Re: RBD cache questions (kernel vs. user space, KVM live migration)

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On Tue, 14 Jan 2014, Gregory Farnum wrote:
> On Tuesday, January 14, 2014, Christian Balzer wrote:
>       Also on that page we read:
>       "Since the cache is local to the client, there?s no coherency if
>       there are
>       others accesing the image. Running GFS or OCFS on top of RBD
>       will not work
>       with caching enabled."
> 
>       This leads me to believe that enabling the RBD cache (or having
>       it enabled
>       somehow in the kernel module) would be a big no-no when it comes
>       to
>       KVM/qemu live migrations, seeing how the native KVM disk caching
>       also
>       needs to be disabled for it to work.
> 
> 
> I'm not sure about this, as I'm not sure what KVM requires here. The kernel
> client is safe for the named systems, period, though -- it looks and
> behaves just like a normal block device. The problem with the userspace
> cache is that it makes the local node behave the same way as a disk's
> RAM cache would -- so if you tell something to be durable, it will be, but
> you don't get coherency between different mounts because tr cache is all
> local.

This is safe.  My understanding qemu issues a flush (or is detached, or 
something) during the migration process so the cache is written back to 
rados before things flip over.  (As an aside, a flush by the guest kernel 
also triggers an rbd flush, so rbd with caching is fully crash safe too, 
just like the cache found in a SATA/SAS disk).

sage


> -Greg
>  
> 
>       Am I correct in that assumption?
> 
>       Regards,
> 
>       Christian
>       --
>       Christian Balzer        Network/Systems Engineer
>       chibi@xxxxxxx        Global OnLine Japan/Fusion Communications
>       http://www.gol.com/
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>       ceph-users mailing list
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> 
> 
> 
> --
> Software Engineer #42 @ http://inktank.com | http://ceph.com
> 
> 
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