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,

Dave

[cid:image001.png_at_01D05044.5C2AEE60]

Dave Herring
DBA
103 JFK Parkway
Short Hills, New Jersey 07078
Mobile 630.441.4404

dnb.com<http://www.dnb.com/>

[cid:image002.png_at_01D05044.5C2AEE60]<http://www.facebook.com/DunBradstreet>[cid:image003.png_at_01D05044.5C2AEE60]<http://twitter.com/dnbus>[cid:image004.png_at_01D05044.5C2AEE60]<http://www.linkedin.com/company/dun-&-bradstreet>[cid:image005.png_at_01D05044.5C2AEE60]<http://www.youtube.com/user/DunandBrad>

From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On Behalf Of Martin Berger Sent: Tuesday, October 8, 2019 2:34 AM
To: dmarc-noreply_at_freelists.org
Cc: oracle-l_at_freelists.org
Subject: Re: LGWR, EMC or app cursors?

CAUTION: This email originated from outside of D&B. Please do not click links or open attachments unless you recognize the sender and know the content is safe.

Hi Dave,

as you asked for tracing, a "normal" 10046 trace can be enabled for logwriter<https://nam03.safelinks.protection.outlook.com/?url=https%3A%2F%2Ffritshoogland.files.wordpress.com%2F2014%2F04%2Fprofiling-the-logwriter-and-database-writer.pdf&data=02%7C01%7Cherringd%40dnb.com%7Cf5bf752ea83c4c4c36fc08d74bc21067%7C19e2b708bf12437597198dec42771b3e%7C0%7C1%7C637061169179075851&sdata=aTuydwpa7%2FrZwDsMemlNoGO%2BVCBOdYqys5T6rU6AG6o%3D&reserved=0>. 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<https://nam03.safelinks.protection.outlook.com/?url=https%3A%2F%2Ffritshoogland.wordpress.com%2F2013%2F08%2F30%2Foracle-io-on-linux-log-writer-io-and-wait-events%2F&data=02%7C01%7Cherringd%40dnb.com%7Cf5bf752ea83c4c4c36fc08d74bc21067%7C19e2b708bf12437597198dec42771b3e%7C0%7C1%7C637061169179075851&sdata=9yHLvzg3kvHMZdsz0FhRs4%2B3sRVKyFAtlF64P3n%2F8%2Bo%3D&reserved=0> 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_at_freelists.org<mailto:dmarc-noreply_at_freelists.org>>: 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

--
http://www.freelists.org/webpage/oracle-l




image/png attachment: image001.png



image/png attachment: image002.png



image/png attachment: image003.png



image/png attachment: image004.png



image/png attachment: image005.png