How it works

SHR (Synology Hybrid RAID) is Synology’s storage volume technology: Storage Manager builds a redundant volume that uses the largest capacity shared across every member disk, then keeps leftover space on larger disks ready for when you upgrade smaller ones. Classic fixed RAID layouts (RAID 1, 5, 6) expect equal-size disks. Put a 4TB and an 8TB in a 2-bay RAID 1 and you typically waste half the larger drive. SHR-1 (one-disk fault tolerance) and SHR-2 (two-disk fault tolerance, needs at least four bays) slice capacity so the volume can grow later by replacing the smallest members with bigger ones, without wiping the array and starting over.

On modern Synology DSM, SHR volumes commonly sit on Btrfs. You get checksums, scrub, and Snapshot Replication from the UI; Synology owns the volume manager, so you do not hand-edit Linux mdadm profiles or build bare Btrfs RAID yourself. SHR is not the same stack as Linux md RAID or ZFS raidz: those live on raw disks with different expansion rules (TrueNAS pools and vdevs, for example). Prefer CMR NAS HDDs for members when rebuild and scrub time matter.

When it matters

It matters when you want to start a 2-bay or 4-bay Synology with mixed or upgradeable disks, expand without destroying the volume, and stay inside DSM rather than running TrueNAS/ZFS on raw disks. Typical home path: buy a DiskStation diskless, fill with the cheapest reliable CMR NAS drives that fit today’s budget, then later replace one smaller disk at a time and let Storage Manager reclaim leftover capacity. Hot-swap trays make those swaps less painful. SHR is a bad fit if you already committed to ZFS pools elsewhere, or if every disk must be identical and you never plan to mix sizes.

Shopping path: a Synology DiskStation that supports SHR-1 (2-bay) or SHR-2 (4-bay+), enough CMR NAS HDDs for your bay count, and a clear habit of offsite or secondary backups (snapshots on the same volume are not a substitute).

A compact 2-bay Synology for SHR-1 home media, a 4-bay Synology with SHR-2 headroom and NVMe cache slots, and a CMR NAS HDD suited as an SHR member.

The Synology DiskStation DS225+ is a quiet 2-bay DSM appliance with 2.5GbE; Storage Manager builds SHR-1 volumes on Btrfs so you can mix sizes and expand by replacing the smaller disk later.

The Synology DiskStation DS923+ is a 4-bay Synology where SHR-1 or SHR-2 is a setup checkbox, with dual M.2 NVMe cache slots and more room to grow by swapping smaller members over time.

The WD Red Plus 4TB CMR (WD40EFPX) is a CMR NAS HDD commonly used as an SHR member; prefer CMR when rebuild and scrub load matter.

Filesystem under modern DSM SHR: Btrfs. Open-pool contrast: ZFS. Drive recording and trays: CMR, Hot-swap. Striping and parity for NAS shopping: RAID levels for your NAS. Appliance pick: Synology DS225+ vs UGREEN NASync DXP4800.