RE: parallel recovery slaves waiting on undo reads
- From: Noveljic Nenad <nenad.noveljic@xxxxxxxxxxxx>
- To: Frits Hoogland <frits.hoogland@xxxxxxxxx>
- Date: Fri, 28 Feb 2020 11:00:33 +0000
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.
Assuming that ‘db file parallel read’ for multiple blocks starts measuring just
before submitting IO operations and stops after the last wait completed, do you
find the following interpretation of "db file parallel read" wait time correct
when multiple blocks are involved:
Async IO: “db file parallel read” wait time = max(IO time)
Sync IO: “db file parallel read” wait time = sum(all IO times); (sum because
the reads are executed sequentially, I think)
Best regards,
Nenad
https://nenadnoveljic.com/blog
From: Frits Hoogland <frits.hoogland@xxxxxxxxx>
Sent: Freitag, 28. Februar 2020 09:53
To: Noveljic Nenad <nenad.noveljic@xxxxxxxxxxxx>
Cc: Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>; oracle-l@xxxxxxxxxxxxx
Subject: Re: parallel recovery slaves waiting on undo reads
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>
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