The inner life of NFS filehandles

January 20, 2009

The NFS protocol uses something called a 'file handle' to identify what file a given operation applies to; this is sort of analogous to how a traditional Unix system internally identifies files by their inode number (well, their device plus their inode number).

In theory a NFS filehandle is opaque and an NFS server can use any scheme it wants in order to uniquely identify files. In practice there are a number of constraints on how a NFS server can form filehandles, based on preserving as many Unix semantics as possible; for example, the filehandle can't change just because someone renamed the file, and it has to be stable over server restarts.

A traditional, straightforward NFS server implementation puts three major things into a filehandle:

  • some identifier of the (server) filesystem that the file is on. Originally this was the (Unix) device that the filesystem was mounted on, but these days you can often set this explicitly (Linux uses the fsid= option in exports(5), for example), or the NFS server will somehow generate a stable number for each different filesystem based on various things.

  • some stable identifier of the file itself that will allow the NFS server to quickly look it up, traditionally the inode number.

  • a generation count for the inode number, to detect when a NFS filehandle refers to an older version of an inode.

The generation count is needed because traditional Unix filesystems can reuse the inode and thus the inode number of a deleted file, and when a file is deleted you want all the old NFS filehandles for it to stop working instead of suddenly referring to whatever new file inherits the inode. So it is the combination of the inode number and the generation count that uniquely identifies an abstract 'file' in a particular NFS-exported filesystem.

(Of course this is only necessary if your filesystem reuses its equivalent of inode numbers. If it does not, you don't need generation counts.)

Historically, bugs about when inode generation counts did or did not get updated have been a fruitful source of peculiar NFS problems. For example, I believe that there was once a Unix system that updated the generation count when you truncated a file.

Written on 20 January 2009.
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Last modified: Tue Jan 20 01:02:25 2009
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