Not sure how relevant this is, because you are looking for the reason your
recovery worker processes do a lot of IO I assume.
Below is a description of the what happens when you see db file parallel read:
With newer oracle versions (12+), you’ll see plan lines indicating the word
‘BATCHED’. I believe it’s these points where oracle knows it has to read
multiple non-adjacent blocks that it is getting these all at once. However, I
read indications it might be happening outside of the ‘BATCHED’ lines, and is
implemented at any time it knows multiple non-adjacent blocks are needed, which
would have been read serially in the past.
This can be implemented on the OS level as asynchronous IO requests via the
regular asynchronous IO mechanism (io_submit-io_getevents), or uses a
synchronous version to submit a a batch of IO requests: preadv. The mechanism
of requesting multiple non-adjacent blocks has its own wait event: db file
parallel read, which is a reasonable accurate description of what it actually
does: it wants to read data from multiple places at the same time.
From a performance perspective, the problem with this wait event is that the
timing of the wait event has no absolute meaning: waiting for a single IO is
something different that waiting for let’s say 70 IO requests submitted at the
same time. p2 tells you the amount of oracle blocks, p3 the amount of requests.
Frits Hoogland
http://fritshoogland.wordpress.com ;<http://fritshoogland.wordpress.com/>
frits.hoogland@xxxxxxxxx <mailto:frits.hoogland@xxxxxxxxx>
Mobile: +31 6 14180860
On 27 Feb 2020, at 12:51, Noveljic Nenad <nenad.noveljic@xxxxxxxxxxxx> wrote:
The half of the reads are indeed on only three index partitions:
SQL> select obj#,dataobj#,value,100*ratio_to_report(value) over() perc from
v$segstat where statistic_name = 'physical reads' order by value desc fetch
first 5 rows only ;
OBJ# DATAOBJ# VALUE PERC
---------- ---------- ---------- ----------
4294967295 35894092 10644378 18.892159
4294967295 35894903 9141720 16.2251686
4294967295 35894891 8487867 15.0646786
4294967295 35894096 2833553 5.02912748
4294967295 35894095 2719772 4.82718344
Executed on the primary database:
SQL> select data_object_id, object_type from dba_objects where data_object_id
in ( 35894092, 35894903, 35894891, 35894096, 35894095 );
DATA_OBJECT_ID OBJECT_TYPE
-------------- -----------------------
35894092 INDEX PARTITION
35894095 INDEX PARTITION
35894096 INDEX PARTITION
35894891 INDEX PARTITION
35894903 INDEX PARTITION
Many thanks,
Nenad
https://nenadnoveljic.com/blog/ ;<https://nenadnoveljic.com/blog/>
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx <mailto:oracle-l-bounce@xxxxxxxxxxxxx>
<oracle-l-bounce@xxxxxxxxxxxxx <mailto:oracle-l-bounce@xxxxxxxxxxxxx>> On
Behalf Of Jonathan Lewis
Sent: Donnerstag, 27. Februar 2020 11:08
To: oracle-l@xxxxxxxxxxxxx <mailto:oracle-l@xxxxxxxxxxxxx>
Subject: Re: parallel recovery slaves waiting on undo reads
In general I would expect it to be indexes that are the target for "db file
parallel read", but possiby parallel recovery processes are allowed to behave
completely differently.
Regards
Jonathan Lewis
________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on behalf
of Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>
Sent: 27 February 2020 10:04
To: oracle-l@xxxxxxxxxxxxx
Subject: Re: parallel recovery slaves waiting on undo reads
Can you get access to v$segstat / v$segment_statistics to see what's
changing most ?
Regards
Jonathan Lewis
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