CODEWITHSUNDEEP
synchronizationrace-conditionnfscentos5
WilliamKF
WilliamKF

Reputation: 43179

How to force NFS mounted disk to sync to current state that was updated by a different machine?

I am running into an issue as follows in my C++ application running on Centos 5:

On machine A there is an NFS mounted drive from machine C that contains a file:

/nfs/mounted/drive/path/directory/file

My application renames the directory from machine A:

mv /nfs/mounted/drive/path/directory /nfs/mounted/drive/path/directory.old

The application on machine A then ssh(s) over to machine B to run an application I created which starts by first checking for the file's existence:

/nfs/mounted/drive/path/directory/file

The file is found to be present (it is a race condition and machine B is still seeing the old state) and therefore machine B proceeds as if the file is present, but then machine B subsequently fails when it goes to open the file and it is not really there.

I tried to resolve this by calling sync() in my application on machine B before it checks for the file. I also tried doing echo 2 > /proc/sys/vm/drop_caches but neither of these caused my application on machine B to immediately see the current state of the file being gone.

What can I do in my C++ application on machine B before it checks for the file to ensure the data is the most up to date status? I do not want to explicitly test the file in question by opening it, but instead I wish to cause the disk to be in current state. I would like to do this forced sync in a relatively fast manner from C++, as in taking less than around a quarter of a second elapsed time.

Upvotes: 0

Views: 1856

Answers (1)

kofemann
kofemann

Reputation: 4423

NFS has a file handle per file system object. As you rename a directory, the filemhandle did not change and lookup gives you positive result. To avoid such situation better to create a new directory, move all files and directories from original directory and remove the source directory. By this you will invalidate the old file handle.

Upvotes: 1

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