Hello,
Le 30/08/2016 à 14:08, Steffen Weißgerber a écrit :
Hello,
after correcting the configuration for different qemu vm's with rbd disks
(we removed the cache=writethrough option to have the default
writeback mode) we have a strange behaviour after restarting the vm's.
For most of them the cache mode is now writeback as expected. But some
neverthless use the disks in writethrough mode (at least an 'info block'
reports that on the qemu monitor). This does also not change when
configuring cache=writeback explicitly.
On our 6 node KVM Cluster we have the same behaviour for the problematic
vm's on all hosts which are configured equally with qemu 2.5.1, ceph 0.94.7
and kernel 4.4.6.
The ceph cluster has version 0.94.6.
For me it seems to be a problem specific to the rbd's. Is there a way to
check the cache behaviour of a single rbd?
To my knowledge, there is not such a thing as an RBD's cache mode.
The librbd cache exists in the client's memory, not on the Ceph
cluster's hosts. Its configuration is to be put in the ceph
configuration file, on each client host.
Setting QEMU's disk cache mode to "writeback" informs the guest's OS
that it needs to explicitly flush dirty data to persistent storage when
needed.
Regards
Steffen
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