Re: parallel recovery slaves waiting on undo reads
Date: Thu, 27 Feb 2020 10:07:47 +0000
Message-ID: <LNXP265MB15620F0067EDE21067BBE4A9A5EB0_at_LNXP265MB1562.GBRP265.PROD.OUTLOOK.COM>
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_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk> Sent: 27 February 2020 10:04
To: oracle-l_at_freelists.org
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
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on behalf of Noveljic Nenad <nenad.noveljic_at_vontobel.com> Sent: 27 February 2020 08:37
To: Mark W. Farnham; rich242j_at_gmail.com; oracle-l_at_freelists.org; '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_at_rsiz.com>
Sent: Donnerstag, 27. Februar 2020 01:06
To: Noveljic Nenad <nenad.noveljic_at_vontobel.com>; rich242j_at_gmail.com; oracle-l_at_freelists.org; 'Andy Sayer' <andysayer_at_gmail.com>
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_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org> [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Noveljic Nenad Sent: Wednesday, February 26, 2020 6:50 PM To: rich242j_at_gmail.com<mailto:rich242j_at_gmail.com>; oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org>; Andy Sayer Subject: RE: parallel recovery slaves waiting on undo reads
>is this Active Data Guard?
No.
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 {_at_times["bytes"] = quantize(arg2);}' -p 10020
bytes
value ------------- Distribution ------------- count
4096 | 0
8192 |_at_@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 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 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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