Re: Unknown Exadata Cell Parameter - "_cell_fc_toresilver_limit_chdrs"

I don't know what this parameter is supposed to controll, but looking at the stats you produced I would have guessed that it was something to limit the RATE at which discs were resilvered rather than having something to do with the time between checks of something.

Do you see a lot of this type of activity going on all the time, or does it only happen in bursts occasionally. If it's bursts from time to time (and you haven't seen any indication that someone has decided to reconfigure or rebalance your ASM discs) then perhaps you have some failing or failed hardware that Oracle keeps trying to work around, so I'd check for any reports that might be available about the state of your ASM discs and what your disc groups look like. I don't do anything with Exadata hardware, so I can't offer any detailed suggestions about that.

Another thought about when this happens - can you find some correlation between (e.g.) activity that loads new data partitions and the resilvering - maybe it's a simple side effect of data files growing, and Oracle doing something to grow them on one half of redundant pairs and then catching up on the other half by resilvering.

Regards
Jonathan Lewis

On Thu, 14 Dec 2023 at 20:22, 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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