Oracle Undocumented Parameters
select ksppinm
from x$ksppi
where substr(ksppinm,1,1) = '_';
-----------------------------------------------------------------------
The following is a list of undocumented parameters.
1. _db_block_cache_protect
On VMS, the DB_BLOCK_CACHE_PROTECT mechanism has been made much
faster. During normal use, having it turned on shouldn't be
noticeable (the degradation is less than 1%). Developers who
link non-share will need PSWAPM privilege to use this feature.
When DB_BLOCK_CACHE_PROTECT is turned on, developers may either
use the VMS mailboxes with the M (MEMORY_LOG) command
or they may just examine the ring buffer in the PGA (index
SMPREI_, array SMPREB_) to determine what buffer requests have
been made recently.
DB_BLOCK_CACHE_PROTECT will prevent certain corruption from getting to
disk; although, it may crash the foreground of the instance. It will help
catch stray writes in the cache. When you try to write past the buffer
size in the sga, it will fail first with a stack violation.
It seems that the db_block_cache_protect has a significant performance
overhead. Preliminary testing shows that it has considerable overhead
(a single update took twice as long with the parameter set to TRUE).
2. _db_block_compute_checksums
There is another new init.ora parameter, DB_BLOCK_COMPUTE_CHECKSUMS, that
controls whether a checksum is put into every block before the block is
written to disk. The default is FALSE. A block read validates an
exiting checksum whether or not this option is enabled. A block is marked
as corrupt if a checksum fails.
It helps determine corruption due to hardware problems. The incarnation
number and the sequence number are added to the end of the block to help
catch corruption.
If the problem (corruption) is in the middle of the block
this test will not detect it. To detect this problem a checksum may be
generated in the block header before every write and verified on every
read.
3. _db_block_hash_buckets= "Number of database block hash buckets"
The number of hash buckets is
a) by default to be set to a prime number;
b) coerced to a prime if there is an init.ora parameter setting.
The value, if not a prime number > 2, is rounded up to the next highest
prime.
I would tend not to change it unless there is latch contention on the hash
chains. raising it to equal the number of buffers would clearly remove any
contention (basically, this is just saying that each buffer lives on its
own hash chain). Having it set too small would mean that we might have to
scan over lots of buffers to find the one we want. I think the default is to
make it 1/4 of the total number of buffers
4. _db_block_multiple_hashchain_latches
"Use one latch per hash chain"
5. _db_handles
"System-wide simultaneous buffer operations"
6. _db_handles_cached
"Buffer handles cached each process"
7. _wait_for_sync
" Wait for sync on commit "
Wait_for_sync is an oracle generic parameter which, when set to
false, will allow the system to complete commits without waiting
for the redo-log buffer flushes to complete.
8. _db_block_max_scan_cnt="Maximum number of buffers to inspect when
looking for free buffer"
DB_BLOCK_MAX_SCAN_CNT is an init.ora parameter which specifies
the number of unavailable buffers a process should scan before signaling
DBWR to write dirty buffers from the buffer cache to disk.
9. _db_writer_scan_depth
"Number of LRU buffers for dbwr to scan when looking for dirty buffers"
10a. _db_writer_scan_depth_increment
"Add to dbwr scan depth when dbwr is behind"
10b. _db_writer_scan_depth_decrement
Subtract from dbwr scan depth when dbwr is working too hard
11. _db_large_dirty_queue
"Number of buffers which force dirty queue to be written
12. _db_block_write_batch
Number of blocks to group in each DB Writer IO
specifies the no of blocks to be written to the disk in one write operation.
Should be increased till write wait time and write complete waits starts
to increase.
DBWR Free Low is the number of times DBWR is invoked because a user
process found at least DB_BLOCK_WRITE_BATCH/2 buffers on the dirty list.
This parameter specifies the number of blocks which should be written to
disk at one time.
This parameter should only be increased until the statistics
Write Complete Waits and Write Wait Time show growth. Write Complete
Waits is the number of times a process waited for DBWR
to write a current block before making a change to a buffer.
13. _db_block_cache_clone
"Always clone data blocks on get (for debugging)"
This parameter setting has a significantly adverse affect on performance
and we were told to run without it.
