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[skip-ci] docs: document exclusive lock on files open for write over NFS #8252

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kofemann merged 1 commit into from
Oct 9, 2026
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[skip-ci] docs: document exclusive lock on files open for write over NFS #8252

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12 changes: 12 additions & 0 deletions docs/TheBook/src/main/markdown/config-nfs.md
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Expand Up @@ -125,6 +125,18 @@ In this example, hosts in the dcache.org may read and write, while host external

If there are multiple path specifications, the shortest matching path wins. If there are multiple host/subnet specifications, the most precise specification wins.

## Concurrent write access

A file that is currently open for writing is exclusively locked: while such an
open file exists, the NFS door denies any other attempt to open the same file,
regardless of whether that attempt requests read or write access. Any such
attempt fails with `Permission denied` (`EACCES`).

When the writing client closes the file, the lock is released and further opens
succeed. This prevents two concurrent writers from creating competing replicas
of the same file (with the same pnfsid but different contents) and reflects the
fact that dCache treats file content as write-once.

## Configuring NFSv4.1 door with GSS-API support

Adding `sec=krb5` into `/etc/exports` is not sufficient to get
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