I have a couple of questions about caching:
I have 5 VM-Hosts serving 20 VMs.
I have 1 Ceph pool where the VM-Disks of those 20 VMs reside as RBD Images.
1) Can i use multiple caching-tiers on the "same" data pool?
I would like to use a local SSD OSD on each VM-Host that can serve
as "application accelerator" local-cache for the VM-Disks.
I can imagine data corruption if other VM-Hosts write to the same
Ceph data pool but not using the same caching-tier.
I imagine no data corruption if i know no other VM-Hosts will access
that Ceph object (VM-Disk / RBD image).
I would need to flush the cache of that VM-Host when i shutdown the
VM on it, before i can start the VM on a different VM-Host.
Or is Ceph perhaps smart enough that it would notify the above
Caching-Tier to evict a cached object when there is a change on that
object not changed by that caching-tier?
2) RBD Cache is useless for hosting Oracle databases?
If Oracle is doing a O_SYNC and RBD Cache would flush on O_SYNC,
then there would be nothing cached. Correct?
3) Would a caching tier be smart enough to flush dirty/modified objects
on "idle" i/o?
(when client i/o is not busy ceph will use that time to sync to backend)
I know it will flush on at a certain capacity (50%) or on a certain
age (600sec), but can it also flush on a certain busy/idle percentage or
auto-magically/intelligently?
Thanks,
Samuel Terburg
Panther-IT BV
www.panther-it.nl
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