK that the mc admin CLI and external tooling use to talk to the admin API.	github.com/minio/madmin-go/v3
Console	The embedded web UI from github.com/minio/console. Mounted by cmd/api-router.go when console is enabled.
Grid	The binary multiplex RPC framework used for low-overhead intra-cluster traffic.	internal/grid/
DSync	Distributed mutex protocol implemented in internal/dsync. Drives cmd/lock-rest-server.go.	internal/dsync/
Namespace lock	The cluster-wide lock primitive every object operation goes through. Wraps DSync and the local locker.	cmd/namespace-lock.go
Metacache	A streaming list-objects cache: list operations write entries to per-bucket caches, served back with continuation tokens.	cmd/metacache*.go
Data scanner	Background sweep of every object that updates usage caches, lifecycle decisions, and heal hints.	cmd/data-scanner.go
Bandwidth monitor	Throughput accounting per bucket/replication target, exposed in metrics.	internal/bucket/bandwidth/
Veeam	Veeam Backup & Replication has special HEAD/GET semantics; MinIO carries Veeam-specific code paths.	cmd/veeam-sos-api.go
Callhome	Optional opt-in telemetry that posts diagnostics back to MinIO's SUBNET service.	cmd/callhome.go, internal/config/callhome/
Speed test	Built-in synthetic PUT/GET benchmark exposed via the admin API.	cmd/speedtest.go, cmd/perf-tests.go
Endpoint ellipsis	The {a...b} notation that expands into multiple endpoints when starting the server.	cmd/endpoint-ellipses.go
Bootstrap peer	Lightweight peer endpoint used at startup to negotiate quorum before the full storage REST is up.	cmd/bootstrap-peer-server.go