Thank you very much Gopal and Mark. Your explanation really helped me to
understand it better. Thank you.
I will try to dig more into the buffer busy wait part and try to see
pctfree and minimize_records_per_block options. Btw From the current_obj#
in v$actie_session_history i see the top contributor is pointing to a non
partitioned table(In which single row INSERT is happening from multiple
session) followed by an index partition(index is on a sequence generated
column) and here UPDATE happening on that indexed column.
Regards
Pap
On Wed, May 12, 2021 at 9:01 PM K Gopalakrishnan <kaygopal@xxxxxxxxx> wrote:
Pap-
ASM manages the storage at the host level and ASSM manages the storage at
Segment level.
Within the segments, you have extents and ASSM manages extents
(allocation, size etc)
ITLs manage block level concurrency issues.
On the other hand, Freelists maintain the list of blocks which are
eligible for INSERTs in non ASSM managed extents.
With ASSM, there are no freelists as the insert eligible blocks are
managed by bitmap blocks in segment headers.
There are multiple levels of bitmap blocks (Level 1 - Leaf BMB/ Level 2 -
Branch BMB and Level 3 (root BMB)) in a tree foramt manages the space
inside the segments.
Most of these things are automated and seldom require any changes. You can
still set PCTFREE here as that is honored.
PCTUSED used to decide when to link/unlink the blocks from the freelist
chain. This is replaced by different groupings in ASSM.
BTW you mention you are encountering Buffer Busy Waits and this could be
totally a different issue altogether.
In this case, you have to reduce the block level concurrency - (rather
than improving it with ITL fixes).
You can either increase the PCTFREE to higher values or use Jonathan
Lewis's trick (minimize_records_per_block).
-Gopal
On Tue, May 11, 2021 at 10:41 PM Pap <oracle.developer35@xxxxxxxxx> wrote:
Thank you so much for this detailed explanation.
If I get it correctly, it's the ASM tablespace(which we do have) manages
the dynamic increase and decrease of ITL or initrans based on concurrency
so we don't' need to set them explicitly. Also it should be looked into
only if by we see the ITL related waits. Correct if my understanding is
correct. As you mentioned in case of high concurrency we may need to
increase pctfree to higher value, will that not automatically managed by
ASM and also in such cases do we see any other waits or same ITL waits will
be seen in AWR?
Is freelists also managed dynamically as above and no need manually to be
set to higher on a ASM? (Note-Actually we are encountering buffer busy
waits during high activity period and the object it's pointing to is table
followed by index.)
Regards
Pap
On Wed, 12 May 2021, 8:20 am K Gopalakrishnan, <kaygopal@xxxxxxxxx>
wrote:
Pap-
The parameter INITRANS - defines number of ITL (Interested Transaction
Lists) slots _initially_ created during new blocks allocation to the
segment.
Any transaction that is interested in conducting DMLs on that block,
should fill in some details about that transaction.
This includes setting the Transaction ID, Undo Block Address, Lock Byte
, Cleanout System Change Number and Free Space Credit.
The row level locking in Oracle is implemented with the help of these
ITL information as well.
In other words, treat ITL like your parking space in the office space.
Anyone entering the office must park their car in the parking space
allotted.
You can park your car in the allotted space or any free space in that
building. Alloted parking spaces are fixed ITLs during the creation of the
block.
If allocated parking space is full, you park in the free space in the
building. Once you use this free space, this space will never return data
storage, only for ITLs.
This defaults to 2 per table and 3 per index IIRC, but DBA_TABLES still
show the default as 1 per table and 2 per index.
You can quickly validate this by simple block dumps.
Note that the ITLs can be created when there is enough space (~24 bytes
per ITL) in the variable header space in the data block.
So when there is a demand for additional transaction slots, oracle
automatically creates ITL slots subject to free space in the block.
If there is no space in the block , you might see contention for ITLs
and you will see ITL waits in the session_wait views.
There are some internal validations in oracle to limit the variable part
of the block header that can not exceed 50% of the block size.
i.e you can not create 50 ITLs if the block size is 2K. (50 ITLs require
50x24=1200+ bytes which is clearly over 50% of the block size).
There are many other checks similar to these as well to stop the abuse
:)
Also the requirement of such high ITL depends on the number of active
rows per block.
If the block itself has less than 50 rows, you would neer need 50 ITLs
for that block.
Similar to parking slot analogy, you would not need more parking spots
than the number of residents.
Readers (aka visitors) do not need any ITL entries to record their
transactions.
You do not need to manually set the ITLs, as ITLs will be created on the
need basis.
If there is an issue with the ITLs , you will clearly see this in the
session_waits.
Also segment_stats has specific metrics to show the objects with ITL
contention.
If you are not seeing any of this in the DB, this recommendation from
oracle is totally irrelevant.
So 50 ITLs (per table & 100 ITLs on their indexes) seems such a waste of
block space.
There are much better mechanisms to handle this. If you suspect such a
high rate of concurrency,
please set the PCTFREE to a higher number (say 15%) so that additional
ITLs can be created when required.
The space reserved with PCTFREE can not be used for INSERTS. They are
used for ITL expansion and updates.
-Gopal
On Tue, May 11, 2021 at 11:47 AM Pap <oracle.developer35@xxxxxxxxx>
wrote:
This database was recently moved from HP to exadata X7 and the DB
version also migrated from 11.2.0.4 to 19.9.0.0.0. It's an OLTP database.
In the past we saw some contention and got a recommendation from Oracle to
set INITRANS value of certain objects(both table and indexes) to as high
as 50. And now the team is thinking to set the initrans value to ~10 for
almost all the objects(~around ~200+ objects). Want to understand , what is
the negative side of setting it to this non default value considering this
is an OLTP database exposed to many concurrent DML/query and we
encounter "buffer busy waits" many times?
Regards
Pap
