Re: Exadata writeback Flash Cache usable space

Hi

Since writeback can contain dirty data it needs mirroring, since the dirty data in writeback mode contains database modified blocks I think it should follow the same redundancy as the disk groups.

I just found this yesterday, a bit dated but I think it applies

https://uhesse.com/2014/08/06/why-write-through-is-still-the-default-flash-cache-mode-on-exadata-x-4/

Thanks

On Sun, Jul 18, 2021 at 3:07 PM Rajesh Aialavajjala < r.aialavajjala_at_gmail.com> wrote:

> I do not think that this is necessarily the case.
>
> The flashcache mode being set to "WriteBack" should not reduce the
> capacity of the flash cache available by a third. Also - I do not think
> that there's a dependency or link between the redundancy of the ASM disk
> groups and the flashcache mode.
>
> Each HC cell is provisioned with 25.6 TB of "raw" PCI flash...
>
> From an environment that has "WriteBack Flash Cache Enabled" - I see a
> reported cache size of 23 odd TB for each cell (this is an X8M-2)
>
> # cellcli -e list flashcache detail
> effectiveCacheSize: 23.28692626953125T
> id:
> size: 23.28692626953125T
> status: normal
>
> Additions, comments, corrections are welcome.
>
> Thanks,
>
> --Rajesh
>
> On Sat, Jul 17, 2021 at 4:22 AM Ls Cheng <exriscer_at_gmail.com> wrote:
>
>> Hi
>>
>> We are deploying a new Exadata x9m quarter rack with HC disks next week
>> and the flashcache will be configured using writeback instead of
>> writethrough.
>>
>> In writeback mode the flash is mirrored whereas in writethrough if I
>> understood correctly there is no mirroring. So I have a doubt, the quarter
>> rack has 76.8 TB raw flash space, what is the usable space with writeback
>> enabled? Since we are using High Redundancy I expect 25.6TB usable flash
>> cache space, correct?
>>
>> Thanks
>>
>>
>>

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