Adding SSD Cache on UNAS Pro 8
UniFi Drive makes it easy to boost your UNAS performance by installing M.2 NVMe SSDs and adding an SSD cache to your storage pool. SSD cache reduces random read and write latency, making frequently used files load faster. This guide walks you through SSD setup, status indicators, and cache behavior. If you're looking to optimize your UNAS for optimal performance and efficient storage, click here.
Recommended Use Cases
- Virtual machine storage: Optimized for accelerating the random I/O patterns common in datastores.
- Frequently accessed files: Delivers faster access to frequently opened small files.
Note: Large sequential files bypass the SSD cache because continuous writes would quickly consume space and cause unnecessary wear, reducing SSD lifespan.
Requirements
- UNAS Pro 8
- M.2 NVMe SSDs
- For best performance, use two identical models.
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M.2 SSD Tray
- Required to install M.2 SSDs. Insert the tray into your UNAS Pro 8 while it is powered off.
Adding SSD Cache to an Existing Storage Pool
SSD cache cannot be added if your storage pool already consists of SSDs, or if it's a legacy storage pool created with UniFi OS versions earlier than 4.3.
- Navigate to Settings > Control Plane > Storage > select an M.2 SSD and click Assign SSD Cache to Storage Pool.
- Choose an SSD cache type:
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Read-Only
- Requires: 1 SSD
- RAID type: RAID 0
- What it does: Accelerates reads only.
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Read-Write
- Requires: 2 SSDs
- RAID type: RAID 1
- What it does: Accelerates both reads and writes with redundancy. If one SSD fails, cached write data is preserved.
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Read-Only
Removing SSD Cache from Storage Pool
- Navigate to Settings > Control Plane > Storage > select an M.2 SSD and click Remove SSD Cache.
- In read-write cache mode, cached data is written back to the storage pool before removal. Then, the SSDs are formatted.
Switching Between SSD Cache Types
- Navigate to Settings > Control Plane > Storage > select an M.2 SSD and choose the SSD cache type you want to switch to.
- Read-Only → Read-Write: Requires 2 SSDs (RAID 1).
- Read-Write → Read-Only: Cached data is written back to the storage pool, then the type is switched.
SSD Status Indicators
- Healthy: The SSD is working normally and can be used as SSD cache.
- Limited Usable Size: The SSD is larger than the smallest SSD in the RAID group, so part of its capacity isn't used.
- Lifespan Ending Detected: The SSD is nearing the end of its life and can no longer be used as cache.
- Reading Error Detected: Multiple read errors occurred; the SSD cannot be used as cache.
- SMART Read Errors Detected: Health monitoring (SMART) reported errors; the SSD cannot be used as cache.
- Severe Errors Detected: Critical uncorrectable errors found; the SSD cannot be used as cache.
- Issue Detected: One or more issues (lifespan, read, SMART, or severe errors) found; the SSD cannot be used as cache.
- UniFi Data Found: The SSD contains data from another UniFi Console. Confirm the data is no longer needed, then format the SSD before reuse.
- Failed: The SSD is damaged and cannot be used as cache.
If an SSD shows a warning state (e.g., Lifespan Ending Detected, Reading Error Detected, SMART Read Errors Detected, Severe Errors Detected, or Issue Detected), shut down the UNAS Pro 8 and replace it promptly to prevent failure.
Impact on SSD Cache Behavior
When an SSD is used as an SSD cache, its status can affect how the cache behaves.
In Read-Write Cache Mode
- If one SSD fails: Cached data is written back to the storage pool, the failed SSD is removed, and the type switches to Read-Only.
- If both SSDs fail: The SSD cache stops working, and the storage pool enters a Failed state.
To prevent data loss, shut down the UNAS Pro 8 and replace the failed SSD immediately.
In Read-Only Cache Mode
- Single SSD cache: If it fails, the cache is disabled. Replace the SSD immediately.
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Two SSD cache:
- If one SSD fails: Cache continues to work. Replace the failed SSD immediately.
- If both SSDs fail: Cache is disabled. Replace the SSDs immediately.