Storage Pool Capacity Exhausted: Free Up Space Now

Quick answer

Delete the largest stale files, then resync the pool. If that doesn't free enough space, add a new drive or replace a small one with a larger drive.

Why it happens

Storage pools — whether you're using Windows Storage Spaces, a Synology NAS, or a custom ZFS array — allocate space in chunks. When you fill them past a certain threshold (typically 85-90%), performance tanks and writes start failing. The trigger is usually the same: you added a lot of data without checking, or a single application (like a media server or backup agent) generated huge log files or temporary caches. I've seen this hit people who set up a Plex server on a Synology DS220+ and forgot to limit the size of their transcoded cache. Suddenly, the pool shows 0 bytes free and the DSM sends a critical alert.

Fix it — step by step

These steps work for most storage pool systems. Adjust names and paths for your specific GUI (Windows Storage Spaces, Synology DSM, QNAP QTS, TrueNAS).

1. Identify what's eating space.
   Open the storage pool management page. Look for a breakdown by share or volume. On Windows, open Server Manager → File and Storage Services → Volumes. On Synology, go to Storage Manager → Storage Pool. Note the total size and used percentage. If it's above 90%, you'll need to delete data.
2. Find the biggest files and folders.
   Use a tool like WinDirStat on Windows, du on Linux, or the built-in File Station on Synology. Sort by size descending. A common culprit: @tmp or #recycle folders. On Synology, check shr_id files in @eaDir — those are thumbnail caches that can bloat to gigabytes. Delete anything you don't need.
3. Empty the Recycle Bin.
   Most NAS systems have a per-share recycle bin. It's off by default, but if it's on, deleted files sit there until you manually purge. On Synology, go to Control Panel → Shared Folder → select each share → Edit → Enable Recycle Bin → click Empty. On Windows Storage Spaces, check the $Recycle.Bin folder on each drive.
4. Run a resync or repair.
   After deleting files, the pool's metadata might not reflect the free space immediately. Trigger a resync. On Windows Storage Spaces, that means right-clicking the pool in Server Manager and selecting Repair. On Synology, go to Storage Manager → Storage Pool → click the three dots → Repair. Wait for it to finish. After the resync, the free space should update.
5. Check for hidden snapshots.
   If you have scheduled snapshots, they reserve space for point-in-time copies. On TrueNAS, go to Storage → Pools → click the gear icon → Snapshots. On Synology, check Snapshot Replication. Delete old snapshots you don't need. Keep the last 3-5 for safety, but purge anything older than a month.

Alternative fixes if deleting isn't enough

If you've cleaned up everything you can and still need more space, try these in order:

• Expand the pool with an existing drive. Some file systems (Btrfs, ext4) can grow a pool by adding an entire unused drive. On Synology, click Add Drive under the Storage Pool page. On Windows, go to Server Manager → click the pool → Add Physical Disk. The new drive must be as large as the smallest drive in the pool for parity pools.
• Replace a small drive with a larger one. If you have a 2TB drive in a 4-drive pool, swap it with a 4TB or 8TB drive. On Synology, use Storage Manager → HDD/SSD → Replace Drive. The pool will rebuild itself after you insert the new drive. This takes hours but doesn't require a backup (though you should have one).
• Change the allocation unit. This is a last resort. On Windows Storage Spaces, if you're using thin provisioning, you can reclaim space from empty blocks. Open PowerShell as admin and run: Repair-VirtualDisk -FriendlyName "YourPoolName" -ReconcileStorage. This forces a metadata cleanup. It's not guaranteed to free space, but I've seen it recover 5-10% on some pools.

Prevention tip

Set up a capacity alert at 80% and a critical alert at 90%. On Synology, go to Control Panel → Notification → Advanced → Storage. On Windows, use System Center Operations Manager or a simple PowerShell script that emails you when Get-VirtualDisk shows usage above 80%. Then, every month, schedule a 10-minute cleanup — delete logs older than 90 days, purge recycle bins, and remove old snapshots. That habit alone will keep your pool out of the red.