no space left on device -- but there is space left on device

Error in the logs:
```
lvl=error msg="failed to write blob" err="open /mnt/fast2/BLOBS/0x765180c2144cc82e2fde91f5c197332264f3922f43fc04b10fa915a48411c096: no space left on device"
```
But device has plenty of space:
```
# df -h | grep fast2
/dev/nvme4n1p2 2.8T 178G 2.5T 7% /mnt/fast2
```

Start by tracing the blob-write path that produces the logged error, then compare its filesystem checks with the reported df output. The issue names no files or tests, so first identify the relevant entry point and a way to reproduce the failure. Done means the cause of the incorrect no-space error is established and the behavior is corrected or clearly documented.

Written by the indexing model from the issue text.