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@xxxxxxxxx> 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@xxxxxxxxx> 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
