It looks like one of my replies to this got lost in transmission - can't
even find it in my Sent box.
The point may come where you have to dump a couple of minutes of redo log
file to see what operations are appearing as this may give some idea of
what's going on.
There's an example of syntax for dumping a small volume based on SCNs.
https://jonathanlewis.wordpress.com/2019/06/11/redo-dumps/
Regards
Jonathan Lewis
On Fri, Apr 24, 2020 at 10:41 AM Paul Houghton <Paul.Houghton@xxxxxxxxxxxxx>
wrote:
Thanks for the responses.
I have a third “identical” database on another server which is also slow,
so I thought I would play with that. It also generates more redo, and redo
I/O seems to be the bottleneck.
I took the pfile from the “fast” database and applied it to this one (Just
had to change the name, and the location of some files). Redo is still
generated at twice the rate. This rules out standby as a cause of the
issue. I suppose I need to investigate Jonathans suggestion to see whether
SLOB is doing something different between the two servers, but it really
shouldn’t because the configuration is identical. Also any workload seems
slower, and potentially redo I/O bound. I will have to do that on Monday.
Nothing looks strange in AWRs calculations. Redo does seem to be the
limiting factor, so if I could get the slow db to generate less redo like
the fast one, it would have a big impact on performance. I checked the redo
block size and it is 512 on both. Investigating whether 4K is better for
the SAN is another rabbit hole, but isn’t a cause of difference here.
Statistic
Fast DB
Slow DB
Db block changes
79,758,656
27,719,598
redo size
32,053,006,128
34,648,068,868
user commits
592,015
170,871
user rollbacks
2
1
*From:* Mark W. Farnham <mwf@xxxxxxxx>
*Sent:* 23 April 2020 19:14
*To:* harel.safra@xxxxxxxxx; Paul Houghton <Paul.Houghton@xxxxxxxxxxxxx>
*Cc:* oracle-l@xxxxxxxxxxxxx; jonathan@xxxxxxxxxxxxxxxxxx;
tim.evdbt@xxxxxxxxx
*Subject:* RE: Redo per transaction inconsistency when running SLOB
· a physical standby database. The faster one doesn’t
seems like the likely difference to me. Did I miss the protection mode?
How far away (ping latency) is the standby?
Do you have the ability to set up on the new box with the only change
being no physical standby?
*From:* oracle-l-bounce@xxxxxxxxxxxxx [
mailto:oracle-l-bounce@xxxxxxxxxxxxx ;<oracle-l-bounce@xxxxxxxxxxxxx>] *On
Behalf Of *Harel Safra
*Sent:* Thursday, April 23, 2020 12:31 PM
*To:* Paul.Houghton@xxxxxxxxxxxxx
*Cc:* oracle-l@xxxxxxxxxxxxx
*Subject:* Re: Redo per transaction inconsistency when running SLOB
Hi,
Any chance the slower one with the standby is set to force logging while
the faster one isn't? Are there nologging operations in the database?
Harel
On Thu, Apr 23, 2020 at 2:29 PM Paul Houghton <Paul.Houghton@xxxxxxxxxxxxx>
wrote:
Hi
We have some new hardware which seems slower from an IO perspective than
the old hardware, so I downloaded SLOB to investigate, and ran it on two
databases which are copies of each other. Both on RHEL7 OS with Oracle
12.2.0.1 and the April critical patch. Both are in archive log mode.
Looking into the AWR, it seems the slower one on the new hardware is
generating more redo per transaction than the old one. I set up SLOB
identically in both databases (./setup.sh IOPS 8) and ran it identically
(./runit.sh 8). The tablespace is a smallfile and I specified it’s
location, but otherwise I used the defaults. I set up the admin user as
“sys/sys as sysdba” and for this run changed the run time to 900 seconds.
There are some differences in configuration. The slow one is supposed to
be more like production than the faster one, so it has:
· bigger buffers the log buffer is 10M as opposed to 5M in the
faster one
· uses huge pages, where the fast one doesn’t
· LOST_WRITE_PROTECT is NONE in both environments.
· a physical standby database. The faster one doesn’t
There are applications running against these databases, but I can’t see
that there was any application SQL that would make this much difference.
I am looking in the load profile section of the AWR report that is
generated. Redo size per transaction is 54K for the faster one, and 203k
for the slower one, so about 4x as much. What could be causing this?
I’d be grateful for some pointers as to where I can look to see what is
causing the extra redo.
Thanks
PaulH
