Database Reference
In-Depth Information
FROM sys.x$bh
WHERE obj= &&obj
AND dbablk= &&block
AND state!=0 ;
Enter value for obj: 75742
Enter value for block: 180
STATE MODE_HELD LE DBARFIL DBABLK CR_SCN_BAS CR_SCN_WRP CLASS
---------- ---------- -- -------------- ---------- ---------- ---------- ----------
cr 0 00 1 75742 649314930 3015 1
cr 0 00 1 75742 648947873 3015 1
cr 0 00 1 75742 648926281 3015 1
cr 0 00 1 75742 648810300 3015 1
cr 0 00 1 75742 1177328436 3013 1
CR buffer generation is a special case, and no global cache locks are acquired to protect CR buffers. It is possible
to create a CR storm if there are long-pending transactions on highly accessed objects. Hence, it is prudent to
schedule batch processes updating heavily accessed tables to a less busy timeframe.
GC CR Grant 2-Way/Gc Current Grant 2-Way
gc cr grant 2-way
and
gc current grant 2-way
wait events are encountered if the requested block is not resident in any
buffer cache. The FG process requests LMS process for a block, but the block is not resident in any buffer cache. So,
LMS process replies with a grant message to the FG process to read the block from disk. The FG process reads the
block from disk and continues the processing.
The following line shows that for the block access with file_id=4 and lock_id=180, a grant response was received
by the FG process from LMS process. The subsequent line shows that a physical read was performed to read the block
from disk.
nam='gc cr grant 2-way' ela= 402 p1=4 p2=180 p3=1 obj#=75742
nam='db file sequential read' ela= 553 file#=4 block#=180 blocks=1 obj#=75742
Excessive waits for these events imply either that the buffer cache is undersized or that the SQL statements are
aggressively flushing the buffer cache. Identify SQL statements and objects suffering from waits for the events, and
tune those SQL statements.
The DRM feature is designed to reduce the occurrences of waits for the grants.
GC CR Block Busy/GC Current Block Busy
Busy events indicate that LMS performed additional work due to concurrency-related issues. For example, to build
a CR block, LMS process may have to apply undo records to reconstruct a block consistent with query SCN. While
transferring the block back to the FG process, LMS will mark the block transfer to have suffered from
gc cr block
busy
event or
gc current block busy
event depending upon the type of block transfer.
GC CR Block Congested/GC Current Block Congested
If the LMS process did not process a request within 1 ms after the receipt of a request, then LMS process marks the
response that block suffered from congestion-related wait events. There are many reasons for congestion-related wait
events, such as that the LMS process is overwhelmed by numerous global cache requests, LMS process is suffering
from CPU Scheduling delays, LMS process has suffered from another resource starvation (such as memory), etc.
