Hi Jason,
Le 22/03/2016 14:12, Jason Dillaman a écrit :
We actually recommend that OpenStack be configured to use writeback cache [1]. If the guest OS is properly issuing flush requests, the cache will still provide crash-consistency. By default, the cache will automatically start up in writethrough mode (when configured for writeback) until the first OS flush is received.
Phew, that was the good reasoning then, thank you for your confirmation. :)
I interpret native as kernel-managed I/O, and as the RBD through librbd
isn't exposed as a block device on the hypervisor, I configured threads
I/O for all our guest VMs.
While I have nothing to empirically back up the following statement, I would actually recommend "native". When set to "threads", QEMU will use a dispatch thread to invoke librbd IO operations instead of passing the IO request "directly" from the guest OS. librbd itself already has its own IO dispatch thread with is enabled by default (via the rbd_non_blocking_aio config option), so adding an extra IO dispatching layer will just add additional latency / thread context switching.
Well, if only that would be possible...
Here's the error message from libvirt when starting a VM with
native+writeback:
"""
native I/O needs either no disk cache or directsync cache mode, QEMU
will fallback to aio=threads
"""
librbd has a setting called 'rbd_op_threads' which seems to be related to
AIO.
When does this kick in?
This is related to the librbd IO dispatch thread pool. Keep it at the default value of "1" as higher settings will prevent IO flushes from operating correctly.
Yes, a lot of questions where the internet gives a lot of answers.
Some feedback would be nice!
Thanks,
Wido
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[1] http://docs.ceph.com/docs/master/rbd/rbd-openstack/#configuring-nova
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