Pretty much that is what I need :-)
At the moment of checking on that db there were 12 undo segments and
assuming the db was kept with them all the numbers are quite consistent.
I've checked the maxconcurrency in v$undostat for a week before the
incident - with the peak from the incident it was 4.4 on average and 2 as
median, so exactly as You said kept very low.
The transaction peak was obtained within less then 20 minutes - jump from
around 100 tx to 100000 tx, so again as You said very high slope. And 12x34
is 408, which is quite close to 331 in maxconcurrency.
But I thought oracle db would create more undo segments, when needed.
While here it seems it keeps with the number, which as I understand, is
established during the db opening or the txn pressure is so high, the db
is not able to init a new undo segment at certain point in time fast enough
to be ready for increased number of txn and at this point some number of
sessions would report ORA-01554 - which one is true?
And many thanks :-)
Remigiusz
2018-06-29 11:34 GMT+02:00 Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>:
Taking a quick guess:
An (automatic) undo segment has 34 slots in the transaction table (the
thing in the undo segment header).
If you and up with a small number N of undo segments then you can't have
more than 34 * N concurrent transactions active because each one needs a
transaction table slot and the next attempt to start a transaction would
see ORA-01554
It seems a little unlikely that this would happan in typical system since
Oracle would usually end up stealing an extent from an existing undo
segment to use it as the first extent of a new undo segment. But it
wouldn't be hard to set up a mechanism that started with a small number of
undo segments in a relatively small undo tablespace - started some
transactions (then leave them idle forever after the first change - e.g.
they might have come in from a remote database) that made it impossible to
free any extents, then did a lot of work to allocate all the extents (but
still left a little space in allocated extents for continuting
transactions), then started lots of transactions until you hit the error.
Regards
Jonathan Lewis
________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on
behalf of Remigiusz Sokołowski <remigiusz.sokolowski@xxxxxxxxx>
Sent: 29 June 2018 10:23:02
To: oracle-l@xxxxxxxxxxxxx
Subject: ORA-01554: transaction concurrency limit reached reason:no undo
segment found with available slot params:0, 0 :
Hi,
lately I saw the error ORA-01554 and wonder in which circumstances it is
possible to hit this error.
Theoretically this is quite clear - the db hit the roof on concurrent
transactions, but:
- the database (12.1.0.2) is auto undo management
- transactions are set to 1647, transactions_per_rollback_segment to 5,
but I believe those are simply default settings and according to docs those
settings are ignored for automatic undo management
- in docs it is stated the undo on AUTO is limited mostly by the undo
tablespace size (but here this size was not depleted - overall there are
~524000 blocks with ~64000 used between midnight and 15 when the error was
hit according to v$undostat and maximum query lasting below 1h)
- also I would rather expect ORA-01652 on undo tablespace
From what I've found it seems the db creates undo segments automatically
and if no more space then assigns transactions to existing ones - but this
is very imprecise.
Anyone able to elaborate on that?
Regards
Remigiusz
--
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Remigiusz Sokołowski <remigiusz.sokolowski@xxxxxxxxx<mailto:remigiusz.
sokolowski@xxxxxxxxx>>
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