In one of the recent PSUs Oracle implemented a functionality for reclaiming
wasted space in LOBs. The space background processes (“wxxx”) are doing this
work. The number space background processes is controlled by the parameter
_max_spacebg_slaves . The default value 1024 is excessive and likely to cause
problems. Set it to a much lower value, like 10 for example. I did this on all
my databases.
Best regards,
Nenad
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> On Behalf
Of dmarc-noreply@xxxxxxxxxxxxx
Sent: Dienstag, 22. Dezember 2020 21:38
To: oracle-l@xxxxxxxxxxxxx
Subject: High read io from w00x (SMCO) processes?
Our storage group contacted me this week complaining that one of our database
servers had started experiencing a much higher i/o load than previously (as of
last Friday). Looking at the database in question did not show especially high
activity.
pidstat -dl showed that the vast majority of i/o activity is coming from w00x
processes and is from reads rather than writes.
A few things that might be useful:
Oracle version 12.1.0.2.0
The database uses NCLOBs/securefiles
_enable_space_preallocation=3
There was unusually high DB activity on the database which touched the NCLOBs
on the Friday when the issue started. But the high i/o issue continued over the
weekend (when DB activity was very low) and all day Monday and Tuesday (high DB
activity during the day, low DB activity during the night).
So my questions are
1. Is it possible that SMCO is engaged in some very long running cleanup from
the DB activity on Friday? Could is somehow be stuck on something?
2. Is there any way to trace what work the w00x processes are doing?
3. Any merit to the tempting notion of setting _enable_space_preallocation=0,
waiting a bit, and then setting it back to 3?
4. Any other notions?
As always, thanks in advance for any advice/suggestions.
Best,
Jay
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