Re: LGWR, EMC or app cursors?

From: Andy Sayer <andysayer@xxxxxxxxx>
To: dmarc-noreply@xxxxxxxxxxxxx
Date: Thu, 10 Oct 2019 18:50:43 +0100

Storage folks may not be trustworthy, or may not be checking the right
things. Remember not every system is as well instrumented as your Oracle DB!

If you are seeing 30+ seconds for an individual log file parallel write
then the IO is almost certainly going missing. You can “handle” this by
decreasing your disk timeout (it sounds like it’s 30 seconds which is way
too pessimistic for modern storage speeds). This just means that your 30
second IO requests will be retried sooner.

You will probably see this occurring more than once and on more than just
your redo files (slow redo writes are more noticeable because everything
will wait on that eventually).

While the application can probably do less work, it’s not the application’s
fault it sometimes has to spend half a minute doing nothing.

Thanks,
Andy

On Thu, 10 Oct 2019 at 18:10, Herring, David <dmarc-noreply@xxxxxxxxxxxxx>
wrote:

The storage folks are assuring me that there are no issues at that level
and that any shared resources are NOT seeing the same thing.  I have to
take their word for it.

As for oswatcher details, yes, it's running and I've reviewed "ps" data
looking for the LGWR process.  For WCHAN around one of the times we had a
problem, LGWR lists "SYSC_s" and "asm_ma".  I'll see if I can track down
what that means.  OSW runs every 30 sec so obviously there's a lot of
detail missing when the problem happens.

Regards,

<https://www.google.com/maps/search/103+JFK+Parkway+%0D%0A+Short+Hills,+New+Jersey+07078?entry=gmail&source=g>

Dave

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*Dave Herring*

DBA

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Mobile 630.441.4404

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*From:* oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> *On
Behalf Of *Tiwari, Yogesh
*Sent:* Wednesday, October 9, 2019 11:13 PM
*To:* oracle-l@xxxxxxxxxxxxx
*Cc:* Tiwari, Yogesh <Yogesh.Tiwari@xxxxxxxxxxxxxx>
*Subject:* RE: LGWR, EMC or app cursors?

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Dave,

If you are sharing SAN with other databases hosts, probably they should
also complain at the same time. Is this true for you?

Further, you might want to capture WCHAN for LGWR(s) processes, when it
goes into this hang. This can give clues, if it is really SAN, or OS issue.

Tools like oswatcher(TFA) do capture complete “ps” detail, which can help
in this situation. Do see that as an option?

Thanks,

*Yogi *

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*From:* oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> *On
Behalf Of *Herring, David
*Sent:* 10 October 2019 08:11
*To:* martin.a.berger@xxxxxxxxx; dmarc-noreply@xxxxxxxxxxxxx
*Cc:* oracle-l@xxxxxxxxxxxxx
*Subject:* RE: LGWR, EMC or app cursors?

First, thanks to a number of you who replied.  The problem still exists
but at least I know a bit more about it.  I read through Frits' post and
it's quite interesting and informative, but didn't help.  One potentially
serious problem in debugging this is "strace" or any type of system trace
is not available on these servers, my guess is that the security team felt
having access to that is a "no-no".  Of course I seriously doubt it'd be
realistic to strace LGWR and let it run for hrs., waiting for the problem
to occur (potentially large performance impact, let alone a giant
tracefile).

Unfortunately I'm back to trying to figure out details on exactly what
LGWR is doing during it's "log file parallel write".  Per Andy's suggestion
I validated that the column SEQ# in ASH doesn't change during the duration
of the problem for LGWR, so it's one huge wait.  In fact seconds before the
one example that I’m trying to tear apart I see LGWR waiting on the same
event but it's a different SEQ# so it got some work done, then just spun
for nearly 30 seconds while all other DML sat and waited on "log file
sync".  LGWR finally gets it's work done, everything back to normal.

I'm going to go back to the full issue bridge list (we have calls on this
daily with SMEs covering all areas) and see if I can get a 100%
confirmation that they've validated all components inbetween LGWR and the
physical disk.

