RE: Unknown Exadata Cell Parameter - "_cell_fc_toresilver_limit_chdrs"

I don’t have any documentation on that, but reading the alphacrypt, I’d say that if you have a bad “plex” (that is old school called a mirror, as in image, even though it doggone well better not be reversed), and oracle is rebuilding it from one or two other still valid images, this is some limit at the cell and fiber channel? level of the rates from change directives? to process.  

It could be something entirely different, but that matches your comment. Unless you have something being rebuilt at the media image level, this shouldn’t matter (if I have the meaning anything like correct.)  

The old notion was that if someone shut Oracle down to rebuild plexes on the physical media, then it was pedal to the metal. But if you were up and running (with either two or one remaining good plexes), then Oracle would be rebuilding the broken plexes but not be piggy and starve the users for i/o.  

I could be completely wrong.  

mwf  

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Laurentiu Oprea Sent: Thursday, December 14, 2023 4:55 PM To: dinch.osman_at_gmail.com
Cc: ORACLE-L (oracle-l_at_freelists.org) Subject: Re: Unknown Exadata Cell Parameter - "_cell_fc_toresilver_limit_chdrs"  

Out of curiosity, did you tried to measure the performance (defined as duration) of a set queries with and without this parameter set?  

Thanks  

On Thu, Dec 14, 2023, 22:22 Osman DINC <dinch.osman_at_gmail.com> wrote:

Hi all Oracle enthusiasts,  

Do you have any information about the "_cell_fc_toresilver_limit_chdrs" parameter?

It is set on my environment (X7-2) cellinit.ora. It is an undocumented and non-default configuration.  

I could not find any information about it, but it is commented as "It is set to reduce Internal IO on exadata cells". We do not have prior knowledge about it.  

Also I have screenshots of AWR reports - Exadata Statistics - Top IO Reasons by MB section.

(Before and after parameter change)  

Before _cell_fc_toresilver_limit_chdrs parameter is set:

https://drive.google.com/file/d/1PEcjUYAuG8SNTq1vfXLl0fyoeoffSLhq/view?usp=drive_link  

After "_cell_fc_toresilver_limit_chdrs"=6000000

https://drive.google.com/file/d/12tDsDvt22oFyEaQtRdMsXcCYduw9992A/view?usp=drive_link  

According to the screenshots, It reduces Internal IO done by storage servers dramatically, but there may be a trade-off which i do not know.  

I want to clear this parameter as it is a non-default configuration. It was set on image version 20.1.5.0.0.201209 and it has been retained for years.  

Exadata image version is 22.1.17 now.

If anybody can share more information about what this parameter does, I will be glad.    

Regards,

Osman DİNÇ.    

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