> 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 > """ > Learn something new everyday: looking at QEMU's internals, that flag actually only makes a difference for local IO backends (files and devices). Therefore, no need to set it for librbd volumes. > >>> 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 > >>> _______________________________________________ > >>> ceph-users mailing list > >>> ceph-users@xxxxxxxxxxxxxx > >>> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > >>> > >> _______________________________________________ > >> ceph-users mailing list > >> ceph-users@xxxxxxxxxxxxxx > >> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > >> > > > > [1] http://docs.ceph.com/docs/master/rbd/rbd-openstack/#configuring-nova > > > > > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > -- Jason Dillaman _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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