NAS RAID types explained: how RAID 1, 5, 6, 10 and SHR protect your data

NAS RAID types at a glance

Synology’s documentation describes RAID (Redundant Array of Independent Disks) as combining multiple drives into a single storage space, each RAID type trading off performance, capacity and reliability differently. On a NAS you pick the RAID type when you create a storage pool, and it decides how many drives you need, how many can fail before data is lost, and how much raw space is left for your files.

N is the number of drives in the pool. Source: Synology Knowledge Center, Choose a RAID Type (DSM 7) (opens in a new tab), read on October 28, 2026. The level names are generic; SHR is Synology’s own system. Which types your NAS offers depends on the model and bay count: see its product specifications page.

RAID is not a backup

Synology’s SHR article states that RAID, classic or SHR, is not a backup solution. RAID keeps a pool running when a drive dies, not an older copy of a file: Synology describes RAID 1 as writing identical data to all the drives simultaneously, so a file you delete or overwrite is gone from every drive at once. For that, you need a separate backup.

How each NAS RAID type works

NAS RAID 0: striping, no protection

RAID 0 divides data into blocks spread across 2 or more drives to enhance performance, and you keep the sum of all drive sizes. Synology states that it has no fault tolerance: one drive failure loses all the data, and a RAID 0 pool cannot be expanded by adding drives or by replacing them with larger ones. Synology positions it for non-critical systems: data you can recreate, never your only copy.

NAS RAID 1: mirroring

RAID 1 writes identical data to every drive, most often 2 of them. Two drives survive 1 failure; Synology’s table adds that three drives survive 2 and four survive 3. Reads get faster, writes stay similar to a single drive. Capacity is the smallest drive, whatever the drive count: two identical drives give one drive’s worth of space, and the other half holds a copy. Added drives only add mirrors, never capacity.

How RAID 5 works

RAID 5 uses block-level striping with parity data distributed across all member drives. Parity is extra information that lets the array rebuild a missing piece: Synology states that the failed drive’s data is reconstructed from the parity on the remaining drives. RAID 5 needs at least 3 drives, survives the loss of 1, and gives you (N − 1) × the smallest drive.

One drive’s worth of space always goes to parity, so the share you keep grows with the bay count. The weak spot is the degraded state: while a drive is down, Synology states that read and write performance are severely impacted, and a second failure before the repair finishes loses the data. Synology positions RAID 5 where space and cost matter more than performance.

How RAID 6 works

RAID 6 adds a second layer of parity, storing redundant data equal to the size of two drives. It needs at least 4 drives, survives 2 failures, and gives you (N − 2) × the smallest drive. Synology states that its performance is lower than RAID 5 because of the extra protection. The gain: after one failure, the array can lose a second drive during the repair and keep your files.

How RAID 10 works

Synology describes RAID 10 as combining drives into groups of two in which data is mirrored, with the performance of RAID 0 and the protection of RAID 1. It needs an even number of drives, 4 or more, and only half the total space holds data. It survives several failures if none share a pair: with four drives, losing one drive from each pair is fine, losing both of one pair is not. Synology points it at applications with high I/O demand, such as database servers.

Synology Hybrid RAID (SHR)

Synology calls SHR its automated RAID management system, designed for users new to RAID. The difference shows with mixed drive sizes. Classic RAID builds the pool on the smallest drive: in Synology’s example, if the smallest drive is 500 GB, every other drive can contribute only 500 GB. SHR divides each drive’s storage space into smaller chunks and creates additional redundant storage, so space classic RAID leaves idle on larger drives gets used.

SHR-1 needs at least 2 drives to survive 1 failure; SHR-2 needs at least 4 drives and survives 2. Synology gives no formula for SHR capacity, only “optimized by the system”, and offers a RAID Calculator (opens in a new tab) for the exact figure with your drives. Some Synology models do not support SHR, or support it only in part.

Basic (one drive on its own) and JBOD (drives joined into one space) provide no redundancy. RAID F1 is a RAID 5 variant for all-flash arrays on specific Synology models, needing at least 3 drives and surviving 1 failure.

How much space you keep: usable capacity by bay count

With identical drives, Synology’s formulas become a count of drives’ worth of space. The table applies them to common NAS bay counts: drives kept for data, share of raw capacity, and failures survived.

How the cells are computed: RAID 5 keeps N − 1 drives, RAID 6 keeps N − 2, RAID 10 keeps N / 2, RAID 1 keeps one, RAID 0 keeps all N; the share is drives kept divided by N, rounded to one decimal where needed (5 / 6 = 83.3%, 4 / 6 = 66.7%). Synology’s table lists RAID 1 with 2 to 4 drives only.

• On a 2-bay NAS the only protected classic type is RAID 1, at 50%.
• On a 4-bay NAS, moving from RAID 5 to RAID 6 costs one drive’s worth of space (3 of 4 down to 2 of 4) and buys survival of a second failure. RAID 10 keeps the same 50% as RAID 6, but survives two failures only in different pairs.
• The cost of RAID 6 shrinks as bays are added: 50% kept with 4 drives, 75% with 8, the same share RAID 5 keeps with 4.