Regards,

<https://www.google.com/maps/search/103+JFK+Parkway+%0D%0A+Short+Hills,+New+Jersey+07078?entry=gmail&source=g>

Dave

[image: cid:image001.png@01D05044.5C2AEE60]

*Dave Herring*

DBA

103 JFK Parkway
<https://www.google.com/maps/search/103+JFK+Parkway+%0D%0A+Short+Hills,+New+Jersey+07078?entry=gmail&source=g>

Short Hills, New Jersey
<https://www.google.com/maps/search/103+JFK+Parkway+%0D%0A+Short+Hills,+New+Jersey+07078?entry=gmail&source=g>
07078
<https://www.google.com/maps/search/103+JFK+Parkway+%0D%0A+Short+Hills,+New+Jersey+07078?entry=gmail&source=g>

Mobile 630.441.4404

*dnb.com
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*From:* oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> *On
Behalf Of *Martin Berger
*Sent:* Tuesday, October 8, 2019 2:34 AM
*To:* dmarc-noreply@xxxxxxxxxxxxx
*Cc:* oracle-l@xxxxxxxxxxxxx
*Subject:* Re: LGWR, EMC or app cursors?

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Hi Dave,

as you asked for tracing, a "normal" 10046 trace can be enabled for
logwriter
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.

You will not get SQL statements, but normal trace information regarding
WAITs.

The event log file parallel write is somehow tricky. Frits wrote a nice blog
post
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about it.

It's important to understand that it represents multiple IOs (that's the
parallel).

"EMC and sysadmins have confirmed there are no disk errors and from

their standpoint the disks are waiting on Oracle."

I assume you have a (or two) FiberChannel SAN which connects EMS and your
DB-host.  Please ask them for measurements on those switches also.

The argument is simple: If the host claims it waits on the disks
(according to iostat) and EMC claims it's waiting on Oracle, have a closer
look at the components in between.

hth,

Martin

Am Mo., 7. Okt. 2019 um 17:20 Uhr schrieb Herring, David <
dmarc-noreply@xxxxxxxxxxxxx>:

Folks, I've got a bit of a mystery with a particular db where we're
getting a periodic 25-30 pause between user sessions and LGWR processes and
can't clearly identify what's the cause.

   - The database is 11.2.0.4, RHEL 7.5, running ASM on EMC.
   - Sometimes once a day, sometimes more (never more than 5) times a day
   we see user processes start waiting on "log file sync".  LGWR is waiting on
   "log file parallel write".
   - At the same time one of the emcpower* devices shows 100% busy and
   service time 200+ (from iostat via osw).  mpstat shows 1 CPU at 100% on
   iowait.  It's not always the same disk (emcpowere1, a1, h1, …), not always
   the same CPU.  EMC and sysadmins have confirmed there are no disk errors
   and from their standpoint the disks are waiting on Oracle.
   - During this time LGWR stats in ASH are all 0 - TIME_WAITED, DELTA*
   columns.  Only after the problem goes away (about 25 secs) these columns
   are populated again, obviously the DELTA* columns 1 row later.  LGWR's
   session state is WAITING so I assume the column value observations are due
   to LGWR waiting, as it won't write stats until it can do something.

I am stuck trying to find out, really prove who is the culprit or what
exactly the wait is on.  Is LGWR waiting on user sessions and user sessions
are waiting on LGWR and all that causes the disk to be 100%?  Can I enable
some sort of tracing on LGWR and would that point to exactly what he's
waiting on to prove where the problem is?

Regards,

Dave

References:
- LGWR, EMC or app cursors?
  - From: Herring, David
- Re: LGWR, EMC or app cursors?
  - From: Martin Berger
- RE: LGWR, EMC or app cursors?
  - From: Herring, David
- RE: LGWR, EMC or app cursors?
  - From: Tiwari, Yogesh
- RE: LGWR, EMC or app cursors?
  - From: Herring, David

Re: LGWR, EMC or app cursors?

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