Added synology DSM to the knowledge base

knowledge base/synology diskstation manager.md

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+# Synology DiskStation Manager
+
+## Rsync
+
+Requirements:
+
+1. the rsync service is enabled under _Control Panel > File Services > rsync_
+1. the user has the right permissions for the shared folder under either
+
+   - _Control Panel_ > _Shared Folders_ > _Shared Folder_ edit window > _Permissions_, or
+   - _Control Panel_ > _User & Group_ > _User_ or _Group_ edit window > _Permissions_
+
+Examples:
+
+```shell
+# From a shared folder on a NAS to a local one.
+rsync \
+  --archive --hard-links \
+  --append-verify --partial \
+  --progress --verbose \
+  --compress --no-motd \
+  --exclude "@*" --exclude "#*" \
+  "user@nas:/volume1/shared_folder/" \
+  "path/to/local/folder/" \
+  --delete --dry-run
+
+# Sync all snapshotted data to a folder
+find /volume1/@sharesnap/shared_folder \
+  -maxdepth 1 -mindepth 1 \
+  -type d
+| xargs -I {} -n 1 -t \
+    rsync \
+      -AXahvz --chown=user --info=progress2 \
+      --append-verify --partial \
+      --no-inc-recursive --no-motd \
+      {}/ \
+      /volume2/destination/folder/
+```
+
+## Snapshots
+
+Use the **Snapshot Replication** package available in the Package Center for better control and automation.
+
+Gotchas:
+
+1. when the _Make snapshot visible_ option in a shared folder's settings in _Snapshot Replication_ is ticked:
+
+   - the `#snapshot` folder is created in the shared folder's root directory
+   - the default snapshots directory for that shared folder is mounted on it in **read only** mode:
+
+     > ```plaintext
+     > /dev/mapper/cachedev_0 on /volume1/Data/#snapshot type btrfs (ro,nodev,relatime,ssd,synoacl,space_cache=v2,auto_reclaim_space,metadata_ratio=50,block_group_cache_tree,subvolid=266,subvol=/@syno/@sharesnap/Data)
+     > ```
+
+## Deduplicate blocks in a volume
+
+Requirements:
+
+1. `docker` needs to be installed from the package manager, as it is simpler to run a container than installing `duperemove` and all its dependencies on the machine
+1. the container needs to be run in privileged mode (`--privileged`) due to its actions on the disk
+
+Gotchas:
+
+1. the container might fail on very large datasets, usually due to Out Of Memory (OOM) issues; to avoid this:
+
+   1. offload the hashes from RAM using a hash file (`--hashfile "/volume1/NetBackup/duperemove.tmp"`)
+   1. use smaller datasets where possible, like a shared folder and just one of its snapshots instead of all of them
+
+1. `duperemove` can dedupe blocks only if acting on folders in a _rw_ mount; when deduplicating snapshots, use their _rw_ mount path `/@syno/@sharesnap/shared_folder` instead of their _ro_ version `/volumeN/shared_folder/#snapshot`
+
+Examples:
+
+```shell
+# small/medium dataset
+# 2 folders in a shared folder
+sudo docker run --privileged \
+  --rm --name duperemove \
+  --mount "type=bind,source=/volume1/Data,target=/sharedfolder" \
+  michelecereda/duperemove:0.11.2 \
+    -Adhr \
+    "/sharedfolder/folder1" "/sharedfolder/folder2"
+
+# large dataset
+# 1 shared folder and all its snapshots
+sudo docker run --privileged \
+  --rm --name duperemove \
+  --mount "type=bind,source=/volume1,target=/volume1" \
+  michelecereda/duperemove:0.11.2 \
+    -Adhr \
+    --hashfile "/volume1/NetBackup/duperemove.tmp" \
+    "/volume1/Data" "/volume1/@sharesnap/Data"
+```
+
+## Sources
+
+- [Configuring deduplication block on the Synology]
+
+[configuring deduplication block on the synology]: https://onedrive.live.com/?authkey=%21ACYMJq62iJaU7HY&cid=1E8D74207941B8DD&id=1E8D74207941B8DD%21243&parId=1E8D74207941B8DD%21121&o=OneUp