diff --git a/server/lib/room/channel.ts b/server/lib/room/channel.ts
new file mode 100644
index 00000000..4bd8e257
--- /dev/null
+++ b/server/lib/room/channel.ts
@@ -0,0 +1,114 @@
+import { RegisterServerOptions } from '@peertube/peertube-types'
+import { getProsodyDomain } from '../prosody/config/domain'
+import * as path from 'path'
+import * as fs from 'fs'
+
+/**
+ * Stores that given room is related to given channel.
+ * Can throw an exception.
+ * @param options server options
+ * @param channelId channel ID
+ * @param roomJIDLocalPart room JID (only the local part)
+ */
+async function setChannel2Room (
+  options: RegisterServerOptions,
+  channelId: number,
+  roomJIDLocalPart: string
+): Promise<void> {
+  const logger = options.peertubeHelpers.logger
+  logger.info(`Calling setChannel2Room for channel ${channelId} and room ${roomJIDLocalPart}...`)
+
+  _checkParameters(channelId, roomJIDLocalPart)
+
+  const prosodyDomain = await getProsodyDomain(options)
+
+  {
+    const [channel2roomDir, channel2room] = await _getFilePath(
+      options, channelId, roomJIDLocalPart, prosodyDomain, 'channel2room'
+    )
+    await fs.promises.mkdir(channel2roomDir, {
+      recursive: true
+    })
+    await fs.promises.writeFile(
+      channel2room,
+      ''
+    )
+  }
+
+  {
+    const [room2channelDir, room2channel, room2channelFile] = await _getFilePath(
+      options, channelId, roomJIDLocalPart, prosodyDomain, 'room2channel'
+    )
+    await fs.promises.mkdir(room2channelDir, {
+      recursive: true
+    })
+
+    // The video's channel could have changed. We must delete any deprecated file.
+    const previousFiles = await fs.promises.readdir(room2channelDir)
+    for (const filename of previousFiles) {
+      if (filename !== room2channelFile) {
+        const p = path.resolve(room2channelDir, filename)
+        logger.info('Cleaning a deprecated room2channelFile: ' + p)
+        await fs.promises.unlink(p)
+      }
+    }
+
+    await fs.promises.writeFile(
+      room2channel,
+      ''
+    )
+  }
+}
+
+function _checkParameters (channelId: number | string, roomJIDLocalPart: string): void {
+  channelId = channelId.toString()
+  if (!/^\d+$/.test(channelId)) {
+    throw new Error(`Invalid Channel ID: ${channelId}`)
+  }
+
+  if (!/^[\w-.]+$/.test(roomJIDLocalPart)) { // channel.X or video uuid
+    throw new Error(`Invalid ROOM JID: ${channelId}`)
+  }
+}
+
+async function _getFilePath (
+  options: RegisterServerOptions,
+  channelId: number | string,
+  roomJIDLocalPart: string,
+  prosodyDomain: string,
+  way: 'channel2room' | 'room2channel'
+): Promise<[string, string, string]> {
+  channelId = channelId.toString()
+
+  const roomJID = roomJIDLocalPart + '@' + prosodyDomain
+
+  if (way === 'channel2room') {
+    const dir = path.resolve(
+      options.peertubeHelpers.plugin.getDataDirectoryPath(),
+      'channel2room',
+      channelId
+    )
+    return [
+      dir,
+      path.resolve(dir, roomJID),
+      roomJID
+    ]
+  } else if (way === 'room2channel') {
+    const dir = path.resolve(
+      options.peertubeHelpers.plugin.getDataDirectoryPath(),
+      'room2channel',
+      roomJID
+    )
+    return [
+      dir,
+      path.resolve(dir, channelId),
+      channelId
+    ]
+  } else {
+    throw new Error('Invalid way parameter')
+  }
+}
+
+export {
+  setChannel2Room
+}
diff --git a/server/lib/routers/api/room.ts b/server/lib/routers/api/room.ts
index 6208d7f5..8be6cdf1 100644
--- a/server/lib/routers/api/room.ts
+++ b/server/lib/routers/api/room.ts
@@ -6,6 +6,7 @@ import { getCheckAPIKeyMiddleware } from '../../middlewares/apikey'
 import { Affiliations, getVideoAffiliations, getChannelAffiliations } from '../../prosody/config/affiliations'
 import { fillVideoCustomFields } from '../../custom-fields'
 import { getChannelInfosById } from '../../database/channel'
+import { setChannel2Room } from '../../room/channel'
 
 // See here for description: https://modules.prosody.im/mod_muc_http_defaults.html
 interface RoomDefaults {
@@ -78,6 +79,9 @@ async function initRoomApiRouter (options: RegisterServerOptions, router: Router
           },
           affiliations: affiliations
         }
+
+        await setChannel2Room(options, channelId, jid)
+
         res.json(roomDefaults)
       } else {
         // FIXME: @peertube/peertype-types@4.2.2: wrongly considere video as MVideoThumbnail.
@@ -127,6 +131,9 @@ async function initRoomApiRouter (options: RegisterServerOptions, router: Router
           },
           affiliations: affiliations
         }
+
+        await setChannel2Room(options, video.channelId, jid)
+
         res.json(roomDefaults)
       }
     }
diff --git a/support/documentation/content/en/technical/data/_index.md b/support/documentation/content/en/technical/data/_index.md
index f13d4710..3fe40d6d 100644
--- a/support/documentation/content/en/technical/data/_index.md
+++ b/support/documentation/content/en/technical/data/_index.md
@@ -54,3 +54,30 @@ it stores it in `videoInfos/local/video_uuid.json` (where `video_uuid` is the vi
 The `channelConfigurationOptions` folder contains JSON files describing channels advanced configuration.
 Filenames are like `1.json` where `1` is the channel id.
 The content of the files are similar to the content sent by the front-end when saving these configuration.
+
+## channel2room and room2channel
+
+Some parts of the plugin need a quick way to get the channel id from the room id, or the all room id from a channel id.
+We won't use SQL queries, because we only want such information for video that have a chatroom.
+
+So we have 2 folders: `channel2room` and `room2channel`.
+When a chatroom is created, we create 2 empty files:
+
+* `channel2room/channel_id/room_id@muc_domain`
+* `room2channel/room_id@muc_domain/channel_id`
+
+Where:
+
+* `muc_domain` is the room's domain (should be `room.your_instance.tld`)
+* `channel_id` is the channel numerical id
+* `room_id` is the local part of the room JID
+
+So we can easily list all rooms for a given channel id, just by listing files in `channel2room`.
+Or get the channel id for a room JID (Jabber ID).
+
+Note: we include muc_domain, in case the instance domain changes. In such case, existing rooms
+could get lost, and we want a way to ignore them to avoid gettings errors.
+
+Note: there could be some inconsistencies, when video or rooms are deleted.
+The code must take this into account, and always double check room or channel existence.
+There will be some cleaning batch, to delete deprecated files.