A Linux process can receive ENOSPC even when df -h shows free blocks: the failed operation may need an inode or directory allocation, may be blocked by a quota or reserved-space policy, or may be writing to a different filesystem than the one you checked. Start by identifying the exact operation, destination path, and filesystem used by the process. The title’s “48 hours” is not independently verified, and without details such as the filesystem, process identity, mount namespace, and quota state, no single cause can be established.
What “free space” does—and does not—tell you
df -h reports block capacity for the filesystem containing the path you give it. Its output does not prove that every kind of allocation is available to a particular process. GNU df can show inode capacity with -i; POSIX also distinguishes space available to unprivileged users from system-reserved space. See the df(1) manual and the POSIX df(1p) specification.
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| What to check | Why free blocks may not settle the question | Useful evidence |
|---|---|---|
| Block capacity | The path may be on a different filesystem from the one you inspected. | df -h /exact/path and mount information for that path. |
| Inodes and directory allocation | Creating a file requires metadata as well as data blocks; inode pressure or a filesystem-specific directory limit may prevent creation. | df -i /exact/path, plus filesystem-specific configuration. |
| Quota and reservation policy | A user or group can be unable to allocate space even when the filesystem has capacity available elsewhere. | The writer’s UID/GID and quota state; reserved-block configuration where applicable. |
| Container or overlay backing store | A merged container path does not by itself identify the filesystem supplying writable storage. | The watcher’s mount namespace and the upper layer’s backing filesystem. |
| Allocated but untraversed files | A pathname traversal can miss storage still allocated to a file held open after unlinking. | Confirm that measurements use the same filesystem and namespace, then inspect open files. |
These are candidate explanations, not diagnoses. The failing syscall and its complete error can help distinguish them: open(2) documents quota exhaustion as EDQUOT, distinct from a device-space error.
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First, establish what failed and where
Record the failed operation, exact destination, timestamp, full error text, process identity, and container identity if applicable. A message summarized as “no space” can obscure whether the process failed to create a file, extend one, or perform another allocation. Inspect the destination itself rather than assuming the root filesystem or a remembered mountpoint is relevant.
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df -h /path/to/destination
df -i /path/to/destination
findmnt -T /path/to/destinationfindmnt is a suggested operational check; confirm it is installed on the affected system. Compare the results with the process’s view of the path if it runs in a container or a separate mount namespace.
Check the writer’s quota and access to reserved space
Identify the UID and GID under which the watcher runs, and check quota state for that identity and filesystem. Linux documents EDQUOT when a file creation exceeds a user’s block or inode quota in open(2); quota tools can show usage and limits. Filesystem-wide free blocks do not rule out an identity-specific limit.
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On ext-family filesystems, reserved blocks and the identities allowed to use them can also affect allocation. The ext4(5) manual describes a 5% reserved-space default, along with resuid and resgid controls. That figure is a documented default, not a universal setting: verify the actual filesystem and configuration before relying on it.
Look for inode pressure and filesystem-specific limits
If df -i /path/to/destination shows no usable inodes, investigate a large number of small files and the filesystem’s inode behavior. A healthy block-availability figure from df -h does not answer this question; inode information is the relevant view described by df(1).
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Also check for allocation constraints specific to the filesystem in use. For example, ext4’s max_dir_size_kb option can cause ENOSPC when a directory exceeds its configured size limit, as documented in ext4(5). Do not assume this option applies to other filesystems or is configured on the affected host.
In a container, identify the writable layer’s backing filesystem
For Docker, another container runtime, or an overlay mount, inspect mount information from the watcher’s mount namespace and determine which filesystem backs its writable upper layer. OverlayFS presents a layered view: the merged directory alone does not establish the upper layer’s available capacity. The Linux kernel OverlayFS documentation describes the layered model; runtime and mount configuration determine the actual backing store.
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Reconcile differences between df and du
df reports filesystem-level usage, while du estimates space through directory traversal. Before interpreting a mismatch, make sure both commands are examining the same filesystem and namespace. Their measurements answer different questions; see the du(1) manual for its usage-estimation behavior.
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Use evidence to narrow the cause
- If the exact path’s
df -ioutput shows inode pressure, investigate inode exhaustion and small-file growth. - If the writer’s quota is exhausted, identify the applicable user, group, or other configured quota rather than treating filesystem-wide free blocks as available to that writer.
- If ext4 is in use, verify reserved-block settings, permitted identities, and any configured directory-size limit.
- If the process runs in a container or separate mount namespace, compare its mount view with the host and identify the writable layer’s backing filesystem.
- If
dfanddudisagree, first align their path, filesystem, and namespace; then check for allocated files no longer visible by pathname.
Until those checks identify the resource and policy involved, free blocks on one df display are not enough to explain—or dismiss—the watcher’s ENOSPC.
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