From d5e80fe89c8ef9b67cd0366410761ce0be6dfda2 Mon Sep 17 00:00:00 2001 From: dCache Bot Date: Fri, 9 Oct 2026 09:55:38 +0000 Subject: [PATCH] [skip-ci] docs: document exclusive lock on files open for write over NFS Assisted-by: deepseek-v4.1-flash/deepseek --- docs/TheBook/src/main/markdown/config-nfs.md | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/docs/TheBook/src/main/markdown/config-nfs.md b/docs/TheBook/src/main/markdown/config-nfs.md index af594c3ec15..c6d92cb9bd1 100644 --- a/docs/TheBook/src/main/markdown/config-nfs.md +++ b/docs/TheBook/src/main/markdown/config-nfs.md @@ -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