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3.3.8 Procedure 8: CREATE - Create a file Connected: An Internet Encyclopedia
3.3.8 Procedure 8: CREATE - Create a file

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3.3.8 Procedure 8: CREATE - Create a file

3.3.8 Procedure 8: CREATE - Create a file

SYNOPSIS

      CREATE3res NFSPROC3_CREATE(CREATE3args) = 8;

      enum createmode3 {
           UNCHECKED = 0,
           GUARDED   = 1,
           EXCLUSIVE = 2
      };

      union createhow3 switch (createmode3 mode) {
      case UNCHECKED:
      case GUARDED:
           sattr3       obj_attributes;
      case EXCLUSIVE:
           createverf3  verf;
      };

      struct CREATE3args {
           diropargs3   where;
           createhow3   how;
      };

      struct CREATE3resok {
           post_op_fh3   obj;
           post_op_attr  obj_attributes;
           wcc_data      dir_wcc;
      };

      struct CREATE3resfail {
           wcc_data      dir_wcc;
      };

      union CREATE3res switch (nfsstat3 status) {
      case NFS3_OK:
           CREATE3resok    resok;
      default:
           CREATE3resfail  resfail;
      };

DESCRIPTION

Procedure CREATE creates a regular file. On entry, the arguments in CREATE3args are:

where

The location of the file to be created:

dir

The file handle for the directory in which the file is to be created.

name

The name that is to be associated with the created file. Refer to General comments on filenames on page 30.

When creating a regular file, there are three ways to create the file as defined by:

how

A discriminated union describing how the server is to handle the file creation along with the appropriate attributes:

mode

One of UNCHECKED, GUARDED, and EXCLUSIVE. UNCHECKED means that the file should be created without checking for the existence of a duplicate file in the same directory. In this case, how.obj_attributes is a sattr3 describing the initial attributes for the file. GUARDED specifies that the server should check for the presence of a duplicate file before performing the create and should fail the request with NFS3ERR_EXIST if a duplicate file exists. If the file does not exist, the request is performed as described for UNCHECKED. EXCLUSIVE specifies that the server is to follow exclusive creation semantics, using the verifier to ensure exclusive creation of the target. No attributes may be provided in this case, since the server may use the target file metadata to store the createverf3 verifier.

On successful return, CREATE3res.status is NFS3_OK and the results in CREATE3res.resok are:

obj

The file handle of the newly created regular file.

obj_attributes

The attributes of the regular file just created.

dir_wcc

Weak cache consistency data for the directory, where.dir. For a client that requires on the post-CREATE directory attributes, these can be found in dir_wcc.after.

Otherwise, CREATE3res.status contains the error on failure and CREATE3res.resfail contains the following:

dir_wcc

Weak cache consistency data for the directory, where.dir. For a client that requires only the post-CREATE directory attributes, these can be found in dir_wcc.after. Even though the CREATE failed, full wcc_data is returned to allow the client to determine whether the failing CREATE resulted in any change to the directory.

IMPLEMENTATION

Unlike the NFS version 2 protocol, in which certain fields in the initial attributes structure were overloaded to indicate creation of devices and FIFOs in addition to regular files, this procedure only supports the creation of regular files. The MKNOD procedure was introduced in the NFS version 3 protocol to handle creation of devices and FIFOs. Implementations should have no reason in the NFS version 3 protocol to overload CREATE semantics.

One aspect of the NFS version 3 protocol CREATE procedure warrants particularly careful consideration: the mechanism introduced to support the reliable exclusive creation of regular files. The mechanism comes into play when how.mode is EXCLUSIVE. In this case, how.verf contains a verifier that can reasonably be expected to be unique. A combination of a client identifier, perhaps the client network address, and a unique number generated by the client, perhaps the RPC transaction identifier, may be appropriate.

If the file does not exist, the server creates the file and stores the verifier in stable storage. For file systems that do not provide a mechanism for the storage of arbitrary file attributes, the server may use one or more elements of the file metadata to store the verifier. The verifier must be stored in stable storage to prevent erroneous failure on retransmission of the request. It is assumed that an exclusive create is being performed because exclusive semantics are critical to the application. Because of the expected usage, exclusive CREATE does not rely solely on the normally volatile duplicate request cache for storage of the verifier. The duplicate request cache in volatile storage does not survive a crash and may actually flush on a long network partition, opening failure windows. In the UNIX local file system environment, the expected storage location for the verifier on creation is the metadata (time stamps) of the file. For this reason, an exclusive file create may not include initial attributes because the server would have nowhere to store the verifier.

If the server can not support these exclusive create semantics, possibly because of the requirement to commit the verifier to stable storage, it should fail the CREATE request with the error, NFS3ERR_NOTSUPP.

During an exclusive CREATE request, if the file already exists, the server reconstructs the file's verifier and compares it with the verifier in the request. If they match, the server treats the request as a success. The request is presumed to be a duplicate of an earlier, successful request for which the reply was lost and that the server duplicate request cache mechanism did not detect. If the verifiers do not match, the request is rejected with the status, NFS3ERR_EXIST.

Once the client has performed a successful exclusive create, it must issue a SETATTR to set the correct file attributes. Until it does so, it should not rely upon any of the file attributes, since the server implementation may need to overload file metadata to store the verifier.

Use of the GUARDED attribute does not provide exactly-once semantics. In particular, if a reply is lost and the server does not detect the retransmission of the request, the procedure can fail with NFS3ERR_EXIST, even though the create was performed successfully.

Refer to General comments on filenames on page 30.

ERRORS

NFS3ERR_IO NFS3ERR_ACCES NFS3ERR_EXIST NFS3ERR_NOTDIR NFS3ERR_NOSPC NFS3ERR_ROFS NFS3ERR_NAMETOOLONG NFS3ERR_DQUOT NFS3ERR_STALE NFS3ERR_BADHANDLE NFS3ERR_NOTSUPP NFS3ERR_SERVERFAULT

SEE ALSO MKDIR, SYMLINK, MKNOD, and PATHCONF.


Next: 3.3.9 Procedure 9: MKDIR - Create a directory

Connected: An Internet Encyclopedia
3.3.8 Procedure 8: CREATE - Create a file

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