Tape mount analysis on z/OS: what SMF 21 tells you

SMF 21 tells you that a tape volume was demounted, on which drive, and how many read errors, write errors and start subchannel operations it saw while mounted. For tape mount analysis on z/OS it gives a count per volume and drive, but not the wait of the job nor what happened inside a virtual tape library.

This article explains what an SMF 21 record contains, how to count mounts with it, where its limits are, and which other sources complete the picture.

What SMF 21 is

SMF record type 21 (X’15’) is the Error Statistics by Volume record, or ESV. z/OS writes it when a data set on magnetic tape is demounted. It holds statistics for the whole volume over the time it was mounted, however many data sets were read or written and however many times they were closed.

The IFHSTATR utility formats and prints these records. Many sites also load them into their own performance database, next to the other SMF types.

One detail matters for counting: a type 21 record is also written when EREP runs and when EOD is issued. More than one record can exist for a single mount.

What an SMF 21 record contains

The record describes a volume and the drive it was mounted on. The table below groups its content by use.

Counting mounts with SMF 21

A mount count from SMF 21 is simple in principle: one demount, one record. In practice, four steps keep the count honest:

1. Remove the extra records written by EREP runs and EOD, so that each mount counts once.
2. Split by device type, so that virtual and real drives are not added together.
3. Count per system, then add up: each LPAR writes its own records.
4. Count per hour, not per day. Mount peaks during the batch window are what drive the number of drives you need.

The result answers questions such as “how many mounts per hour during the night?” or “which volumes are mounted again and again?”. It is a good start for a tape mount analysis, and often the first evidence that a workload should move from real to virtual tape, or the other way round.

What SMF 21 does not tell you

SMF 21 is about volumes and drives. Several questions of a full tape mount analysis need other data:

• Who asked for the mount. Correlate with job and data set records (SMF 30, 14 and 15) or with your tape management system.
• How long jobs waited for mounts. The ESV record has no wait time. The RMF device activity report shows the share of time a tape device spent with a mount pending (% MT PEND).
• What happened inside a virtual tape library. On a TS7700, SMF 21 describes the virtual volume. Whether the mount was a cache hit or needed a recall from physical tape is in the library’s own statistics.
• What is on the tapes. Active and scratch volumes, file counts and fill rates come from the tape management catalogue.

Other sources for a complete picture

The TS7700 keeps point-in-time and historical statistics. The BVIR facility retrieves them to the host, and the IBM Tape Tools utility VEHSTATS turns the historical statistics into reports on mounts, data transfer and use of the library.

How zPulse approaches tape

Let us be precise: zPulse does not read SMF 21, nor any raw SMF record. zPulse Tape works from the reports that already summarise tape activity:

• RMF Magnetic Tape Device Activity or CMF Tape Activity Report. zPulse charts virtual tape mounts per LPAR over time, with a filter by device type, and the virtual tape I/O by LPAR with its response, connect, disconnect, IOSQ and pending time.
• VEHSTATS files from a TS7700. The VEHSTATS Report tool shows, among other tabs, mounts, physical mounts, cache throttling, the cluster overview and virtual drives.
• Tape management catalogue reports from DFSMSrmm, CONTROL-T or CA-1. A tape study shows active and scratch volumes and the used and free capacity for each tape type.

A scheduled tape study can send a standard report with the catalogue charts, a tape performance mail and a VEHSTATS mail, each with the covered period in its subject. The zPulse overview shows how tape fits with the Disk, CPU and Space modules.

To see this on your own data, request a free study: send your tape reports, VEHSTATS files and catalogue reports, and a Technical Storage expert walks you through the results.