Synology SHR-2: Dual Redundancy with Mixed Drive Sizes

SHR-2 vs SHR-1 vs RAID 6

SHR-2 is to RAID 6 what SHR-1 is to RAID 5. With identical drives, SHR-2 and RAID 6 give the same usable capacity. With mixed drives, SHR-2 recovers space in tiers that RAID 6 would leave unused.

• Minimum drives: Synology requires at least four drives for an SHR-2 pool.
• Capacity: with identical drives, (n − 2) × the drive size. With mixed sizes, Synology's own RAID Calculator counts SHR-2 capacity only where at least four drives reach a size tier; space above the fourth largest drive is unused.
• Protects against: any two failed drives.

When to Use SHR-2

SHR-2 is worth considering when:

• You have four or more bays and the data would be hard to replace
• You want the pool to survive a second failure while a rebuild is running
• You have, or plan to have, mixed drive sizes

With identical drives on a 4-bay NAS, SHR-2 gives two drives of usable layout (the same as RAID 6): 4 × 8 TB gives 16 TB, before DSM takes about 10 GB per drive and 4% (Btrfs) or 2% (ext4) of the volume. The RAID calculator works out mixed sizes for you.

RAID keeps a NAS running through a drive failure. It is not a backup: deletion, ransomware, a failed rebuild or a dead NAS can still take everything, so keep a separate copy.

Frequently Asked Questions

How much less space does SHR-2 give than SHR-1?

One more drive's worth goes to the second parity. With identical drives on a 4-bay NAS, SHR-1 gives three drives of usable layout and SHR-2 gives two. On a 6-bay NAS, SHR-1 gives five and SHR-2 gives four.

How does an SHR-2 pool grow?

On an SHR-2 pool of equal drives, capacity grows only after four drives have been replaced with larger ones, one at a time. Whether an existing SHR-1 pool can be converted to SHR-2 depends on your DSM version and drive count; check Synology's Storage Manager documentation before you plan on it.