Re: LGWR, EMC or app cursors?

Hi David,
Might also want to check if these time coincide with snapshots being taken on your drives. Snapshots at some storage systems may sometimes freeze IO to get to a synchronized state.

Harel

On Mon, Oct 7, 2019 at 7:17 PM Herring, David <dmarc-noreply_at_freelists.org> wrote:

> Chris, yeah, seems that way. I had a problem a while back where
> operations would timeout at seemingly random times and found out
> $LD_LIBRARY_PATH included a NAS that no longer existed. "strace" showed
> that valid paths were picked for a while, then the invalid NAS would get
> hit and there'd be a pause.
>
>
>
> Anyway, the EMC config is all multipath. I'll check with storage/sysadmin
> teams (pretty much everything is outsourced) to review it all again.
>
>
>
> Regards,
>
>
>
> Dave
>
>
>
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> *Dave Herring*
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> *From:* oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> *On
> Behalf Of *Chris Taylor
> *Sent:* Monday, October 7, 2019 10:27 AM
> *To:* dmarc-noreply_at_freelists.org
> *Cc:* oracle-l_at_freelists.org
> *Subject:* Re: LGWR, EMC or app cursors?
>
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>
> Always the same database/machine?
>
>
>
> Almost sounds like a path is down/unavailable from the machine to the
> storage but the OS doesn't know it isn't responding. I'm not as
> familiar with EMC Power and I think EMC Power uses something besides
> mutlipath drivers (but I might be mistaken).
>
> If you're not getting any path errors, it might be worthwhile to have
> someone go into the cage and replace all the Fiber cables connecting from
> this server to the storage (they can test the cables I believe and see if
> one is faulty/noisy).
>
>
>
> Thanks,
> Chris
>
>
>
>
>
> On Mon, Oct 7, 2019 at 10:20 AM Herring, David <
> dmarc-noreply_at_freelists.org> wrote:
>
> 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
>
>

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