Question: What is a buffer pin, and
what does the buffer is pined count mean in an AWR
report? I have over 83,000 buffer is pinned count
per second and I don't know if I should be concerned.
Statistic
Total per Second
per Trans
-------------------------------- ------------------
-------------- -------------
buffer is not pinned count
63,687,417
17,589.5
249.8
buffer
is pinned count
300,751,787
83,063.1
1,179.8
Answer:
A buffer pin is like a latch (cache buffer chains), and the
buffer is pinned count defines a estimate of the efficiency
of rare re-visits to the same data buffer block. In plain English,
a session may "pin" a handle to a data block to re-visit the
data block at a later time without having to perform a second logical read.
Oracle does not always pin a buffer block, and these
types of non-latched reads are recorded as "consistent gets
- examination" and "buffer is not pinned count" in the AWR report. See my notes on
consistent gets.
Oracle has several hidden parameters that define the buffer
pin operations:
_buffer_handles_cached:
This defines the total number of concurrent pins allowed and
the default is 5.
_db_handles:
This
is he total number of "pins" that can be reserved by any
database session.
These are hidden and undocumented parameters and should only
be changed with the permission of MOSC.
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