80 Customizing IBM Tivoli Workload Scheduler for z/OS V8.2 to Improve Performance
Figure 4-11 zFS in a colony address space
4.3.5 zFS aggregates
A zFS aggregate is a data set that contains zFSs. The aggregate is a VSAM
Linear Data Set (VSAM LDS) and is a container that can contain one or more
zFS. An aggregate can only have one VSAM LDS, but it can contain an unlimited
number of file systems. The name of the aggregate is the same as the VSAM
LDS name.
Sufficient space must be available on the volume or volumes, because multiple
volumes can be specified on the DEFINE of the VSAM LDS. DFSMS decides
when to allocate on these volumes during any extension of a primary allocation.
VSAM LDSs greater than 4 GB can be specified by using the extended format
and extended addressability capability in the data class of the data set. For
allocation of VSAM data sets for zFS aggregates, see “Allocating a zFS” on
page 84.
After the aggregate is created, formatting of the aggregate is necessary before
any file systems can exist in it. A zFS is a named entity that resides in a zFS
aggregate. It contains a root directory and can be mounted into the USS
hierarchy. Although the term file system is not a new term, a zFS resides in a zFS
aggregate, which is different from an HFS.
Chapter 4. Optimizing the UNIX System Services environment 81
zFS aggregates come in two types:
Compatibility mode aggregates
Multiple file system aggregates
Compatibility mode aggregates
A compatibility mode aggregate can contain only one zFS, making this type of
aggregate more like an HFS. This is flagged in the aggregate when it is created.
The name of the file system is the same as the name of the aggregate, which is
the same as the VSAM LDS cluster name. The file system size (called a quota) in
a compatibility mode aggregate is set to the size of the aggregate. Compatibility
mode aggregates are more like an HFS data set, except that they are VSAM
linear data sets instead of HFS data sets. We recommend that you start using
compatibility mode aggregates first, because they are more like the familiar HFS
data sets. Figure 4-12 shows a compatibility mode aggregate.
Figure 4-12 zFS compatibility mode aggregate
Multiple file mode aggregates
A multi-file mode aggregate enables the administrator to define multiple zFSs in
a single aggregate. This type of aggregate can contain one or more zFSs. This
allows space that becomes available when files are deleted in one file system to
be made available to other file systems in the same aggregate data set or space
sharing.
82 Customizing IBM Tivoli Workload Scheduler for z/OS V8.2 to Improve Performance
Space sharing
Space sharing means that if you have multiple file systems in a single data set,
and files are removed from one of the file systems, which frees DASD space,
another file system can use that space when new files are created. This new type
of file system is called a multi-file-system aggregate.
The multiple file system aggregate OMVS.EBIS.zFS, shown in Figure 4-13, can
contain multiple zFSs. This makes it possible to do space sharing between the
zFSs within the aggregate.
The multiple file system aggregate has its own name. This name is assigned
when the aggregate is created. It is always the same as the VSAM LDS cluster
name. Each zFS in the aggregate has its own file system name. This name is
assigned when the particular file system in the aggregate is created. Each zFS
also has a predefined maximum size, called the quota.
Figure 4-13 zFS multiple file mode aggregates
Metadata cache
The zFS has a cache for file system metadata, which includes directory contents
and the data of files that are smaller than the aggregate block size. The setting of
this cache size is important to performance because zFS references the file
system metadata frequently.

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