IBM 000-8697 Manual Do Utilizador

IBM Informix OnLine Database Server Administrator’s Guide

The regular buffers store dbspace pages read from disk. The status of the
regular buffers is tracked through the buffer header table. Within shared
memory, regular buffers are organized into

LRU

 buffer queues. (Refer to

for further information about the

LRU

queues.) Buffer allocation is

managed through the use of latches and lock access information. Buffer infor-
mation is available through four options of tbstat:

■

 and -B options display general buffer information.

■

 displays

LRU

 queue statistics.

■

 displays information about OnLine user processes that are

sharing or waiting for buffers.

Each regular buffer is the size of one OnLine page, specified as

BUFFSIZE

 in

the configuration file. In general, OnLine performs

I/O

in full-page units, the

size of a regular buffer. The two exceptions are

I/O

 performed from big

buffers and

I/O

 performed from blobspace buffers. (Refer to

further information about blobspace buffers.)

For every 100 regular buffers, OnLine allocates one additional big buffer.
OnLine creates at least one big buffer, even if

BUFFERS

is less than 100. A big

buffer is a single buffer that is the size of eight pages.

The function of the big buffers is to increase performance on large writes and
reads. For example, OnLine tries to use a big buffer if it is writing a dbspace
blob into shared memory or performing a series of sequential reads. After
disk pages are read into the big buffer, they are immediately allocated to
regular buffers in the buffer pools. The big buffers are also used in sorted
writes and during checkpoints, if possible. (Refer to

 for more

details about sorted and chunk writes.)

Multiple big buffers cannot be combined for contiguous

 I/O

 reads that are

greater than eight pages. The number of big-buffer reads is displayed as part
of the tbstat -P output.