14. _controlfile_enqueue_timeout
/* use parameter value (default is 900) */
/* System Parameter: control file enqueue timeout in seconds */
15. _db_no_mount_lock
add hidden parameter to not acquire mount lock
If hidden int.ora parameter _db_no_mount_lock is set to TRUE
then no mount locks are acquired when the the database is mounted
exlusive. This allows two databases with the same name to be
simultaneously mounted. Note that this circumvents the mechanism
to prevent two simultaneous startups of the same database, and is
thus a dangerous parameter to set. This only affects ports that
ar compiled with the SYS_DFS option enabled (i.e. VMS only).
It does not get a mount lock on the database name during startup.
This allows 2 instances with the same name to run on one machine
16. _log_checkpoint_recovery_check
Check redo log after checkpoints.
Add debugging code to check the red log after a checkpoint. This
code is intended to help find a problm that is generating ora-600 [3020]
during recovery. This code is enabed with a new init.ora parameter:
_log_checkpoint_recovery_check=XXX, where XXX is the number of redo
blocks to check. This is called in LGWR after every checkpoint. If the
init.ora parameter "_log_checkpoint_recovery_check" is zero (default)
it does nothing. If it is a positive value then that many blocks of
redo are scanned to see that the data file blocks on disk could be
recovered if there was an immediate crash. This code was introduced
to catch an elusive bug that results in OERI(3020) errors
occasionally during crash recovery.
17. _switch_on_stuck_recovery
REDO GENERATION HAS BEEN CHANGED SO THAT WE SHOULD NEVER ENCOUNTER
A CHANGE IN THE FUTURE OF THE BLOCK. APPLYING REDO IN SCN ORDER NOW
INSURES THAT CHANGES ARE APPLIED IN INCARNATION/SEQUENCE # ORDER. THERE
IS A LOT OF CODE IN RECOVERY THAT IS NO LONGER NEEDED BECAUSE OF THIS
CHANGE. THIS INCLUDES THE CODE TO BACKUP AND RESCAN AFTER REPAIRING
CORRUPTIONS. THE CODE IS BEING LEFT IN TO AVOID LARGE CHANGES JUST BEFORE
V7.0 PRODUCTION RELEASE. THE FOLLOWING WILL INSURE THE UNNEEDED CODE
IS NEVER EXECUTED UNLESS REDO WAS GENERATED BY A BETA RELEASE OR THE SECRET
INIT.ORA PARAMETER _SWITCH_ON_STUCK_RECOVERY IS SET TO TRUE.
Check redo Log contents after checkpoint. This is called in LGWR after
every checkpoint. If this parameter is zero (default) it does nothing.
If it is a positive value then that many blocks of redo are scanned to see
that the data file blocks on disk could be recovered if there was an
immediate crash. This code was introduced to catch an elusive bug that
results in OERI(3020) errors occasionally during crash recovery.
Checkpoint recovery check: this is the number of redo blocks that
kcracl will verify after every LGWR generated checkpoint. Defaults to zero
for no checking. When opening the named offline log for redo application
and to recalculate future change thread switching this parameter is used.
18. _log_io_size=redo log IO chunk size (blocks/write)
/* System Parameter: IO chunk size */
1. that the value is o/s dependent.
2. if left at 0, the value will be automatically determined for each log
file.
19. _log_buffers_debug
/* debugging: fill redo buffers with [well known] junk after writes */
"debug redo buffers (slows things down)"
20. _log_debug_multi_instance
/* debugging : pretend multi-instance */
"debug redo multi instance code"
21. _log_entry_prebuild_threshold
/* redo entry pre-build threshold */
/* this is a bad idea for a uniprocessor , and is only helpful for a
multiprocessor when there is latch contention */
LOG_ENTRY_PREBUILD_THRESHOLD determines the maximum size of a redo entry
to prebuild before the copy to the log buffer. Increasing this parameter
reduces the time that the redo copy latch is held. This parameter should
not be modified if it is a single processor environment or there will be
memory contention.
22. _disable_logging
If this is true, redo records will not be generated -- no recovery is possible
if the instance crashes. It is mainly used for getting good benchmarking
results.
Default is false
23. _log_blocks_during_backup
TRUE value implies before any change is made to a db_block in the buffer
cache, a *complete image* of the block is copied to the redo
redo log. (This explains why excessive redo would be generated for
datafiles excessive redo would be generated for datafiles in hot backup
mode.) There is a new init.ora parameter, LOG_BLOCKS_DURING_BACKUP,
that controls whether block images ar written to the redo log during
hot backup.
