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StashRaid

Storage Parameters

Usable Capacity

12 TB

Fault Tolerance

4

Array Resilience

Architecture

RAID 5 • Parity

Fault Tolerance

1 Concurrent Failures Allowed

Current Configuration

RAID 5 (Parity)

12 TB Usable

Total RAW: 16 TB

Disk 01DATA
Disk 02DATA
Disk 03DATA
Disk 04PARITY

Total Drives

4 x 4 TB

Efficiency

75% Utilized

Usable: 12 TBWasted to redundancy: 4 TB

Pros & Cons

  • PRO: Good balance of capacity/speed/safety

  • PRO: Cost-effective redundancy

  • CON: Slower writes due to parity calculation

    This setup prioritizes redundancy but has associated performance or capacity costs.

RAID Expansion Guide

Running out of storage is inevitable — but rebuilding your entire array from scratch doesn't have to be. Modern RAID systems offer several ways to expand capacity without data loss or downtime. This guide covers which RAID types support online expansion, how each method works, and step-by-step instructions for the most popular platforms.

Why Array Expansion Matters

When you first built your NAS, you sized it for your needs at the time. But media libraries grow, backup retention policies expand, and before you know it your array is at 85% capacity. The traditional approach — copy everything off, rebuild, copy back — is time-consuming, risky, and requires enough extra storage to hold a full copy of your data.

Live expansion lets you add drives to an existing array and grow the filesystem in-place. Depending on your RAID software and hardware, you may be able to add one drive at a time, replace drives with larger ones, or attach entirely new vdevs to a pool. The best approach depends on the RAID system you're using.

Expansion Methods Comparison

RAID TypeExpandable?MethodRisk LevelDowntime
Synology SHR YesHot-swap drives or add new baysLowNone (degraded during rebuild)
ZFS (mirror vdevs) YesAdd entire new vdevsLowNone
ZFS RAID-ZLimitedAdd new vdev (can't expand a RAID-Z vdev)MediumNone (add vdev) / Full (rebuild)
mdadm (Linux SW RAID) YesAdd drive, grow array, reshapeMediumNone (degraded during reshape)
Hardware RAID 5/6 DependsOCE (controller must support it)HighNone to full (vendor-dependent)
Hardware RAID 0/1/10NoFull backup + rebuild requiredHighFull rebuild window
Btrfs RAID Yesbtrfs device add + balanceMediumNone (degraded during balance)

Expansion Risks & Considerations

Data Loss Risk

Any operation that touches every block on your drives carries inherent risk. A power failure during a reshape, a firmware bug in your controller, or an undetected bad sector can corrupt the entire array. Always have a verified backup before expanding.

Extended Degraded State

During a reshape or expand operation, your array has reduced or no redundancy. If a second drive fails during this window, you will lose data. Schedule expansion at a time when the array can remain online uninterrupted.

Performance Impact

Array expansion consumes significant I/O bandwidth. Reads and writes during a reshape can be 50–80% slower than normal. Consider throttling the reshape speed or running it during off-peak hours.

Time Estimates

A RAID 5 reshape adding one drive to a 4× 8 TB array: ~18–30 hours. A SHR1 rebuild after swapping to larger drives: ~8–20 hours per drive. ZFS adding a new vdev: minutes (data redistribution is ongoing).

Step-by-Step Expansion Guides

Synology SHR Expansion

  1. Verify compatibility — Open Storage Manager > HDD/SSD and confirm the new drives are supported.
  2. Insert the new drive — Hot-swap into an empty bay or replace a smaller drive if all bays are full.
  3. Check storage pool — DSM detects the new drive. Go to Storage Manager > Storage Pool, click the pool, then click Action > Add Drive.
  4. Select RAID type — If you have mixed drive sizes, SHR automatically optimizes capacity allocation.
  5. Wait for the rebuild — The array rebuilds with redundancy. This can take 8–24 hours depending on drive size. The pool remains accessible.
  6. Verify — Once complete, check that capacity reflects the new drive. Run a data scrub for peace of mind.

