RE: parallel recovery slaves waiting on undo reads
Date: Wed, 26 Feb 2020 23:49:38 +0000
Message-ID: <39394_1582760994_5E570421_39394_6021_1_e3f8bfab30d743f3b4376cd64610242f_at_vontobel.com>
>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 {_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 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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