SHR vs RAID 5: When Synology Hybrid RAID Actually Wins

This is the comparison neither side will make honestly, and it cuts in both directions. With four identical drives, SHR is exactly RAID 5. Not approximately, exactly. Same usable capacity, same fault tolerance, same everything, because SHR on a matched set collapses into a single parity group. Anyone selling you SHR as free capacity on matched drives is wrong. And on a mismatched set the picture inverts completely: SHR can return double the usable space at the same fault tolerance, which no vendor calculator on the other side will show you.

What is the difference between SHR and RAID 5?

RAID 5 truncates every member to the size of the smallest drive, stripes data across them, and dedicates one drive worth of space to parity. Usable capacity is (n minus 1) times the smallest drive. Anything above that smallest size on the larger drives is unreachable, and it does not appear anywhere in the interface. You paid for it and the array cannot use it.

SHR slices the same drives horizontally at every distinct drive size and gives each horizontal band its own parity group. A band present on k drives with p drives of parity yields (k minus p) times the band height in usable capacity when k is greater than p, and is stranded entirely when it is not, because there are not enough drives in that band to protect it.

That is the whole mechanism. Everything below follows from it arithmetically.

How does the band algorithm work in practice?

Take 4, 8, 12, 12 TB, which is a realistic set for anyone who has upgraded drives over a few years. Here is what SHR does with it, band by band.

Computed with the same band algorithm behind the RAID capacity calculator. Figures are decimal TB as printed on the drive label. Your NAS reports 24 TB usable as 21.83 TiB, which is the same bytes counted in binary, explained on the TB against TiB chart.

SHR returns 12 TB more usable capacity on drives you have already bought, at exactly the same single-drive fault tolerance. The 20 TB that RAID 5 strands is roughly $730 of drive at current 12 TB pricing, thrown away for no protection benefit at all.

When are SHR and RAID 5 identical?

Whenever every drive is the same size. With four matched drives there is only one band, that band sits on all four drives, and (4 minus 1) times the drive size is precisely the RAID 5 formula. There is no clever allocation left to do.

The table below runs both schemes across a range of real drive sets so the pattern is visible. Read the top two rows first, then watch the gain open up as the drives diverge.

Stranded capacity is space you paid for that the array cannot address. Both schemes tolerate exactly one drive failure in every row. The three-drive row shows SHR still working below four bays, which is where the two-bay and three-bay Synology units live.

Two things stand out. The gain scales with how far apart the drives are, not with how many you have. And SHR strands capacity too, in the rows where one drive is much larger than every other, because a band that only one drive reaches has nothing to pair with for parity. SHR is better arithmetic, not magic.

Where does SHR pay you a second time?

On the upgrade path, and this is the part that matters most to someone choosing a platform today rather than choosing a layout for drives already in hand.

Under RAID 5, replacing drives one at a time gains you nothing at all until the last one is replaced, because the array is still truncated to whatever the smallest remaining drive is. Under SHR each replacement adds a new band, and the second replacement already returns capacity.

Each step is one drive swapped and one full rebuild, which is why an upgrade path is a real cost in exposure as well as money. See the rebuild time chart for what each of those rebuilds costs in hours.

Read the middle rows. After two of four drives are replaced, RAID 5 still gives you the same 12 TB it gave you at the start, and SHR gives you 20. That is a year or two of usable capacity you get earlier, purchased with the same money. It is the single strongest practical argument for the platform, and it is invisible on any comparison that only looks at a freshly built array.

What does SHR cost you?

Three things, and they are worth stating plainly because the rest of this page is favourable.

• Vendor lock-in on the enclosure. SHR runs on Synology hardware. If you later want TrueNAS, Unraid or a QNAP, the pool does not move across, and the migration is a full copy out and back.
• No in-place conversion. Choosing RAID 5 now and wanting SHR later means recreating the volume, so the decision is effectively permanent at pool creation.
• Complexity when things go wrong. A single-band array is easy to reason about. A five-band array with different drive counts per band is harder to picture during a recovery, though the underlying tools are the same standard Linux ones.

That last point deserves a correction to a common fear. SHR is not a proprietary on-disk format. It is built from standard Linux software RAID and the Logical Volume Manager, with Btrfs or ext4 on top, so a set of drives can be assembled on an ordinary Linux machine with the standard tools if the enclosure dies. That is a genuinely better position than a proprietary hardware RAID card, where a dead controller can mean a dead array.

Which should you actually choose?

Four cases, and the answer is different in each.

• You are buying four identical drives today and will replace them all at once in five years. Choose either. They are the same thing. Buy on price, on the software you prefer, and on whether the box has the processor you need.
• You expect to upgrade drives one at a time. Choose SHR, and choose Synology to get it. This is most home users, and the upgrade table above is the reason.
• You already own mismatched drives. Choose SHR. The gain column is real capacity in drives you have already paid for.
• You want ZFS features such as checksums, snapshots and send or receive. Choose neither. RAIDZ truncates to the smallest drive exactly as RAID 5 does, so it loses the mixed-drive argument outright, and you are choosing it for data integrity rather than for capacity.

One decision sits above all of these and is not about SHR at all. At four or more drives with capacities past roughly 8 to 10 TB, the parity count matters more than the parity scheme. SHR-2 and RAID 6 both tolerate two failures, and the reason to want that is rebuild exposure rather than capacity, which is worked through in RAID 5 against RAID 6.

How do you check this for your own drives?

Run your exact set through the RAID capacity calculator, which evaluates every layout at once, including SHR, SHR-2, RAIDZ1, RAIDZ2, RAIDZ3 and the standard levels, and shows usable, parity and stranded capacity for each. It is the only calculator we know of that is not owned by a hardware vendor, which is why it will happily tell you that SHR gains you nothing on the drives you described.

The arithmetic is simple enough to verify by hand, which is the point of publishing it. Sort the drives, take each distinct size as a boundary, and for each band multiply the height by the number of drives that reach it minus the parity count. Add the bands. If your NAS reports a figure roughly 9 percent below what you compute, that is binary against decimal units and not a fault.

Related reading

• RAID levels explained for the vocabulary and every other layout
• RAID 5 against RAID 6 for the parity count decision
• Best 4-bay NAS for the hardware that runs SHR
• How to migrate to a bigger NAS for the drive-by-drive upgrade in practice
• Usable capacity chart for precomputed figures at every bay count

Deciding on a specific model? We review the Synology DS1825+ review and Synology DS925+ review in full.