Interested Transaction Lists (ITL) Waits

When I’ve generated AWR report, there I’ve noticed “Segments by ITL Waits” at this moment I‘ve not had any idea what was that. But after searching it on the internet I found very good information…Now, I want to share it with you.

First of all let’s discuss what ITLs is:

When a row is locked by a transaction, this information is placed in the block(header), where this row exists. The portion of the block header, which saves this information(transaction address and rowid) is called ITLs(Interested Transaction Lists).

ITLs contains several slots, initial number of slots are defined by the parameter INITRANS, which is indicated during the creation of the object (table, index) and can be increased till MAXTRANS.

Example:
CREATE TABLE test_tab
( COL1 NUMBER,
  COL2 VARCHAR2(50))
INITRANS 1
MAXTRANS 1

When one transaction tries to modify row, one slot will be filled by the info about this transaction(Assume that transaction is not finished, means commit or rollback is not performed). If another transaction will try to change the row within this block, this process will wait till the first transaction will not end.

Question is why?

Because for the second transaction there(in the block header) must be allocated another free slot, but free slot doesn’t exist and also slots number can’t increase because of  MAXTRANS is 1 which equals to the number of already allocated slot. At this time increasing MAXTRANS will solve the problem, to change MAXTRANS do as follows:

–Maximum 10 slots can be allocated(10 transaction can be placed)

Alter table test_tab MAXTRANS 10;

What is ITL wait?

When slot can’t be increased and free slots still not available… Following transaction will wait until any slot will not be available, as we have had previously… This wait is called ITL waits. We solved this waits by increasing MAXTRANS but there may happen another problem. That ,for example we have initially 1 slot(INITRANS is 1) and can be allocated maximum 10 slots (MAXTRANS is 10), info  about one transaction is placed in one slot, then  comes another transaction and its info can’t be placed in the next slot(causes waiting) but as we know we can have 9 other available slots, still can’t be placed, why?

Let me draw some pictures so that you can easily guess what happens.

–Newly created block with initially one slot(during creation table we indicated INITRANS 1) looks like this:

Empty Block with one ITL slot

–Three rows are inserted in the block.

Full block with one ITL slot

–One transaction modified one row and did not commit. The Info is written to the available slot.

Full block with one alocated ITL slot(locking,filling ITL slot)

–Another transaction comes and wants to modify row, consequently info about this transaction should be placed in another free slot(do not forget we have initially one slot and number of this slots can be increased till 10)

Full block with one alocated ITL slot and not enought space for another slot

Note:In this case there is not empty space for new slot, that is why it can’t be allocated… If we have had indicated initially more slots another transaction would be able to be placed, as you can guess to solve this problem you should indicate bigger INITRANS during creation of object, or alter object and change this parameter…For the blocks which are already full as I can guess it will not affect, but for future it will be usefull. However this also means that there is less space in the block for actual data.
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