Default is TRUE for VM, but port specific with the default defined in
sparams.h. This may beset to FALSE if the Oracle block size equals
the O/S physical sector sie or if it is otherwise ensured that hot backup
reads consistent versios of blocks even if those blocks are being written
at the time. Put anther way, this may be set to FALSE on machines that
can guarantee the aomicity of a single block I/O request.
Default is true
Problem is due to split blocks.
24. _allow_resetlogs_corruption
Added new secret init.ora parameter to override error 1138.
When set to TRUE the
resetlogs option will be allowed even if there are hot backups that need
more redo applied. Unless you are certain that absolutely all redo, includig
the online logs, has been applied, then a full export and import mst be
done to insure the database is internally consistant.
from 6.0 code
/* if we crashed/shutdown during a hot backup it is over now and we
** are no longer logging blocks. If they did a manual recovery
** followed by a NORESETLOGS then the hot backup recovery flag can
** be cleared. The hot backup recovery flag may alsoneed clearing
** if RESETLOGS was allowed because of _allow_resetlogs_corruption
** parameter. */
from the c file kcv.c 7.0 code
/* if the resetlogs option is in effect we save the highest checkpoint
** scn that we see, to be used as the incomplete recovery scn. It only
** gets used if a resetlogs is done without any recovery. Only backup
** control file recovery will be allowed and it always rewrites the
** incomplete recovery scn. We use the highest scn we can find so that
** new resetlogs scn will be greater than anything in the database.
** This may not be strictly true if the user sets
** _allow_resetlogs_corruption, and there is a fuzzy file. Note that
** since resetlogs does not look at the log file headers, datafile
** checkpoints are all we have to look at.
25. _reuse_index_loop
"number of blocks being examine for index block reuse"
/* secret system parameter to control how agressive we should walk the free
** list when attempting to reuse block - default is 5.
** Set to 0 for fast index operation which is susceptible to growth,
** Set to > 5 for slower index op but more agressive in reusing blocks */
Controls the amount of work done when looking for a block to reusse
for n index entry. The value determines the number of blocks to
check on the freelist when looking for a reusable block.
26. _mts_load_constants
/* hidden init.ora to set server load balancing constants */
/* fill in load balancing parameters (from _mts_load_constants) */
* PID Controller - calculate control on number of servers using:
* control = Kp * err + Kd * delta(err) + Ki * sum(err)
* where Kp = proportional, Kd = derivative constant, Ki = integral constant
* Kp,Kd,Ki can be changed with the hidden parameter _mts_load_constants
* in order to tune the system as desired.
This values should only be changed after gathering enough information to
determine that the mts is not optimal.
27. _mts_fastpath
/* hidden init.ora to enable dispatcher fastpath */
default is false
* Return TRUE if the system should implement the dispatcher network
* fastpath. When enabled, the dispatcher fastpath causes network i/o
* to be buffered and only flushed when absolutely necessary. When not
* enabled, flushes will occur for every dirty virtual circuit buffer.
*** The following parameters are from the Kernel SQL Library manager
28. _kgl_multi_instance_lock
Only for debugging. all the _kgl_multi_instance_xxxx
"whether KGL to support multi-instance locks"
Default is 0
29. _kgl_multi_instance_pin
"whether KGL to support multi-instance pins"
Default is 0.
30. _kgl_multi_instance_invalidation
"whether KGL to support multi-instance invalidations"
Default is 0.
31. _row_cache_instance_locks
Kernel SQL Row cache management component, number of row cache instance
locks
default is 100
32. _row_cache_buffer_size
"size of row cache circular buffer"
default is 200
33. _messages
" message queue resources - dependent on # processes "
The asynchronous message mechanism allows processes to send
messages to each other. A process may send a message to a
specified other process (always without waiting for a reply),
may wait for a message to arrive on its queue, and may obtain
the next message. Messages sent to detached processes are
reliably delivered. Messages sent to foreground processes are
reliably delivered as long as the process is active. The
mechanism also permits sending of a simple "reply", which is a
one-byte message without queuing. It should normally be used
to reply to asynchronous messages, and this is a safer
technique than regular messages for responding to foreground
processes. This mechanism is not used in single process mode.
34. _cpu_count
ksb - Kernel Service Background processes
"number of cpu's for this instance"
CPU_COUNT has to be set on some platforms in order for Oracle to take
advantage of multi-processor system, on others it does not have effect on
performance since load balancing between processors is handled by the o/s.
