The first time I set up a Synology DS220+ for a friend, we had two mismatched drives sitting on the table: a leftover 4TB from an old desktop and a shiny new 8TB. He wanted both in there, working together, no waste. That single moment is where the whole RAID versus SHR question lives.
Standard RAID would have thrown away half of that 8TB. SHR would not. That is the short version, but the choice deserves a proper look before you commit, because changing your mind later means backing everything up and starting over.
What each one actually is
RAID (Redundant Array of Independent Disks) is the decades-old set of standards for combining drives so they either go faster, survive a failure, or both. On a home Synology you mostly meet RAID 1, RAID 5, RAID 6, and RAID 10. It is a general storage concept, not a Synology invention.
SHR stands for Synology Hybrid RAID. It is Synology's own layer sitting on top of standard RAID and Linux logical volumes. Under the hood it is still doing RAID 1 or RAID 5 style parity, but it slices your drives into regions so it can use odd, mismatched capacities that plain RAID would refuse to touch.
The mixed-drive problem, explained with real numbers
Say you put a 4TB and an 8TB drive into a two-bay unit and pick RAID 1. RAID 1 mirrors, so it can only mirror as much as the smallest drive holds. You get 4TB usable, and 4TB of that 8TB drive just sits there, unreachable, forever.
Now pick SHR with those same two drives. SHR mirrors the overlapping 4TB region for protection and, because a two-drive SHR cannot protect the leftover, it presents the layout honestly. In a two-bay setup the practical usable space still lands near 4TB, so the win is smaller here than people expect.
The real payoff shows up with three or more bays. Drop a 4TB, 6TB, and 8TB into a DS923+ under SHR and you get roughly 10TB usable with single-drive protection. Try that mix under RAID 5 and it treats every drive as 4TB, handing you about 8TB. That is a genuine 2TB you paid for and would otherwise lose.
It gets worse the more lopsided your drives are. Picture a four-bay box with three old 4TB drives and one new 16TB one. RAID 5 caps everything at 4TB and leaves 12TB of that big drive dead. SHR reclaims a big chunk of it, and the gap between the two choices can run to double figures in terabytes.
Side by side
| Factor | Classic RAID (1/5/6) | SHR / SHR-2 |
|---|---|---|
| Mixed drive sizes | Caps every drive at the smallest | Uses most of the extra capacity |
| Usable space (mixed disks) | Lower, often much lower | Higher |
| Fault tolerance | 1 drive (RAID 5), 2 drives (RAID 6) | 1 drive (SHR), 2 drives (SHR-2) |
| Expansion by swapping drives | Supported but wastes odd sizes | Grows cleanly as you upsize disks |
| Moving the array to another brand | Standard, widely readable | Synology only |
| Best for | All-identical drives, portability | Home users mixing and upgrading |
Where standard RAID still wins
Portability is the honest reason to pick classic RAID. An SHR volume is tied to Synology's implementation. If your NAS dies and you want to pull the drives into a QNAP or a plain Linux box, an mdadm-based RAID 5 is far more likely to be readable than an SHR pool.
The second reason is buying all your drives at once. If you fill a four-bay unit with four identical 8TB drives on day one, SHR and RAID 5 give you the exact same usable space and the exact same protection. There is no mixed-size advantage to capture, so either choice is fine and RAID 5 is the more universal label.
Some people also just prefer knowing exactly what is happening under the covers. Plain RAID 5 is a well-documented standard, and if you like being able to look up your layout in decades of Linux mdadm documentation, that transparency has real value. SHR adds a Synology-specific abstraction layer that is convenient but slightly more of a black box.
How to actually decide
Run through these questions in order. The first honest "yes" usually settles it.
- Are your drives different sizes, or will they be after your next upgrade? If yes, lean SHR. This is the case for most home users, who buy drives a couple at a time as prices drop.
- Do you plan to grow the array by swapping one drive at a time for a bigger one? SHR handles this gracefully and expands the volume once the second matching-or-larger drive lands.
- Might you one day move these drives into a non-Synology box? If that is a real possibility, standard RAID keeps your options open.
- How many bays do you have? With two bays the SHR advantage is small. With four or more it can be several terabytes.
A word on redundancy levels
Whatever you choose, decide how many drive failures you want to survive. Single-drive protection (RAID 5 or SHR) is the common home pick and it is fine for two to four bays.
Once you get to five or six large drives, the rebuild after a failure takes many hours and stresses the surviving drives. That is when SHR-2 or RAID 6, which tolerate two simultaneous failures, start to earn their keep. Losing a second drive mid-rebuild is rare, but with 16TB or 20TB disks the rebuild window is long enough to worry about.
Can you change your mind later?
Partly. You can convert an SHR pool to SHR-2 if you add drives, and DSM lets you migrate some RAID levels in place. But you cannot freely flip between classic RAID and SHR without destroying and rebuilding the storage pool, which wipes the data on it.
That is exactly why this is a setup-time decision. A mistake I see constantly is someone picking RAID 5 by reflex, filling it with mixed drives, then discovering months later they left 4TB on the table and cannot reclaim it without a full rebuild. Synology documents the supported paths in its SHR knowledge base article, and it is worth a two-minute read before you click Apply.
My rule of thumb
For nearly every home user I set up, I pick SHR. The typical home story is drives bought over time in whatever size was cheapest that month, and a plan to keep growing. SHR is built for exactly that untidy reality.
I only reach for standard RAID when someone bought a matched set of identical drives and cares about being able to read those drives outside a Synology someday. If neither of those is you, choose SHR, add SHR-2 once you cross five big drives, and get on with using the thing.
Frequently asked questions
Is SHR slower than regular RAID 5?
In everyday home use you will not notice a difference. SHR runs on the same underlying parity math as RAID 5, so read and write speeds are effectively the same for streaming media, backups, and file shares. Any tiny overhead is lost against your network and drive speeds.
Can I switch from RAID 5 to SHR later without losing data?
No. Converting between classic RAID and SHR requires deleting and recreating the storage pool, which erases everything on it. You can migrate between some RAID levels or upgrade SHR to SHR-2, but a full RAID-to-SHR swap means backing up, rebuilding, and restoring.
Does every Synology model support SHR?
Most home and small-business models do, but some higher-end units ship with SHR disabled by default in favor of standard RAID. Check your specific model on Synology's site before planning around SHR, since a handful of enterprise-focused boxes do not offer it.
How many drives can fail before I lose data on SHR?
Standard SHR tolerates one drive failing at a time, the same as RAID 5. SHR-2 tolerates two simultaneous failures, matching RAID 6. Once one drive fails, replace it promptly, because a second failure before the rebuild finishes means data loss.
Will SHR let me read my drives in a Windows PC or another NAS?
Not easily. SHR is a Synology-specific layout, so pulling the drives into Windows or a different brand of NAS usually will not work without specialist recovery tools. If cross-platform readability matters to you, standard RAID is the safer bet.
