4.7 Lock structure

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In the above section we stated that when a block held by an instance is requested by another instance, the original holder makes a copy of the block which becomes the past (previous) image of the block. To keep track of these PIs, Oracle uses locking of blocks. Implementing roles and modes enforce these locks. Refer to Tables 4.1 and 4.2 for an explanation of the roles and modes.

When a lock is acquired for the first time by an instance, only that specific instance has a copy of the block. Therefore, the block is acquired with a local role. If the block was acquired from a remote instance, the holding instance will make a copy of the original block before transmitting the block to the requesting instance. This is called the past image. This indicates that there are two possibilities; one where a PI is maintained and the second where a PI is not maintained. These possibilities for the PI are represented by a Boolean value of ''yes'' or ''no.'' These modes, roles, and instance of the PI all represent the lock structure.

Figure 4.6 represents the three-character lock structure where:

Figure 4.6: GCS lock structure.

• The first character ''M'' indicates the mode:

  N for null

  S for shared

  X for exclusive

• The second character ''R'' indicates the role:

  L for local

  G for global

• The third character ''n'' indicates whether the GRD knows about the PI:

  0for no

  1 for yes

Putting these together there are potentially nine types of lock situations.

Table 4.3 lists the various lock modes and their descriptions.

Table 4.3: Lock Modes

Chars	Mode	Description
NL0	Null local 0	Essentially the same as N with no past image
SL0	Shared local 0	Essentially the same as S with no past images
XL0	Exclusive local 0	Essentially the same as X with no past images
NG0	Null global 0	Global N lock and the instance owns a current block image
SG0	Shared global 0	Global S lock with no past images
XG0	Exclusive global 0	Global X lock with ownership to the block
NG1	Null global 1	Global N lock with the instance past image.
SG1	Shared global 1	Global S lock, with instance owning past image
XG1	Exclusive global 1	Global X lock, and instance owns a past image

Blocks held by a single instance only have a single copy and each is in control of its modification and writing of the changes to disk. However, when another instance requests for the same block, its status changes from a local to a global state. Only in this situation, where one or more other instances also contains a copy of the current block, does the original instance require to keep the original, or PI, of the block.

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