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Bucket Versioning Design Guide

Description of `xl.meta`

xl.meta is a new self describing backend format used by MinIO to support AWS S3 compatible versioning. This file is the source of truth for each version at rest. xl.meta is a msgpack file serialized from a well defined data structure. To understand xl.meta here are the few things to start with

xl.meta carries first 8 bytes an XL header which describes the current format and the format version, allowing the unmarshaller's to automatically use the right data structures to parse the subsequent content in the stream.

v1.0

Entry	Encoding	Content
xlHeader	[4]byte	`'X', 'L', '2', ' '`
xlVersion	[4]byte	`'1', ' ', ' ', ' '`
xlMetaV2	msgp object	All versions as single messagepack object
[EOF]

v1.1+

Version 1.1 added inline data, which will be placed after the metadata.

Therefore, the metadata is wrapped as a binary array for easy skipping.

Entry	Encoding	Content
xlHeader	[4]byte	`'X', 'L', '2', ' '`
xlVersionMajor	uint16	Major xl-meta version.
xlVersionMinor	uint16	Minor xl-meta version.
xlMetaV2	msgp bin array	Bin array with serialized metadata
crc	msgp uint	Lower 32 bits of 64 bit xxhash of previous array contents (v1.2+ only)
inline data	binary	Inline data if any, see Inline Data section for encoding.
[EOF]

v1.0-v1.2 Versions

xl.meta carries three types of object entries which designate the type of version object stored.

ObjectType (default)
LegacyObjectType (preserves existing deployments and older xl.json format)
DeleteMarker (a versionId to capture the DELETE sequences implemented primarily for AWS spec compatibility)

A sample msgpack-JSON xl.meta, you can debug the content inside xl.meta using xl-meta.go program.

{
  "Versions": [
    {
      "Type": 1,
      "V2Obj": {
        "ID": "KWUs8S+8RZq4Vp5TWy6KFg==",
        "DDir": "X3pDAFu8Rjyft7QD6t7W5g==",
        "EcAlgo": 1,
        "EcM": 2,
        "EcN": 2,
        "EcBSize": 10485760,
        "EcIndex": 3,
        "EcDist": [3, 4, 1, 2],
        "CSumAlgo": 1,
        "PartNums": [1],
        "PartETags": [""],
        "PartSizes": [314],
        "PartASizes": [282],
        "Size": 314,
        "MTime": 1591820730,
        "MetaSys": {
          "X-Minio-Internal-Server-Side-Encryption-S3-Kms-Key-Id": "bXktbWluaW8ta2V5",
          "X-Minio-Internal-Server-Side-Encryption-S3-Kms-Sealed-Key": "ZXlKaFpXRmtJam9pUVVWVExUSTFOaTFIUTAwdFNFMUJReTFUU0VFdE1qVTJJaXdpYVhZaU9pSkJMMVZzZFVnelZYVjZSR2N6UkhGWUwycEViRmRCUFQwaUxDSnViMjVqWlNJNklpdE9lbkJXVWtseFlWSlNVa2t2UVhNaUxDSmllWFJsY3lJNklrNDBabVZsZG5WU1NWVnRLMFoyUWpBMVlYTk9aMU41YVhoU1RrNUpkMDlhTkdKa2RuaGpLMjFuVDNnMFFYbFJhbE15V0hkU1pEZzNRMk54ZUN0SFFuSWlmUT09",
          "X-Minio-Internal-Server-Side-Encryption-Seal-Algorithm": "REFSRXYyLUhNQUMtU0hBMjU2",
          "X-Minio-Internal-Server-Side-Encryption-Iv": "bW5YRDhRUGczMVhkc2pJT1V1UVlnbWJBcndIQVhpTUN1dnVBS0QwNUVpaz0=",
          "X-Minio-Internal-Server-Side-Encryption-S3-Sealed-Key": "SUFBZkFPeUo5ZHVVSEkxYXFLU0NSRkJTTnM0QkVJNk9JWU1QcFVTSXFhK2dHVThXeE9oSHJCZWwwdnRvTldUNE8zS1BtcWluR0cydmlNNFRWa0N0Mmc9PQ=="
        },
        "MetaUsr": {
          "content-type": "application/octet-stream",
          "etag": "20000f00f58c508b40720270929bd90e9f07b9bd78fb605e5432a67635fc34722e4fc53b1d5fab9ff8400eb9ded4fba2"
        }
      }
    }
  ]
}

v1.3+ versions

Version 1.3 introduces changes to help with faster metadata reads and updates

Entry	Encoding	Content
xlHeaderVersion	msgp uint	header version identifier
xlMetaVersion	msgp uint	metadata version identifier
versions	msgp int	Number of versions following
header_1	msgp bin array	Header of version 1
metadata_1	msgp bin array	Metadata of version 1
...header_n	msgp bin array	Header of last version
...metadata_n	msgp bin array	Metadata of last version

Each header contains a mspg array (tuple) encoded object:

xlHeaderVersion version == 1:

//msgp:tuple xlMetaV2VersionHeader
type xlMetaV2VersionHeader struct {
 VersionID [16]byte  // Version UUID, raw.
 ModTime   int64     // Unix nanoseconds.
 Signature [4]byte   // Signature of metadata.
 Type      uint8     // Type if the version
 Flags     uint8
}

The following flags are defined:

const (
 FreeVersion = 1 << 0
 UsesDataDir = 1 << 1
 InlineData  = 1 << 2
)

The "Metadata" section contains a single version, encoded in similar fashion as each version in the Versions array of the previous version.

Inline Data

Inline data is optional. If no inline data is present, it is encoded as 0 bytes.

Entry	Encoding	Content
xlMetaInlineDataVer	byte	version identifier
id -> data	msgp map[string][]byte	Map of string id -> byte content

Currently only xlMetaInlineDataVer == 1 exists.

The ID is the string encoded Version ID of which the data corresponds.

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