Use our SHR Calculator to see how capacity changes as you add or replace drives.

TrueNAS / ZFS Pool Expansion

  1. Install the new drives — Physically connect drives and ensure they are visible in the TrueNAS web UI under Storage > Disks.
  2. Create a new vdev — Go to Storage > Pools, click the gear icon on your pool, and select "Add Vdevs."
  3. Choose vdev type — Select Mirror, RAID-Z1, RAID-Z2, or RAID-Z3. You cannot expand an existing vdev — you must add a whole new one.
  4. Select drives — Pick the drives for the new vdev. ZFS stripes data across all vdevs in the pool.
  5. Confirm — The new vdev is added instantly. Space is available immediately, but ZFS may take time to rebalance writes optimally.

Plan your expansion with the ZFS RAID Calculator to determine how many drives and which vdev layout give you the best usable capacity.

mdadm (Linux Software RAID) Expansion

  1. Partition the new drive — Use fdisk or gdisk to create a partition with the same type as your array (fd for Linux RAID).
  2. Add the drive to the array — mdadm --add /dev/md0 /dev/sdb1
  3. Grow the array — mdadm --grow /dev/md0 --raid-devices=5 (for a 4→5 drive expansion)
  4. Reshape — The kernel automatically begins a reshape to incorporate the new drive into the parity layout. Monitor with cat /proc/mdstat.
  5. Expand the filesystem — Once the reshape completes, grow the filesystem: resize2fs /dev/md0 for ext4 or xfs_growfs /mount/point for XFS.
  6. Verify — Check array health with mdadm --detail /dev/md0.

Run the math on your planned layout with the RAID Calculator before committing.

Best Practices for Any Expansion

  • Always have a verified backup before starting an expansion. See our Backup Strategy Guide.
  • Run a SMART long test on all new drives before adding them to the array.
  • Monitor the rebuild or reshape daily. Set up email notifications for drive failures.
  • Don't rush — expansion is a marathon. Throttle reshape speed to 50,000–100,000 KB/s on mdadm to reduce performance impact.

Frequently Asked Questions

Can I add drives to an existing RAID 5 or RAID 6 array?

With software RAID via mdadm on Linux, yes — you can grow a RAID 5 or RAID 6 array by adding new drives and expanding the filesystem. Many hardware RAID controllers also support online capacity expansion (OCE) on RAID 5/6, but the controller must explicitly support it. Check your controller documentation before buying additional drives.

How long does RAID expansion take?

Expansion time depends on array size, drive speed, RAID level, and the method used. For a 4-drive RAID 5 expanding to 5 drives, expect 6–24 hours on typical 8 TB drives. During a reshape/resilver, performance is degraded — the array remains accessible but reads and writes will be slower. Always plan expansion for a maintenance window if possible.

Can I expand a ZFS pool by mixing drive sizes?

ZFS vdevs are constrained by the smallest drive in the vdev. If you have a 3-way mirror of 4 TB drives and add an 8 TB drive to the vdev, only 4 TB of that 8 TB drive is usable. However, you can add a completely new vdev with any mix of drives — for example, add a 4-drive RAID-Z2 vdev of 8 TB drives alongside an existing 3-drive mirror vdev of 4 TB drives.

Does Synology SHR expansion require all drives to be the same size?

No — Synology SHR (Synology Hybrid RAID) is specifically designed to handle mixed drive sizes. When you replace drives with larger ones, SHR automatically rebalances data to use the new capacity. You can start with 4× 4 TB drives, swap to 8 TB drives one at a time, and end up with the full usable capacity of the larger drives without ever having to rebuild from scratch.

Plan Your Next Expansion

Not sure which RAID level or expansion strategy fits your needs? Use our interactive calculators to compare capacity, redundancy, and cost for any configuration.

RAID Calculator SHR Calculator ZFS Calculator