VTOC-CI-CA--datasets

vtoc 用来在特定的卷上面查找data sets,每一个卷都具有的format：label+pointer（pointer to vtoc)
vtoc (vtoc_entry1,.................)  dataset1............
votc包含了所在卷上面datasets的属性信息，如大小，位置，
votc index 则是vtoc entry 的索引，通常按照字母进行排序，没有vtoc index 的entry包含了
一个dataset_name 以及vtoc entry_pointer

z/OS requires a particular format for disks, which is shown in . Record 1 on the first track of the first cylinder provides the label for the disk. It contains the 6-character volume serial (volser) number and a pointer to the volume table of contents (VTOC), which can be located anywhere on the disk.

Figure 1. Disk label, VTOC, and extents

The VTOC lists the data sets that reside on its volume, along with information about the location and size of each data set, and other data set attributes. A standard z/OS utility program, ICKDSF, is used to create the label and VTOC. When a disk volume is initialized with ICKDSF, the owner can specify the location and size of the VTOC. The size can be quite variable, ranging from a few tracks to perhaps 100 tracks, depending on the expected use of the volume. More data sets on the disk volume require more space in the VTOC.

The VTOC also has entries for all the free space on the volume. Allocating space for a data set causes system routines to examine the free space records, update them, and create a new VTOC entry. Data sets are always an integral number of tracks (or cylinders) and start at the beginning of a track (or cylinder).

You can also create a VTOC with an index. The VTOC index is actually a data set with the name SYS1.VTOCIX.volser, which has entries arranged alphabetically by data set name with pointers to the VTOC entries. It also has bitmaps of the free space on the volume. A VTOC index allows the user to find the data set much faster.

VSAM 的相关信息，之所有摘录下来，是因为有图，比较便于理解
What is a Control Interval in VSAM?

Control Interval in VSAM is a unit of data that is transferred when an I/O request is made between auxiliary storage and virtual storage. It contains records, free space and control information. The data records are grouped into control intervals. That is in other words in non-VSAM method the unit of data that is transferred between memory and the storage device is defined by the block which in contrast in VSAM is referred as Control Interval.
VSAM的CI是IO的基本单位，CI的功能同NON_VSAM文件的page或是block相类似，即IO每次传输都是一个CI，
每一个CI（Interval）包含了数据，空闲空间，以及CI（information）
CI的功能同NON_VSAM文件的page或是block相类似，
下面给出了CI内数据结构布局：

The records in the Control Interval are all placed in the beginning of the Control Interval with key of each record followed by the actual record. The end portion of the Control Interval is occupied by control interval descriptor this also has space for specifying the record length of each record in the Control Interval. The space between this beginning part which is occupied by records and the end part of control interval which has descriptor followed by length of record denotes the free space. This is shown diagrammatically below:

Key 1	Record 1
Key 2	Record 2
Key 3	Record 3
Key 4	Record 4
Key 5	Record 5

 

Free Space

 

Length of Record 1

Length of Record 2

Length of Record 3

Length of Record 4

Length of Record 5

 

Descriptor
(End part of control interval)

The Control information also called as CI in short consists of the following information:

• RDF called as Record Descriptor Field
• CIDF called as Control Interval Descriptor Field

The maximum size that is allowed for a Control Interval is 32K. Let us now see about the information stored on the above RDF and CIDF.

A CIDF is present for each Control Interval. The CIDF is a field which has the last 4 bytes of a Control Interval. This has information about the offset and the length of free space in the Control Interval. Having seen about the information in CIDF let us now see the information present in RDF. If the records are of fixed size then each Control Interval has two RDF's with each RDF of length 3 bytes. If the records are of variable size then a separate RDF is available for each record in the Control Interval. The Control Interval in VSAM is created as soon as VSAM dataset is loaded after which records are placed inside it. Having got an idea about Control Interval in VSAM from above explanation and idea about KSDS clusters, RRDS clusters and ESDS clusters from our previous section let us now see how the records are filled in the control interval by each of these different clusters.
CA的定义以及与ＣＩ的关系：
VSAM Control Interval and Control Area

VSAM Control Area - What is VSAM Control Area? A group of control intervals makes up a control area. Since the control intervals are grouped to form control area the way of filling the control area is similar to the way of filling records in control interval as discussed in detail in our previous paragraph. The number of control intervals per control area depends on how much space is reserved when the dataset is created. This is shown diagrammatically below:

Control interval A

Control interval B

Control interval C

Control interval D

Control interval E

Control interval F

Control interval G

Control interval H

Control interval I

Control interval J

Attributes in the index component

The index component has the following attributes as given below:

• index set
• sequence set

The index set lists the largest index within each control area and the sequence set lists the largest index within each control interval.

The records get allocated in Control Interval and Control area in the following manner:

? Check for enough free space within a control interval is made and if such a space is present then the new records gets added in that position.

? If the Check for enough free space within a control interval returns no enough free space within a control interval then a check for the presence of empty control interval within the same control area is made and if such a empty control interval within the same control area occurs the control interval split is made with the record from the control interval which is filled with records being moved to the empty control interval space.

? If in case of Check for enough free space within a control interval returns no enough free space within a control interval and also there is no empty control interval within the same control area then control area split is made and records gets moved to the new control area

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