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From Logan Barfield <>
Subject Libvirt & RBD caching
Date Wed, 18 Feb 2015 20:00:34 GMT
Our current deployment is KVM with Ceph RBD primary storage.  We have
rbd_cache enabled, and use "cache=none" in Qemu by default.

I've been running some tests to try to figure out why our write speeds
with FreeBSD are significantly lower than Linux.  I was testing both
RBD and local SSD storage, with various cache configurations.  Out of
all of them the only one that performed close to our standard Linux
images was local SSD, Qemu cache=writeback, FreeBSD gpt journal

I've been reading on various lists the reasons and risks for
cache=none vs cache=writeback:
- cache=none: Safer for live migration
- cache=writeback: Ceph RBD docs claim that this is required for data
integrity when using rbd_cache

>From what I can tell performance is generally the same with both,
except in the case of FreeBSD.

What is the current line of thinking on this?  Should be using 'none'
or 'writeback' with RBD by default?  Is 'writeback' considered safe
for live migration?

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