When the drives are not the same size

Synology’s expansion article uses a RAID 5 pool of three drives: 2 TB, 1.5 TB and 1 TB. Classic RAID counts each one as the smallest, 1 TB, so the pool keeps (3 − 1) × 1 TB = 2 TB for data. Of the 4.5 TB of raw space (2 + 1.5 + 1), the array uses 3 TB (three drives × 1 TB); the other 1.5 TB on the two larger drives sits idle. That is the space SHR is built to use; for the SHR figure on the same drives, use Synology’s RAID Calculator.

NAS RAID 1 vs RAID 5

Both survive 1 drive failure; they differ in cost and in behavior after a failure.

• Drives: RAID 1 runs on 2; RAID 5 needs 3 or more.
• Space kept: RAID 1 keeps 1 of 2 drives (50%); RAID 5 keeps 2 of 3 (66.7%), 3 of 4 (75%) and more as drives are added.
• After a failure: a RAID 1 mirror still holds a full copy on the surviving drive; RAID 5 rebuilds from parity, with performance severely impacted while degraded.
• Changing later: Synology converts RAID 1 to RAID 5 (one extra drive if the RAID 1 has only two), but not RAID 5 back to RAID 1.
• Drives to use: UGREEN’s compatibility list advises against SMR drives in either, and recommends enterprise or NAS drives for RAID 5.

Pick RAID 1 if you have a 2-bay NAS or want a simple full copy over capacity. Pick RAID 5 if you have 3 or more bays and want more usable space, accepting slower performance while a failed drive is replaced.

Synology SHR vs RAID 5, and SHR vs RAID 1

SHR reaches the same fault tolerance as the classic levels, with a different use of drive space.

• SHR vs RAID 1: Synology states that a second drive gives a single-drive SHR pool one tolerable failure, equivalent to RAID 1 redundancy, but no extra capacity. Counting from that single drive, capacity grows only once you add at least 2 drives, 3 in total; RAID 1 never does.
• SHR vs RAID 5: SHR-1 with 2 or more drives survives 1 failure, like RAID 5. With mixed sizes, RAID 5 counts every drive as the smallest, while SHR uses the leftover space. When you swap in larger drives, Synology states that classic RAID cannot expand until all its drives are replaced, while SHR can expand as soon as two are upgraded.
• SHR-2 vs RAID 6: both need at least 4 drives and survive 2 failures. SHR-1 converts to SHR-2 (DSM 6.1 and above) with 1 or 2 more drives, depending on the layout.

Pick SHR if your Synology NAS supports it and you expect to mix drive sizes or upgrade drives a pair at a time. Pick classic RAID 5 if you prefer manual control of the layout, as Synology recommends for such users, or if your NAS does not offer SHR.

NAS RAID configuration: changing or growing the pool later

Synology lists the conversions DSM allows without losing existing data, and the extra drives each one needs:

Source: Synology Knowledge Center, Change the RAID Type of a Storage Pool (DSM 7) (opens in a new tab), read on October 28, 2026. Some models cannot change RAID 5 to RAID 6, and all data on the drives you add is erased.

Growing a pool by adding drives works for JBOD, RAID 5, RAID 6, RAID F1, SHR and RAID 1 (where it only adds mirrors); for SHR and RAID 1, 5, 6 and F1 each new drive must be at least as large as the smallest drive in the pool (Add Drives to Expand the Storage Pool Capacity (opens in a new tab), read on October 28, 2026). RAID 0 cannot be expanded at all.

NAS RAID options by bay count

• 2 bays: RAID 1, or SHR-1 on a Synology NAS that supports it, survives 1 failure. RAID 0 or JBOD give you both drives’ space with no protection.
• 4 bays: RAID 5 or SHR-1 survives 1 failure (RAID 5 keeps 3 of 4 drives); RAID 6 or SHR-2 survives 2 (RAID 6 keeps 2 of 4); RAID 10 keeps 2 of 4, survives 2 only in different pairs, and is the performance option. The UGREEN DXP4800 Plus is a 4-bay NAS: UGREEN DXP4800 Plus specs explained, and where to buy the UGREEN DXP4800 Plus (opens in a new tab).
• 5 to 8 bays: RAID 6 keeps 60% of raw space with 5 drives and 75% with 8.

Sources

Manufacturer documentation, read on October 28, 2026.

• Synology Knowledge Center, Choose a RAID Type (DSM 7) (opens in a new tab).
• Synology Knowledge Center, What is Synology Hybrid RAID (SHR)? (opens in a new tab)
• Synology Knowledge Center, Change the RAID Type of a Storage Pool (DSM 7) (opens in a new tab).
• Synology Knowledge Center, Add Drives to Expand the Storage Pool Capacity (DSM 7) (opens in a new tab).
• UGREEN, NAS compatibility list (opens in a new tab), HDD notes.
• UGREEN, DXP4800 Plus user manual (opens in a new tab), version ModDate 2024-07-24 (drive bays and storage modes).