Section 3.5. Multiprotocol Access Problems

   

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3.5 Multiprotocol Access Problems

Several vendors offer server solutions that provide simultaneous support for CIFS, NFS, and other NFS clients . These vendors have gone to a lot of effort to deal with some tough technical problems arising from the fact that the clients and the servers may be running different operating systems and different file systems. Note that the problems, if any, most likely occur with the file metadata and not the core data itself. An extremely simple test of where a problem lies is to copy a file from the server to the client and then back to the server (or vice versa). After the file is back at its original location, is the metadata still the same? Have the file access permissions changed? Do some groups that previously had no access have some kind of access now? Are all the timestamps on the files the same?

Here are some examples of the technical issues involved:

• Providing for mapping between the different ways in which user and group accounts and permissions are tracked on different operating systems.

• Providing for differences between file opening and locking semantic differences between different operating systems and file systems.

• Providing for differences between file naming conventions. Different file systems have different ideas about maximum file name lengths, file name case sensitivity, and the characters that are valid for use in a file name.

• Providing for file system differences in data and data structures; for example, one file system may track two times, whereas another may track three times (file last accessed, file last modified, and file creation). Even when the two file systems track the same times, the units of measurement may be different. Another example is units while measuring file offsets. Whereas some file systems may have 32-bit offsets, others may have 16- or 64-bit offsets.

• Addressing lock-mapping problems. A CIFS server enforces locks on a file; that is, if one client has locked a range of bytes, any write operation to any portion of that byte range by another client means that the second client will get an error. NFS servers, however, do not enforce locking. So a choice needs to be made about whether the locking will be enforced, causing an error to be sent to an NFS client.

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