Re: Question on Exadata X8 - IO

The Exadata X8 and X8M storage cells have 14TB disks. With 12 per cell, that's 168TB *per cell*. You haven't mentioned which rack size your X5 machine is, but from the numbers you're showing it looks like maybe a half rack. A half rack of X8M will come with 1PB of total disk, giving you over 300TB of usable space to divide between your RECO and DATA disk groups if you are using HIGH redundancy. That seems plenty for your 150TB database. But if you need more, add another storage cell.

As for performance degradation from using HIGH redundancy, you need to consider that the additional work of that extra write is being taken on by the storage cells. By definition the redundant block copies must go to separate cells. NORMAL redundancy writes to two cells and HIGH goes to three. In aggregate, each write will be as fast as your slowest cell. So any difference in write performance is more a function of the total number of cells you have to share the workload. That difference would be diminished as you increase the number of cells in the cluster.

And of course that difference would be mitigated by the write back cache too because writes to the flash cache are faster than writes to disk.

Honestly, I can't imagine that Oracle would sell you an Exadata machine where any of this would be a problem for you. It would be so undersized from the beginning that your problems with it would be much greater than any marginal difference in write performance from using high redundancy.

Andy

On Fri, Feb 12, 2021 at 10:31 AM Lok P <loknath.73_at_gmail.com> wrote:

> Thanks Much.
>
> I got some doc but missed. below URL also pointing to high redundancy as
> requirement but may be its not compulsory as you stated.
>
> We have the size of our database i.e. (~150TB), so we were thinking to
> have some space reduction using double mirror rather triple mirroring. but
> i was not aware that the disk size itself is a lot bigger in X8 and as you
> stated in X8 we have a bigger size disk and so the mirroring will take a
> lot of time(in case of crash/failure) and thus HIGH redundancy is
> recommended. I think we have to relook into the same. Note- What I see in
> the current X-5 machine, we have ~6TB flash/storage server and ~80TB hard
> disk/storage server. Not sure what that is in case of X8 though.
>
> And another doubt i had was, Is it also true that the IOPS will be
> degraded by some percentage in case of tripple mirror as compared to double
> mirror as because it has to write one more additional copy of data block
> to flash/disk?
>
>
> https://stefanpanek.wordpress.com/2017/10/20/exadata-flash-cache-enabled-for-write-back/
>
> On Fri, Feb 12, 2021 at 8:52 PM Ghassan Salem <salem.ghassan_at_gmail.com>
> wrote:
>
>> Please, can you point to where you saw that write-back is only possible
>> with high-redundancy?
>> High redundancy is very much recommended with X8 due to the size of the
>> disks, and the time it takes to re-mirror in case of disk loss: if you're
>> in a normal redundancy, and you loose a disk, while re-mirroring is being
>> done, you don't have any second copy of that data, and so, if you loose
>> yet-another disk, you're in big trouble. With lower capacity disks, the
>> re-mirroring takes much less time, and so the risk is lower.
>>
>> regards
>>
>> On Fri, Feb 12, 2021 at 3:57 PM Lok P <loknath.73_at_gmail.com> wrote:
>>
>>> 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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