35. _debug_sga
/* Debug SGA, don't make the SGA a global section so we can set watchpoints
36. _enqueue_locks
ksq1 - Kernal Service enQueues (1)
Definitions for enqueues client objects, "locks for managed enqueues"
37. _enqueue_hash
"enqueue hash table length"
38. _enqueue_debug_multi_instance
"debug enqueue multi instance"
KST is a trace facility used for "realtime" tracing of events. Below
are defined KST macros that will enable the tracing of such things as
latch operations, memory assignments, etc. Tracing is done to a per
process circular buffer stored in the SGA. Access to these buffers
is via fixed tables. Tracing is enabled for classes of events,
particular events, and ranges of events.
The tracing state may be dynamically changed with the following syntax
"ALTER TRACING"
- "ON"
- Processes begin logging events using the current enabled events
- "OFF"
- Processes stop tracing
- "ENABLE"
- Add in the events indicated in to those which are
being traced.
- "DISABLE"
- No longer trace events specified by
39._trace_buffers_per_process
Note that by default, tracing is NOT enabled. In order to enable tracing
at instance startup time, add _trace_buffers_per_process = 1
40. _trace_block_size
_trace_xxxxx (xxxxx = buffers_per_process, block_size, archive_start,
flushing, enabled, get_time_every, archive_dest etc.)
These parameters are only there for debugging purposes. Customers
or support will never have to use them.
41. _trace_archive_start
"start trace process on SGA initialization"
42. _trace_flushing
"TRWR should try to keep tracing buffers clean"
43. _trace_enabled
By default, tracing is NOT enabled. In order to enable tracing,
_trace_enabled = true
44. _trace_events
If you wish to enable tracing of waits at instance startup time, you can
either
add the line '_trace_events = "WAT,ALL"' to init.ora or execute
'alter tracing enable "WAT,ALL"' in a sqldba session.
If you wish to limit tracing to specific events, you can use the
the following syntax:
alter tracing enable "WAT,,..."
where "id" is either a specific event number, or an event range
(event number 1 - event number 2).
45. _trace_archive_dest
"trace archival destination"
46. _trace_file_size
"trace file size"
default is 10000 blocks
47. _trace_write_batch_size
"trace write batch size"
default is 32
48. _rollback_segment_initial
"starting undo segment number"
Default is 1. DO NOT SPECIFY 0 AS THAT HAPPENS TO BE THE SYSTEM ROLLBACK
49. _rollback_segment_count
"number of undo segments"
default is 0
50. _offline_rollback_segments
If a rollback segment is not accessible because the file it is in
is offline or corrupted, one can force the system to come up without
the rollback segment by specifying the rollback segment in init.ora
paramater '_offline_rollback_segments'. The system will come up by
estimating the current system commit time since it cannot access
transaction tble in the rollback segment. The system commit number
is a conservative guess based on current time, the database creation
time and the assumed transaction rate of 8000 tps. ONE MUST MAKE
SURE THAT THE SYSTEM TIME IS SET CORRECTLY WHEN FORCING THE SYSTEM UP
USING '_OFFLINE_ROLLBACK_SEGENTS'. A trace is written with information
about the estimated system commit number.
51. _corrupted_rollback_segments
Mark a rollback segment as corrupted.
52. _label_tag_cache_size
/* hidden size of the SGA label tag comparison cache (bitcache) */
"number of tags in the label tag comparison cache"
default is 200
53. _trace_get_time_every
"Number of trace sequence numbers per call to slgcs()"
default is 200
54. _vms_bg_priority
"VMS process priority to give to background processes"
default is 4
55. _sort_use_os_files_as_temporaries
Use O/S files rather than temp segments for sorting.
56. _log_checkpoints_to_alert
Whether checkpoint messages should be written to alert.log or not. Turned
off in benchmarks.
57. _large_memory_system :
Used in internal benchmarks. Doesn't concern us.
"Configure system to use memory and save cpu cycles, paging, etc
default is false
58. _wakeup_timeout
This is WMONs sleeptime between checks of it's queue of processes to wake.
59. _latch_wait_posting
enable posting of processes that may be waiting for a latch after a process
frees the same latch (set this parameter to a value greater than one
this parameter to a value greater than one for it to take effect).
60. _sql_connect_capability_code
allows database links from a pre-7.0.15 release to work with release 7.1.
It is necessary to set this parameter for database links from a V6 ?