IBM 000-8697 Manual Do Utilizador

IBM Informix OnLine Database Server Administrator’s Guide

If the requested page must be read from disk, the user process first locates a
usable buffer in the

FLRU

 queues. (Refer to

.) OnLine selects an

FLRU

 queue at random and tries to acquire the latch associated with the

queue. If the latch can be acquired, the buffer at the “least-recently used” end
of the queue is used. If another process holds the

FLRU

queue latch, the user

process tries to acquire a latch associated with another

FLRU

 queue.

After a usable buffer is found, the buffer is temporarily removed from the
linked list that is the

FLRU

 queue. The user process acquires a latch on the

buffer table hash structure and creates an entry in the buffer table as the page
is read from disk into the buffer.

If the user process reads the buffer without modifying the data, it releases the
buffer as unmodified. If the user process had acquired the buffer with an
update or exclusive lock, other user processes may be waiting to read the
buffer.

The release of the buffer occurs in steps. First, the releasing user process
acquires a latch on the buffer table that enables it to modify the buffer entry.

Next, it looks to see if other user processes are sleeping or waiting for this
buffer. If so, the releasing user process wakes the first process in the wait-list
queue that has a compatible lock-access type. The waiting processes are
queued according to priorities that encompass more than just “first-come,
first served” hierarchies. (Otherwise, user processes waiting for exclusive
access could wait forever.)

If no user process in the wait-list queue has a compatible lock-access type,
any user process waiting for that buffer can receive access.

If no process is waiting for the buffer, the releasing process tries to release the
buffer to the

FLRU

queue where it was found. If the latch for that

FLRU

queue

is unavailable, the process tries to acquire a latch for a randomly selected
FLRU queue. When the

FLRU

queue latch is acquired, the unmodified buffer

is linked to the “most-recently used” end of the queue.

After the buffer is returned to the

FLRU

queue or the next user process in the

wait list is awakened, the releasing process removes itself from the user list
bit map for the buffer and decrements the shared-user count by one.