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ENOSPC (“No space left on device”) does not necessarily mean the filesystem has run out of data blocks. A Linux process can fail to create or extend a file while df -h still shows free blocks because it has exhausted inodes, hit a quota or filesystem-specific limit, cannot use reserved blocks, or is writing to a different filesystem than the one you checked. The title alone does not identify which cause applies. Start by checking the exact failing path from the watcher’s environment and identity.
What the error tells you—and what it does not
ENOSPC reports that the attempted operation could not allocate a required resource. It is not, by itself, proof that all data blocks on the device are occupied. The open(2) manual distinguishes space-related errors from EDQUOT, which indicates an exhausted user or group quota in applicable cases. Capture the full error and the operation that failed rather than treating every allocation failure as “the disk is full.” See the Linux open(2) manual.
The report that a watcher raised the error for 48 hours does not establish its filesystem, destination, UID/GID, namespace, quota, or container setup. Without those details, no single root cause can be inferred.
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df reports capacity for the filesystem containing the path supplied to it. Checking / or a remembered mountpoint may therefore say nothing about a watcher writing elsewhere. Run these commands for the exact destination path:
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df -h /path/to/destination
df -i /path/to/destination
findmnt -T /path/to/destinationThe first command reports block capacity; the second reports inode information. findmnt can help identify the mounted filesystem for the path, but confirm that the utility is installed on the affected system. The df(1) manual documents path-based filesystem reporting and the -i inode display.
Check which allocation resource is exhausted
Inodes, not data blocks
A filesystem can have free data blocks but no available inodes for new files. This is a likely branch when the watcher creates many small files. Compare the output of df -i with df -h for the same destination: inode pressure is shown by the former, not established by free block capacity in the latter. Investigate the relevant filesystem’s inode behavior and the population of small files before concluding that blocks are the problem.
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User, group, or project quota
Filesystem-wide free space does not guarantee that a particular account may use it. Establish the watcher’s effective UID and GID (and project identity where applicable), then inspect quota usage and limits for that identity on the filesystem containing the destination. The open(2) manual documents EDQUOT for exhausted user block or inode quota on file creation. A quota check tied to the wrong account or filesystem will not resolve the question.
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POSIX distinguishes space available to unprivileged users from system-reserved space; a filesystem can therefore have blocks that are not available to the watcher even when a display suggests capacity remains. On ext-family filesystems, the ext4 manual describes a default reserved portion of 5%, but that is a documented default, not a universal setting or a guarantee about this incident. Inspect the actual filesystem’s reservation and mount configuration, including resuid and resgid where relevant. The configured amount and the watcher’s access identity determine whether reserved blocks are usable by it. POSIX availability semantics are described in the POSIX df(1p) manual.
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Filesystem-specific allocation limits
Some constraints are specific to a filesystem or its configuration. For example, ext4’s max_dir_size_kb option can cause ENOSPC when a directory exceeds the configured size limit. Check this only when the destination is on ext4 and the option is relevant; it should not be generalized to other filesystem types. The condition is documented in the ext4 manual.
For containers, inspect the watcher’s mount namespace and backing store
A path that looks like an ordinary directory inside a container may be part of an overlay filesystem. The merged view alone does not establish how much capacity is available to the writable upper layer or which backing filesystem supplies it. Inspect mount information from the watcher’s mount namespace, identify the writable upper layer, and check the filesystem that backs it. A host-side path or a mount inspected from another namespace may describe a different storage resource. The Linux kernel OverlayFS documentation explains the layered model; details depend on the host’s configuration.
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Use a df-versus-du mismatch as a clue, not a verdict
If du accounts for much less usage than df, first confirm that both commands are examining the same filesystem and namespace. du estimates space by traversing visible paths, while df reports filesystem-level usage; their scopes and measurement methods differ. See the du(1) manual.
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A practical diagnostic sequence
- Record the failed operation. Capture the exact syscall or write operation, full destination path, timestamp, complete error, and the watcher’s process or container identity.
- Measure the destination’s filesystem. Run
df -h,df -i, andfindmnt -Tagainst that exact path, from the environment where the watcher runs. - Check the writer’s limits. Determine its UID/GID and relevant project identity; inspect quota use and limits for the destination filesystem.
- Inspect filesystem-specific policy. If the filesystem is ext-family, check reserved-block settings and relevant mount options. For ext4, check whether a directory-size limit applies to the failing destination.
- Verify container storage, if applicable. Inspect mounts in the watcher’s namespace and identify the filesystem backing its writable upper layer.
- Reconcile apparent usage differences. If
dfanddudisagree, confirm their scope matches, then investigate whether deleted-but-open files or another difference in visible paths explains the gap.
The decisive question is not simply whether a display shows free blocks. It is which filesystem and allocation resource the failed operation actually needed, under the watcher’s identity and mount view.
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