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Re: [libvirt-users] Why librbd disallow VM live migration if the disk cache mode is not none or directsync



On 7/29/19 3:51 AM, Ming-Hung Tsai wrote:
I'm curious that why librbd sets this limitation? The rule first
appeared in librbd.git commit d57485f73ab. Theoretically, a
write-through cache is also safe for VM migration, if the cache
implementation guarantees that cache invalidation and disk write are
synchronous operations.

For example, I'm using Ceph RBD images as VM storage backend. The Ceph
librbd supports synchronous write-through cache, by setting
rbd_cache_max_dirty to zero, and setting
rbd_cache_block_writes_upfront to true, thus it would be safe for VM
migration. Is that true? Any suggestion would be appreciated. Thanks.

The commit you refer to is very old and my hunch is that things looked different in 2012. Things might have changed since then and if write-through wasn't safe ~7 years ago, it might be safe now (with some tuning).

Michal


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