Time stamps for PDSs and PDSEs

An MVS PDS or PDSE data set can act as a UNIX directory, when mounted by an NFS client to the z/OS NFS server. Members of the PDS or PDSE data sets are files within the UNIX directory. When the client accesses the directory, UNIX-format file time stamps are expected for each file on the client side. File time stamps in UNIX format are part of the attributes required by the NFS protocol for NFS client/server communication.

Based on the NFS protocol, the z/OS NFS server generates the following UNIX time stamps to send to the client:

atime: the time when the file data was last accessed
mtime: the time when the file data was last modified
ctime: the time when the attributes of the file were last changed.

The z/OS NFS server converts MVS time stamps to UNIX time stamps (and vice versa) to match NFS protocol requirements. The server uses the following main time stamp sources to generate UNIX time stamps for MVS z/OS conventional (legacy) file systems:

DSCB (data set control block)
Master Catalog data set attribute extension (AX) cell
PDSE member attribute extension (AX) cell
ISPF member statistics
TOD (current time_of_day on the server side).

Time stamp generation depends on the NFS operation (such as read, write, or setattr) and the type of data set.

TOD is used to set up current times in internal NFS control blocks if needed.

For PDS or PDSE member create/update access with ISPF, some specific additional statistics for the member are maintained by ISPF. They include the creation date and the last modification date and time. The server supports ISPF statistics for compatibility with TSO/ISPF. The server always creates ISPF statistics for new PDS/PDSE members created by NFS clients. For existing PDS/PDSE member updates by the client, the server creates/updates the member ISPF statistics.

The following tables summarize the time stamp sources for NFS operations when obtaining file attributes for PDS (SMS-managed and non SMS-managed) and PDSE data sets and members.

Table 1. Time stamp sources for PDS and PDSE members
Time stamp sources for members (files)
Data set type	ISPF statistics	atime	mtime	ctime
PDS (note 1)	Available	ISPF_ modification_ date + ISPF_ modification_ time	ISPF_ modification_ date + ISPF_ modification_ time	ctime = mtime
PDS (note 1)	Not available	DSCB_ reference_ date + time_ increment	DSCB_ reference_ date + time_ increment	ctime = mtime
PDSE (note 2)	Available (not used for time generation)	PDSE AX cell	PDSE AX cell	ctime = mtime

Note:

MVS does not maintain time stamps for members of a PDS, only for the PDS data set. MVS creation and reference dates are maintained in the DSCB control block.
The UNIX time stamps for PDS members are generated from the DSCB creation and reference dates of the PDS data set containing the members, if the time stamps cannot be generated from the member's ISPF statistics.
MVS maintains the PDSE member create/change time stamp (mtime) in the PDSE AX cell. The Server uses a FileAccessMethodService (FAMS) call to retrieve/save the member attributes (containing time stamps) from/to the PDSE AX cell.
For a PDSE data set/member, the server generates the UNIX time stamps obtained from the Catalog AX cell/PDSE AX cell. The server supports PDSE member's ISPF statistics just for compatibility with ISPF but does not return them to the client.

Table 2. Time stamp sources for PDS and PDSE data sets (directories)
Time stamp sources for data sets (directories)
Data set type	atime	mtime	ctime
PDS (non-SMS)	DSCB_ reference_ date + time_ increment	DSCB_ reference_ date + time_ increment	ctime = mtime
PDS (SMS)	catalog AX cell	catalog AX cell	ctime = mtime
PDSE (SMS)	catalog AX cell	catalog AX cell	ctime = mtime

In Table 1 and Table 2, time_increment is either the server local TOD or 23:59 hours. If reference_date or creation_date is equal to the server local date, the server local TOD is added. Otherwise, a fixed value of 23 hours and 59 minutes is added. If the reference_date value is zero (that is, the file has not yet been referenced) then atime = mtime = ctime = creation_date + time_increment.

MVS maintains PDSE/SMS-managed PDS atime, mtime time stamps in the Catalog AX cell. The Server uses an SVC26 call to retrieve/save the data set attributes (containing time stamps) from/to the Catalog AX cell.

The server keeps all file time stamps current in internal control blocks in readiness to return time stamps when servicing NFS requests issued from NFS clients. The server updates the time stamp sources (in the DSCB, member ISPF statistics, or attribute extension cells) from internal control blocks only during close file operations depending on file timeout expiration values (attrtimeout, readtimeout, writetimeout) as described in section 'Timeout attributes'.