The source user expected CasaOS “Merge” to turn the small internal eMMC plus an external SSD into one large physical system partition. That is not what the feature does. The merge can present multiple storage locations together under CasaOS's /DATA view, but the Linux root filesystem, package manager, Docker root directory, and individual physical disks still exist underneath.
This explains the source outcome: the UI showed hundreds of gigabytes free in the combined system storage, yet apt-get upgrade later returned no space left on device and the internal chip again looked full. More logical capacity in /DATA did not magically relocate every root-level write onto the SSD.
The User Specifically Asked About CasaOS's Beta Merge Feature
Zima-Giorgio initially suggested moving Docker images and volumes to another storage device. The OP clarified that Docker volumes were not the main problem—they were asking whether CasaOS's Storage UI Merge feature itself was reliable.
Another User Reported a Successful Merge
radioamerica7 replied that they had merged storage and it was “working as expected” with no issues.
That encouraged the OP to try it, but short-term success did not answer the deeper root-filesystem question.
The OP Added an SSD and the Merge Process Completed Easily
Xan later added an SSD through the Y-cable and merged it with CasaOS storage. The UI still showed the internal chip as small, but the combined storage space had hundreds of gigabytes available.
For several days, the setup appeared to work.
Then Root-Level No-Space Symptoms Returned
The OP began seeing familiar symptoms:
- Docker containers not starting correctly;
- PHP errors;
-
apt-get upgradefailing withno space left on device; - the internal chip reporting broken/undefined free-space information.
This is the most important evidence in the thread.
CasaOS Merge Historically Used MergerFS for /DATA Aggregation
IceWhale's CasaOS LocalStorage implementation and public issue history describe the merged view as a MergerFS-style combination of CasaOS's file area plus added storage under /DATA. A historical example shows the Linux root still mounted separately while /DATA spans multiple underlying paths.
See the CasaOS merged-storage architecture.
MergerFS Does Not Increase the ext4 Root Filesystem
Files written to paths outside the merged tree—such as package-manager data, logs, parts of Docker's default root, and normal Linux system files—still consume the physical root filesystem.
That is why apt can run out of space even when the merged /DATA view reports abundant free capacity.
Docker Images/Volumes Need Their Own Migration Plan
This is why Giorgio first pointed the user toward moving Docker images and volumes. App payloads can be large, and changing the visible CasaOS storage pool does not necessarily relocate Docker's root directory or every existing volume.
Before changing Docker storage paths, back up container configuration/AppData and follow a current CasaOS/Docker migration method appropriate to the installed host OS.
The Source Feature Was Explicitly Labeled Beta
The OP repeatedly noted that the merge function was beta. CasaOS's public issue history also contains past merged-storage bugs and design requests. Treat it as a pooling convenience layer, not a substitute for understanding the underlying disk/root layout.
A More Predictable Storage Layout Separates Roles
For a small eMMC/SBC home server, a clearer design is:
- OS/root on a system device with enough free headroom;
- Docker/AppData on SSD or another deliberately chosen storage path;
- large media/downloads on separate bulk storage;
- backup on a different device.
This makes “which physical disk is full?” easier to answer than relying on a pooled UI number.
CasaOS Merge Storage FAQ
Did CasaOS Merge give the source user more visible /DATA capacity?
Yes. The merged view showed hundreds of gigabytes free after adding the SSD.
Did that prevent the internal/root storage from filling again?
No. The OP later hit root-level no-space symptoms including failed apt-get upgrade.
Does merged /DATA mean the Linux root partition physically expands?
No. MergerFS-style pooling combines paths at the filesystem layer; it does not resize the underlying root partition.
