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

[Ming-Hung Tsai]

Sorry, the title and content are wrong. I mean libvirt, not librbd.

Ming-Hung Tsai <mingnus at gmail.com> :
>
> 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.