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@xxxxxxxxx> 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@xxxxxxxxx> 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
