Re: Question on Exadata X8 - IO

Basically i am seeing many doc stating triple mirroring is recommended with "write back flash cache" and some other stating write back flash cache is not possible without HIGH redundancy/triple mirroring. So there is a difference between these two statements because if we decide to go for NORMAL redundancy to save some space and to have some IO benefit(in terms of not writing one more additional data block copy). But we want to utilize the "write back flash cache" option to get benefits on write IOPS. And in this case if restriction is put in place for "High redundancy" we won't be able to do that. Please Correct me if my understanding is wrong?

On Fri, Feb 12, 2021 at 12:53 PM Lok P <loknath.73_at_gmail.com> wrote:

> Seeing in below doc which state its recommended to go for high redundancy
> ASM disk group(i.e. triple mirroring) in case we are using write back flash
> cache as because the data will be first written/stay in flash cache and
> flushed to the disk later stage and in case of failure it has to be
> recovered from mirror copy. But i am wondering , is this not possible with
> double mirroring , will it not survive the data loss in case of failure?
> Want to understand what is the suggested setup which will give optimal
> space usage without compromising IOPS and data loss.
>
>
> https://docs.oracle.com/en/engineered-systems/exadata-database-machine/sagug/exadata-storage-server-software-introduction.html#GUID-E10F7A58-2B07-472D-BF31-28D6D0201D53
>
> Regards
> Lok
>
> On Fri, Feb 12, 2021 at 10:42 AM Lok P <loknath.73_at_gmail.com> wrote:
>
>> Hello Listers, We are moving from exadata X5 to X8 and there are multiple
>> reasons behind it. Few of them are , 1)we are almost going to saturate the
>> existing storage capacity(DB size reaching ~150TB) in current X5. 2)And the
>> current IOPS on X5 is also reaching its max while the system works during
>> its peak load.
>>
>> We are currently having HIGH redundancy(triple mirroring) for our
>> existing X5 machines for DATA and RECO disk group and DBFS is kept as
>> NORMAL redundancy(double mirroring). Now few folks raised questions on the
>> impact on IOPS and storage space consumption, if we use double
>> mirroring(NORMAl redundancy) vs triple mirroring(High redundancy) in the
>> new X8 machine. I can see the benefit of double mirroring(Normal
>> redundancy) being saved in storage space(around 1/3rd in terms of DATA and
>> REDO copies), but then what is the risk wrt data loss, is it okay in a
>> production system? (Note- We do use ZDLRA backup for taking the DB backup.
>> And for disaster recovery we have active data guard physical standby in
>> place which runs in read only mode).
>>
>> With regards to IOPS, we are going with default write back flash cache
>> enabled here. Is it correct that with double mirroring we have to
>> write/read into two places VS in triple mirroring we have to do that in
>> three places , so there will also be degradation in IOPS with triple
>> mirroring/High redundancy as compared to double mirroring? if it's true
>> then by what percentage the IOPS degradation will be there? And then if
>> it's okay if we go for double mirroring as that will benefit us wrt IOPS
>> and also saves a good amount of storage space?
>>
>> Regards
>>
>> Lok
>>
>

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