Idx Writes        (VSAM)

DESCRIPTION
The number of "Idx Writes" represents the total number of physical index writes 
issued to the index file since the Sun MTP system was initiated. A physical write
is a write not accommodated by Sun MTP's internal buffers. This write may, 
however, be handled by the operating systems buffer cache and not actually 
require a physical disk access.


VALUES
"Idx Writes" is a numeric quantity that starts at zero when a Sun MTP system
initiates and increments for each physical write to the index portion of a
dataset.

Note that this datapoint is not set for remote datasets (those with a value set
in the "RSys" field) because Sun MTPScan does not communicate with a remote machine.


INTERPRETATION
This datapoint, together with "Writes", "Reads", and "Idx Reads", provides an
accurate depiction of dataset activity. Multiple highly active datasets on the 
same physical device decreases system throughput. All four of these datapoints 
should be used to determine the extent of a file's activity. Note that the 
"Writes" and "Idx Writes" entail more overhead than their read counterparts. 


CONCLUSIONS
There are a few ways to optimize dataset access performance. There are two
kinds of configurable cache involved with a VSAM file local to a host machine:
the operating system buffer cache and the Sun MTP buffer pool. Consult your
machines System Administrator's Guide for details on how to modify the systems
buffer cache.  For instance, on a UNIX system, the UNIX kernel must be rebuilt
with a new configuration in order to change the size of its buffer cache.
Remember that increasing the buffer cache chews up valuable memory. As for the
Sun MTP cache, the size of it is easily configured in the Sun MTP VSAM Control
Table (VCT). Change the value of "Number of shared buffers", write the VCT to
disk, and restart the system to activate a change.

There are also a few ways to distribute dataset files across hardware devices
to achieve performance gains. The simplest way is to physically place entire
datasets on separate disks. If the disks happen to have separate disk
controllers, so much the better.

Another method to take advantage of I/O bandwidth is to use disk striping. This
file system facility automatically distributes files across multiple devices.
Again, consult your machines System Administrator's Guide for details.

Sun MTP provides another method to boost performance by striping a dataset across
multiple segments, each of which may reside on a separate device. Sun MTP allows
a dataset to have at most 8 segments.  The size of each file segment depends on
the type of system where Sun MTP resides.  On UNIX systems, each segment can
contain up to 2 Gigabytes (G) of information (this limit is imposed by the
UNIX file system). On Windows NT systems, the maximum size of each segment
depends on the maximum file size offered by the file system in place. For the
FAT file system, the maximum file size is 4 Gigabytes. For NTFS, the maximum
file size is 2 Terabytes, although this limit is constrained by hardware limits
rather than the NT operating system. This method proves to be more efficient
than the other two. You may choose to combine Sun MTP segmentation with operating
system striping, but the results are indeterminate since the file organization
becomes extremely complex.

