Re: parallel recovery slaves waiting on undo reads
- From: Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>
- To: "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 27 Feb 2020 10:04:27 +0000
Can you get access to v$segstat / v$segment_statistics to see what's changing
most ?
Regards
Jonathan Lewis
________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on behalf
of Noveljic Nenad <nenad.noveljic@xxxxxxxxxxxx>
Sent: 27 February 2020 08:37
To: Mark W. Farnham; rich242j@xxxxxxxxx; oracle-l@xxxxxxxxxxxxx; 'Andy Sayer'
Subject: RE: parallel recovery slaves waiting on undo reads
These aren’t reads on undo. I reached this wrong conclusion by misinterpreting
P1 for ‘db file parallel read’, as Andy pointed out. The reads are spread
across many data files instead.
I’m seeing many more reads than before, which indicates, I think, that the
transactions are changing more blocks than before. I’m going to compare the
number of changed blocks in different time intervals, and then use logminer to
figure out what exactly changed.
Thanks,
Nenad
https://nenadnoveljic.com/blog/
From: Mark W. Farnham <mwf@xxxxxxxx>
Sent: Donnerstag, 27. Februar 2020 01:06
To: Noveljic Nenad <nenad.noveljic@xxxxxxxxxxxx>; rich242j@xxxxxxxxx;
oracle-l@xxxxxxxxxxxxx; 'Andy Sayer' <andysayer@xxxxxxxxx>
Subject: RE: parallel recovery slaves waiting on undo reads
IF recovery meets up with a large uncommitted transaction, yousa gonna get a
pantload (official metric) of reads on undo.
Allowing the database to open after the forward recovery is complete but before
all the transaction in flight rollback is complete is one of the first things
Oracle’s VLDB group got Oracle to change in the implementation, so you can use
the rest of the database while some batch monolith unwisely written all or
nothing completes.
mwf
From: oracle-l-bounce@xxxxxxxxxxxxx<mailto:oracle-l-bounce@xxxxxxxxxxxxx>
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Noveljic Nenad
Sent: Wednesday, February 26, 2020 6:50 PM
To: rich242j@xxxxxxxxx<mailto:rich242j@xxxxxxxxx>;
oracle-l@xxxxxxxxxxxxx<mailto:oracle-l@xxxxxxxxxxxxx>; Andy Sayer
Subject: RE: parallel recovery slaves waiting on undo reads
is this Active Data Guard?
No.
It’s probably worth seeing if you can catch individual events in
v$session_wait_history (shows the last 10 events per session). Importantly,
see if the longer durations match up to more requests in the same
Yes, that’s exactly the case! Well guessed, Andy! Thanks for mentioning
v$session_wait_history – I’ve never used it until now. Oracle database always
pleasantly surprises with the sophistication of its instrumentation.
SQL> select event,p2,p2text from v$session_Wait_history where sid=545 ;
EVENT P2
P2TEXT
---------------------------------------------------------------- ----------
----------------------------------------------------------------
db file parallel read 30
blocks
db file parallel read 17
blocks
db file parallel read 25
blocks
db file parallel read 30
blocks
db file parallel read 31
blocks
db file parallel read 27
blocks
db file parallel read 25
blocks
db file parallel read 21
blocks
db file parallel read 29
blocks
db file parallel read 28
blocks
sudo -u root /usr/sbin/dtrace -q -n 'pid$target::pread:entry {@times["bytes"] =
quantize(arg2);}' -p 10020
bytes
value ------------- Distribution ------------- count
4096 | 0
8192 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 11215
16384 | 0
As we can see above, the events are for the reads of 20-30 blocks. Each read is
8k. Arithmetically, the average wait time of 18ms is feasible for 20-30 8k
reads, each of which taking a little bit less than 1ms in average.
P1 does not match up to the file#, it is the number of files.
I completely misunderstood the meaning of P1. Thanks for pointing this out.
I don’t think you can get the individual reads requests from a parallel read
from the standard wait interface. There may be an additional event you can
set or you might need to use OS tracing to see what files are being accessed.
I can easily find this out with DTrace if it will lead somewhere.
As a first aid, I could add more parallel slaves and/or increase the buffer
cache.
A general question: why is recovery generating so many reads?
Best regards,
Nenad
https://nenadnoveljic.com/blog/
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