Terminology and concepts
To present all the benefits and details of zFS administration, the following concepts and terminology are introduced:
- Attach
- When a zFS file system is mounted, the data
set is also attached. Attach means that zFS allocates and opens the
data set. This attach occurs the first time a file system contained
in the data set is mounted.
A zFS data set can also be attached (by issuing the zfsadm attach command) without mounting it.
Beginning in z/OS V2R2, only zFS data sets that contain
exactly one file system are allowed to be attached.
However,
there are many restrictions in this case. For example, the zFS data
set would not be available to z/OS® UNIX applications because it was
not mounted. In a shared file system environment, the zFS data set
would be detached, not moved, if the system went down or zFS internally
restarted. You might attach a zFS data set to explicitly grow it (zfsadm
grow) or to determine the free space available (zfsadm
aggrinfo). You must detach the zFS data set (zfsadm
detach) before mounting it. - Catch-up mount
- When a file
system mount is successful on a system in a shared file system environment, z/OS UNIX automatically
issues a corresponding local mount, which is called a catch-up
mount, to every other system's PFS for a zFS read/write mounted
file system that is mounted RWSHARE or for a read-only mounted file
system.
If the corresponding local mount is successful, z/OS UNIX does not function ship from that system to the z/OS UNIX owning system when that file system is accessed. Rather, the file request is sent directly to the local PFS. This is sometimes referred to as Client=N, as indicated by the output of the D OMVS,F operator command, or df -v shell command. If the corresponding local mount is unsuccessful (for instance, DASD is not accessible from that system), z/OS UNIX function ships requests to the z/OS UNIX owning system when that file system is accessed (message BPXF221I might be issued). This is sometimes referred to as Client=Y, as indicated by the output of the D OMVS,F or df -v commands. For examples of the command output, see Determining the file system owner.
- File system ownership
- IBM® defines
a file system owner as the system that coordinates sysplex activity
for a particular file system. In a shared file system environment,
there is also the concept of file system ownership. The
owner of a file system is the first system that processes the mount.
This system always accesses the file system locally; that is, the
system does not access the file system through a remote system. Other
non-owning systems in the sysplex access the file system either locally
or through the remote owning system, depending on the PFS and the
mount mode. The file system owner is the system to which file requests are forwarded when the file system is mounted non-sysplex aware. Having the appropriate owner is important for performance when the file system is mounted read/write and non-sysplex aware. The term z/OS UNIX file system owner refers to the owner of the zFS file system as z/OS UNIX recognizes it. This is typically the system where the file system is first mounted, but it can differ from the zFS file system owner (see zFS file system owner).
- zFS file system owner
- zFS has its own concept of file system ownership, called the zFS file system owner. This is also typically the system where the file system is first mounted in a sysplex-aware environment. File requests to sysplex-aware file systems are sent directly to the local zFS PFS, rather than being forwarded to the z/OS UNIX file system owner. This concept is shown in Figure 1. The local zFS PFS forwards the request to the zFS file system owner, if necessary. The z/OS UNIX file system owner can be different from the zFS file system owner. (In reality, zFS owns aggregates. Generally, we simplify this to say zFS file system owner because zFS compatibility mode aggregates only have a single file system.)
- z/OS UNIX file system owner
- The term z/OS UNIX file system owner refers to the owner of the zFS file system as z/OS UNIX knows it. This is typically the system where the file system is first mounted.
Figure 1. z/OS UNIX and zFS file system ownership
When a file system is not sysplex-aware (that is, mounted as NORWSHARE), file requests are function-shipped by z/OS UNIX to the z/OS UNIX file system owner, and then to the PFS. When a file system is sysplex-aware (that is, mounted as RWSHARE), file requests are sent directly to the local zFS PFS and then function-shipped by zFS to the zFS file system owner,
if necessary. - Function shipping
- Function shipping means that a request is forwarded to the owning system and the response is returned to the requestor through XCF communications.
- Local mount
- A local mount means that z/OS UNIX issues a successful mount to the local PFS, which in this case is zFS. z/OS UNIX does this when either the file system is mounted sysplex-aware for that mode (read/write or read-only) or the system is the z/OS UNIX owner. When a file system is locally mounted on the system, z/OS UNIX does not function ship requests to the z/OS UNIX owning system. To determine if a system has a local mount, see Determining the file system owner.
