RE: Difference in writer response time

In a network sense, where is the client application with respect to the two databases?  

When you are using the “primary” database is the client at the “primary” database location?

When you are using the “DR” database is the client at the “DR” database location?  

Depending on how the application handshakes information to be written and how waits are rolled up, that can make all the difference.  

A very quick test is to actually place a client at the location of the “DR” database location on the LAN with the “DR” database if in fact currently you are considering performance of an application on the “primary” database location in both cases currently.  

Explaining how and why the performance difference can actually be network latency and speed yet appear to be differences in DBWR service rate is beyond the scope of this note.  

mwf  

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Pap Sent: Friday, April 09, 2021 8:29 AM
To: Tanel Poder
Cc: Oracle L
Subject: Re: Difference in writer response time  

Thank You Tanel.  

Not having much understanding with respect to the infra side. But , actually the first thing we want to understand is why there is a difference in performance during that peak period between the two sites. As those are primary and DR so ideally everything should be the same including SAN configurations. We have verified all the DB parameters are mostly the same and are not suspected such as to impact DBWR throughput one vs other.  

But as you said there may be some issue wrt kernel or IO stack, so wanted to understand what exact information we can ask from our storage/SAN team by which we can conclude is there is really a difference in performance of storage IO between both sides?  

And should we ask the IO information only during the issue period to understand the difference as because in normal time both the databases perform similar just that during those peak activity the DR is behaving badly as compared to primary?  

Regards

Pap  

On Fri, Apr 9, 2021 at 5:32 AM Tanel Poder <tanel_at_tanelpoder.com> wrote:

Ok, sorry I didn't realize this was on HP-UX (I guess I'm too spoiled nowadays with "servers" like this one <https://tanelpoder.com/posts/11m-iops-with-10-ssds-on-amd-threadripper-pro-workstation/> under my desk ;-)  

Some comments:

1. The free buffer waits show that DBWR falls behind with syncing dirty buffers to disk (~4000 blocks written per second). So it looks like a DBWR throughput problem (caused by some issue in the kernel or I/O stack)
2. Other than storage hardware latency itself and things like block device queue depths, the filesystem itself may be a bottleneck - are you using VxFS - and if yes, with or without ODM? VxFS without ODM doesn't use concurrent I/O by default, potentially causing write-write contention at file level (POSIX single writer locking)
3. Also, the normal recommendation for production systems is to use filesystemio_options=setall, but I don't remember if HP-UX was special in that sense (or you just forced direct I/O with a VxFS mount option)
4. Run Snapper on DBWRs and see how many of them you have in each instance and are they 100% busy waiting for one of the db file parallel write, db file asynch I/O submit waits (or show all CPU usage - no wait events whatsoever, can be due to an instrumentation bug)

The various "higher level contention" wait events, buffer busy waits, enq: TX contention and in rare cases even some latch contention may be just a symptom of the underlying I/O problems. You need to do wait chain analysis to see beyond the "top level symptoms".  

The ash_wait_chains.sql output is pretty wide, Oracle-L doesn't allow me to send screenshots as images, so here are a couple of tweets with images of such problems (where the high level symptoms show different waits than the root cause):

• https://twitter.com/TanelPoder/status/1318023393304825856
• https://twitter.com/TanelPoder/status/1380304923678498820

Back when I used to deal with HP-UX/Solaris machines with VxFS more, a really common bottleneck in high-performance databases (of that era) was people not using concurrent I/O on that filesystem.  

--

Tanel Poder

#vConf2021: Troubleshooting Very Complex Oracle Performance Problems

https://tanelpoder.com/conference/

 

On Thu, Apr 8, 2021 at 2:05 PM Pap <oracle.developer35_at_gmail.com> wrote:

Thank you Tanel.

 

Its HP machine and its on file system. No ASM. The filesystemsio_options in gv$paramer showing as "asynch" in both databases. and also disk_asynch_io is also set as TRUE. As I see from AWR, Its HP-UX IA(64 bit), 64 core, 8- socket machine in both side.

 

The SAN team confirms not much visible difference between the underlying storage performance between those two sites. So wondering as we don't use data guards could this difference be just because of just the structural difference in table/index structure? But we have all the same volume of data on both sides so not sure how that can play such a big difference. And contrary to that, I see we are doing more work on the Primary(faster) side during that time period as compared to DR(slow side), so how is that possible? I am seeing the direct write is higher on DR i.e. slow side as compared to primary, not sure if that is pointing to something suspicious.

 

I have fetched the wait event section of the AWR from both databases and attaching here(in two tabs as primary and DR wait profile). Although I do see "free buffer waits" in one of the top lists but the major one is 'index contention'(top one is a composite PK index but local index, so less chances of structural difference causing such issues). But again it might be that the DBWR response trend is just a symptom of something else but not the cause as the SAN team is saying no difference there.

 

 

Regards

Pap

 

On Thu, Apr 8, 2021 at 10:34 PM Tanel Poder <tanel_at_tanelpoder.com> wrote:

There's also one usual suspect, I don't see filesystemio_options parameter set, are you running on ASM, not a filesystem, right?

 

Thanks,

Tanel.

 

On Thu, Apr 8, 2021 at 4:06 AM Pap <oracle.developer35_at_gmail.com> wrote:

Hello Lister, We have two databases which are the same in all the hardware configurations along with DB parameters and are in sync/replicated using golden gate. And those are treated as primary and DR for us and both are active. But we are seeing differences in behaviour when our application points to primary VS when it points to DR. We are seeing during specific times of the day when our system is at its peak, in one of the database i.e DR the DBWR response times spikes till ~200+ms slowing down the write queries while in another database the dbwr response time stays ~65ms for a similar amount of data and transaction volume. So wanted to understand what can cause such drastic differences? Note- Its version 11.2.0.4 of Oracle. 





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