How it works

A filesystem snapshot is a point-in-time view of a volume or dataset created with copy-on-write: new writes go to new blocks, so the frozen tree keeps the old data without a full file-by-file duplicate. On Btrfs, snapshots hang off a subvolume; Synology DSM wraps that in Snapshot Replication so you schedule shared-folder snapshots from the UI. On ZFS, snapshots sit on datasets inside a pool (often via TrueNAS). Space use stays small until files change; only the blocks that diverge after the snapshot consume extra capacity. Recovery is local and fast: restore a deleted folder or roll a share back to yesterday without pulling a full archive from cloud or USB.

Snapshots live on the same volume (and usually the same chassis) as the live data. A dead NAS, a wiped array, or ransomware that encrypts the box also hits every local snapshot. That is the core difference from a backup: backups are separate copies you can restore when the primary storage is gone. Snapshot Replication between two Synology units, or ZFS send/receive to another pool, moves snapshot streams off the first box, but the first hop on a single DiskStation is still same-volume recovery, not a full 3-2-1 plan.

When it matters

It matters when someone deletes the wrong folder, a bad update corrupts a share, or you want a quick undo before a risky software change on home storage. Snapshots shine for accidental deletes and rollback. They are weak alone against ransomware that encrypts the whole volume, fire or theft of the chassis, or a failed rebuild that takes the array with it. Keep scheduled snapshots for day-to-day mistakes, then keep at least one copy off the box (USB, cloud, or a second NAS) under a 3-2-1 habit.

Shopping path: a Synology DiskStation with Snapshot Replication on SHR/Btrfs volumes for appliance simplicity, or an x86 NAS that can run TrueNAS for ZFS dataset snapshots. Prefer CMR NAS HDDs when scrub and rebuild time matter. Do not treat the snapshot schedule as your only backup.

A compact Synology with DSM Snapshot Replication, a 4-bay Synology with more snapshot room, and an x86 4-bay that can run TrueNAS for ZFS snapshots.

The Synology DiskStation DS225+ is a quiet 2-bay DSM appliance with 2.5GbE; Storage Manager builds Btrfs volumes under SHR and Snapshot Replication schedules shared-folder snapshots for fast local undo.

The Synology DiskStation DS923+ is a 4-bay Synology where Snapshot Replication is a checkbox on SHR/Btrfs volumes, with dual M.2 NVMe cache slots and more room for media libraries that need frequent local recovery points.

The UGREEN NASync DXP4800 Plus is an x86 4-bay NAS (Pentium Gold, DDR5, 10GbE) that can run TrueNAS SCALE for ZFS dataset snapshots; use it when you want pool-level snapshots instead of Synology Snapshot Replication.

Filesystem under Synology snapshots: Btrfs. Open-pool snapshots: ZFS. Synology volume layout: SHR. Drive recording for members: CMR. Off-box copies: 3-2-1 Backup Strategy for NAS.