Re: LOG_BUFFER

From: Mladen Gogala <gogala.mladen_at_gmail.com>
Date: Wed, 18 May 2016 21:35:04 -0400
Message-ID: <573D1848.2030404_at_gmail.com>

On 05/17/2016 02:59 PM, Ram Cheruvattath wrote:
> Hi
> We have a 12c database that has very high redo generation (4TB/hr).
> Our log_buffer is set to 1GB.
> Is there a downside to setting log_bugger this big?
> Thanks
> Ram

Ram, we have talked about multi-threaded log writer in 12c few days ago. In the meantime, I did some digging and found some new undocumented parameters:

_adaptive_scalable_log_writer_enable_worker_threshold Increase in redo generation rate as a percentage

_adaptive_scalable_log_writer_disable_worker_threshold Percentage of overlap across multiple outstanding writes

_adaptive_scalable_log_writer_sampling_count Evaluate single versus scalable LGWR every N writes

_adaptive_scalable_log_writer_sampling_time Evaluate single versus scalable LGWR every N seconds

_adaptive_scalable_log_writer_enable_worker_aging Per million of redo gen rate when LGWR workers were last used

Of course, there is nothing about these parameters on the support.oracle.com. The good, old times when it was possible to figure out how things work by digging through the Oracle support documents are a long gone era. However, I was able to dig out few things from Jonathan's site:

https://jonathanlewis.wordpress.com/2013/07/04/12c-trivia/ https://jonathanlewis.wordpress.com/2013/06/25/12c/

Not much more than we've already discussed, but still worth reading. I would expect multi-threaded writer to be able to cope with much larger buffers than we have seen in 11G. I don't think that in 12c having 1G log buffer is a bad idea. However, the only sign you will get that your log buffer is too small are log buffer space wait events in V$SYSTEM_EVENT. I guess that there is a feature called "adaptive scalable log writer" which will appear in some kind of an internals book when people at Oracle Corp. decide do tell someone about it.

-- 
Mladen Gogala
Oracle DBA
Tel: (347) 321-1217


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Received on Thu May 19 2016 - 03:35:04 CEST