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3.3.7 Procedure 7: WRITE - Write to file Connected: An Internet Encyclopedia
3.3.7 Procedure 7: WRITE - Write to file

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Prev: 3.3.6 Procedure 6: READ - Read From file
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3.3.7 Procedure 7: WRITE - Write to file

3.3.7 Procedure 7: WRITE - Write to file

SYNOPSIS

      WRITE3res NFSPROC3_WRITE(WRITE3args) = 7;

      enum stable_how {
           UNSTABLE  = 0,
           DATA_SYNC = 1,
           FILE_SYNC = 2
      };

      struct WRITE3args {
           nfs_fh3     file;
           offset3     offset;
           count3      count;
           stable_how  stable;
           opaque      data<>;
      };

      struct WRITE3resok {
           wcc_data    file_wcc;
           count3      count;
           stable_how  committed;
           writeverf3  verf;
      };

      struct WRITE3resfail {
           wcc_data    file_wcc;
      };

      union WRITE3res switch (nfsstat3 status) {
      case NFS3_OK:
           WRITE3resok    resok;
      default:
           WRITE3resfail  resfail;
      };

DESCRIPTION

Procedure WRITE writes data to a file. On entry, the arguments in WRITE3args are:

file

The file handle for the file to which data is to be written. This must identify a file system object of type, NF3REG.

offset

The position within the file at which the write is to begin. An offset of 0 means to write data starting at the beginning of the file.

count

The number of bytes of data to be written. If count is 0, the WRITE will succeed and return a count of 0, barring errors due to permissions checking. The size of data must be less than or equal to the value of the wtmax field in the FSINFO reply structure for the file system that contains file. If greater, the server may write only wtmax bytes, resulting in a short write.

stable

If stable is FILE_SYNC, the server must commit the data written plus all file system metadata to stable storage before returning results. This corresponds to the NFS version 2 protocol semantics. Any other behavior constitutes a protocol violation. If stable is DATA_SYNC, then the server must commit all of the data to stable storage and enough of the metadata to retrieve the data before returning. The server implementor is free to implement DATA_SYNC in the same fashion as FILE_SYNC, but with a possible performance drop. If stable is UNSTABLE, the server is free to commit any part of the data and the metadata to stable storage, including all or none, before returning a reply to the client. There is no guarantee whether or when any uncommitted data will subsequently be committed to stable storage. The only guarantees made by the server are that it will not destroy any data without changing the value of verf and that it will not commit the data and metadata at a level less than that requested by the client. See the discussion on COMMIT on page 92 for more information on if and when data is committed to stable storage.

data

The data to be written to the file.

On successful return, WRITE3res.status is NFS3_OK and WRITE3res.resok contains:

file_wcc

Weak cache consistency data for the file. For a client that requires only the post-write file attributes, these can be found in file_wcc.after.

count

The number of bytes of data written to the file. The server may write fewer bytes than requested. If so, the actual number of bytes written starting at location, offset, is returned.

committed

The server should return an indication of the level of commitment of the data and metadata via committed. If the server committed all data and metadata to stable storage, committed should be set to FILE_SYNC. If the level of commitment was at least as strong as DATA_SYNC, then committed should be set to DATA_SYNC. Otherwise, committed must be returned as UNSTABLE. If stable was FILE_SYNC, then committed must also be FILE_SYNC: anything else constitutes a protocol violation. If stable was DATA_SYNC, then committed may be FILE_SYNC or DATA_SYNC: anything else constitutes a protocol violation. If stable was UNSTABLE, then committed may be either FILE_SYNC, DATA_SYNC, or UNSTABLE.

verf

This is a cookie that the client can use to determine whether the server has changed state between a call to WRITE and a subsequent call to either WRITE or COMMIT. This cookie must be consistent during a single instance of the NFS version 3 protocol service and must be unique between instances of the NFS version 3 protocol server, where uncommitted data may be lost.

Otherwise, WRITE3res.status contains the error on failure and WRITE3res.resfail contains the following:

file_wcc

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

If a client writes data to the server with the stable argument set to UNSTABLE and the reply yields a committed response of DATA_SYNC or UNSTABLE, the client will follow up some time in the future with a COMMIT operation to synchronize outstanding asynchronous data and metadata with the server's stable storage, barring client error. It is possible that due to client crash or other error that a subsequent COMMIT will not be received by the server.

IMPLEMENTATION

The nfsdata type used for the READ and WRITE operations in the NFS version 2 protocol defining the data portion of a request or reply has been changed to a variable-length opaque byte array. The maximum size allowed by the protocol is now limited by what XDR and underlying transports will allow. There are no artificial limits imposed by the NFS version 3 protocol. Consult the FSINFO procedure description for details.

It is possible for the server to write fewer than count bytes of data. In this case, the server should not return an error unless no data was written at all. If the server writes less than count bytes, the client should issue another WRITE to write the remaining data.

It is assumed that the act of writing data to a file will cause the mtime of the file to be updated. However, the mtime of the file should not be changed unless the contents of the file are changed. Thus, a WRITE request with count set to 0 should not cause the mtime of the file to be updated.

The NFS version 3 protocol introduces safe asynchronous writes. The combination of WRITE with stable set to UNSTABLE followed by a COMMIT addresses the performance bottleneck found in the NFS version 2 protocol, the need to synchronously commit all writes to stable storage.

The definition of stable storage has been historically a point of contention. The following expected properties of stable storage may help in resolving design issues in the implementation. Stable storage is persistent storage that survives:

  1. Repeated power failures.

  2. Hardware failures (of any board, power supply, and so on.).

  3. Repeated software crashes, including reboot cycle.

This definition does not address failure of the stable storage module itself.

A cookie, verf, is defined to allow a client to detect different instances of an NFS version 3 protocol server over which cached, uncommitted data may be lost. In the most likely case, the verf allows the client to detect server reboots. This information is required so that the client can safely determine whether the server could have lost cached data. If the server fails unexpectedly and the client has uncommitted data from previous WRITE requests (done with the stable argument set to UNSTABLE and in which the result committed was returned as UNSTABLE as well) it may not have flushed cached data to stable storage. The burden of recovery is on the client and the client will need to retransmit the data to the server.

A suggested verf cookie would be to use the time that the server was booted or the time the server was last started (if restarting the server without a reboot results in lost buffers).

The committed field in the results allows the client to do more effective caching. If the server is committing all WRITE requests to stable storage, then it should return with committed set to FILE_SYNC, regardless of the value of the stable field in the arguments. A server that uses an NVRAM accelerator may choose to implement this policy. The client can use this to increase the effectiveness of the cache by discarding cached data that has already been committed on the server.

Some implementations may return NFS3ERR_NOSPC instead of NFS3ERR_DQUOT when a user's quota is exceeded.

Some NFS version 2 protocol server implementations incorrectly returned NFSERR_ISDIR if the file system object type was not a regular file. The correct return value for the NFS version 3 protocol is NFS3ERR_INVAL.

ERRORS

NFS3ERR_IO NFS3ERR_ACCES NFS3ERR_FBIG NFS3ERR_DQUOT NFS3ERR_NOSPC NFS3ERR_ROFS NFS3ERR_INVAL NFS3ERR_STALE NFS3ERR_BADHANDLE NFS3ERR_SERVERFAULT

SEE ALSO COMMIT.


Next: 3.3.8 Procedure 8: CREATE - Create a file

Connected: An Internet Encyclopedia
3.3.7 Procedure 7: WRITE - Write to file

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