IBM 000-8697 Manual Do Utilizador

IBM Informix OnLine Database Server Administrator’s Guide

The following example shows how the value of

LRU_MIN_DIRTY

 is applied

to the buffer pool to arrive at the number of page buffers in an

MLRU

queue

that, when reached, can signal a suspension of page cleaning:

BUFFERS specified as 8000
LRUS specified as 8
LRU_MIN_DIRTY specified as 50

Number of buffers in the MLRU queue when cleaning can be suspended
is equal to (Total buffers/Number of LRU queues) multiplied by the
percentage specified by LRU_MIN_DIRTY.

Buffers in MLRU = (8000/8) * 50%
Buffers in MLRU = 1000 * 0.50
Buffers in MLRU = 500

Refer to

for a description of data buffer flushing. Refer to

for more details about tuning the values of

LRU_MAX_DIRTY

 and

LRU_MIN_DIRTY

.

OnLine shared-lock buffering allows more than one OnLine user process to
simultaneously access the same buffer in shared memory. OnLine provides
this concurrency without a loss in process isolation by using buffer locks and
three categories of lock access (share, update, and exclusive).

The buffer-acquisition procedure comprises seven steps:

Identify the data requested by physical page number.

Determine the level of lock access needed by the user process for the
requested buffer.

Attempt to locate the page in shared memory.

If the page is not in shared memory, locate a buffer in an FLRU queue
and read the page in from disk.

Proceed with processing, locking the buffer if necessary.

After the user process is finished with the buffer, release the lock on
the buffer.

Wake waiting processes with compatible lock access types, if any
exist.