- Non-sysplex aware (sysplex-unaware)
- A file system is non-sysplex aware (or sysplex-unaware) if the PFS (Physical File System) supporting that file system requires it to be accessed through the remote owning system from all other systems in a sysplex (allowing only one connection for update at a time) for a particular mode (read-only or read/write). The system that connects to the file system is called the file system owner. Other system's access is provided through XCF communication with the file system owner. For a non-sysplex aware zFS file system, file requests for read/write mounted file systems are function-shipped to the owning system by z/OS UNIX. The owning system is the only system where the file system is locally mounted and the only system that does I/O to the file system. See zFS file system owner and z/OS UNIX file system owner.
- OMVS address space
- The
address space used by z/OS UNIX, it runs a program that initializes
the kernel. Starting in V2R2, zFS can be run in the OMVS address space.
- Read-only file system
- A file system that is mounted for read-only access is a read-only file system.
- read/write file system
- A file system that is mounted for read and write access is a read/write file system.
- Shared file system environment
- The shared file system environment refers to a sysplex that has a BPXPRMxx specification of SYSPLEX(YES).
- Sysplex
- The term sysplex as it applies to zFS, means a sysplex that supports the z/OS UNIX shared file system environment. That is, a sysplex that has a BPXPRMxx specification of SYSPLEX(YES).
- Sysplex-aware
- Pertains
to a physical file system that handles file requests for mounted file
systems locally instead of shipping function requests through z/OS UNIX.
- Sysplex-aware PFS
- A physical file system (PFS), for example zFS, is sysplex-aware or non-sysplex aware for a particular mount mode (read-only or read/write) in a shared file system environment. When it is sysplex-aware, the PFS is capable of handling a local mount on the system that is not the z/OS UNIX owning system. The PFS that is sysplex-aware can avoid z/OS UNIX function shipping for that mode. Both HFS and zFS file systems are always sysplex-aware for read-only mounts. HFS is always non-sysplex aware for read/write mounts and always results in z/OS UNIX function shipping from systems that are not the z/OS UNIX owning system. As of z/OS V1R13, zFS always runs sysplex-aware (SYSPLEX=FILESYS) in a shared file system environment. Individual file systems can be non-sysplex aware or sysplex-aware, with the default being non-sysplex aware.
- Sysplex-aware file system
- A file system can be mounted sysplex-aware
or non-sysplex aware. When a file system is mounted sysplex-aware,
it means that the file system is locally mounted on every system (when
the PFS is capable of handling a local mount on every system - that
is, the PFS is running sysplex-aware) and therefore, file requests
are handled by the local PFS. All read-only mounted file systems are
always mounted sysplex-aware (see Figure 1).
HFS read/write mounted file systems are always mounted non-sysplex
aware. This means that file requests from non z/OS UNIX owning
systems are always function-shipped by z/OS UNIX to the z/OS UNIX owning
system where the file system is locally mounted and the I/O is actually
done. Beginning with z/OS V1R11, zFS read/write mounted file systems
can be mounted sysplex-aware or or non-sysplex
aware.
- zFS address space
- Because zFS can run in its own
colony address space or inside the OMVS address space, which is the
address space used by z/OS UNIX, any reference to the zFS address
space will mean the address space in which zFS is running.
- zFS aggregate
- The data set that contains a
zFS file system is called a zFS aggregate. A zFS aggregate is a Virtual
Storage Access Method (VSAM) linear data set. After the zFS aggregate
is defined and formatted, a zFS file system is created in the aggregate.
In addition to the file system, a zFS aggregate contains a log file
and a bitmap describing the free space. A zFS aggregate has a single
read/write zFS file system and is sometimes called a compatibility
mode aggregate. Compatibility mode aggregates are similar to HFS. Restriction: zFS does not support the use of a striped VSAM linear data set as a zFS aggregate. If you attempt to mount a compatibility mode file system that had previously been formatted and is a striped VSAM linear data set, it will only mount as read-only. zFS does not support a zFS aggregate that has guaranteed space.
- zFS file system
- Refers to a hierarchical organization of files and directories that has a root directory and can be mounted into the z/OS UNIX hierarchy. zFS file systems are located on DASD.
- zFS Physical File System (PFS)
- Refers to the code that runs in the zFS address space. The zFS PFS can handle many users accessing many zFS file systems at the same time.
- ZFS PROC
- The PROC that is used to start ZFS. It
is typically called ZFS. If ZFS is running in the OMVS address space,
then this refers to the OMVS PROC.