SFS Addresses the Meta Data Access Time within the Run and Reporting Rules

By Judy Piantedosi
Digital Equipment Corporation
Nashua, NH

Published June, 1995; see disclaimer.

During the November 1994 and January 1995 meetings, the SFS Subcommittee addressed the issue of Meta-data posting. Previously it was reported that certain NFS implementations delayed the posting of metadata updates where only the modification time on a write operation was changed.

Since this behavior appeared to be in violation of the SFS Run and Reporting rules, an investigation into the origin of the software was requested. The software was traced back to an old version of the ONC 3.2 reference port and remained in the ONC reference code through its last release ONC 4.2.

Basically, this code sets a flag to update the inode synchronously if:

• it is a write operation, and
• the write has increased the file size or otherwise caused a new block allocation (hole filling).

However, if the write operation overwrites existing blocks of data, the writing of the data is done synchronously but the writing of the inode modify time is done asynchronously.

During the Annual SPEC Meeting in January 1995, the SFS Subcommittee reviewed the information collected on this issue. Although data within the file was always written synchronously, it was felt that writing the metadata inode modify time asynchronously could result in data corruption. Additionally, it was felt that this implementation did violate SPEC's requirement that the NFS Server must synchronously write data/Meta-data to stable storage on an update. The SFS Subcommittee and OSSC moved to clarify the definition of stable storage by ratifying the following amendment to the run and reporting rules:

R95.01.03 (Run rules, ACCESS time update):

SPEC accepts the common practice of updating the ACCESS time of a file asynchronously for the read and readdir operations. However, SPEC reaffirms that the MODIFY time must be updated synchronously.

Since this behavior appeared to be in violation of the SFS Run and Reporting rules, an investigation into the origin of the software was requested.

Thus, all vendors submitting results in the June 1995 SPEC Newsletter (or any subsequent SPEC Newsletter) must assert that their implementation adheres to this ruling. Since the origin of this code is located in a common source pool (ONC V3.2 through ONC V4.2), it is recommended that vendors using this reference port check their implementation for this behavior.

Judy Piantedosi is a Principle Software Engineer for Digital Equipment Corporation in Nashua, N.H. specializing in NFS performance. She is currently involved with SPEC's new System File Server Steering Committee (SFSSC).

Copyright (c) 1995 Standard Performance Evaluation Corporation