[sheepdog-users] Sheepdog image fills only one node

[Alexandre DERUMIER]

Hi,
thanks for the explain, I was also testing sheepdog and having same problem.

BTW, if sheepdog is not on same host that kvm, the writeback cache is useless,
as the cache is on the sheepdog node and not kvm node.
the wiki is not very clear about this case.

----- Mail original ----- 

De: "Christoph Hellwig" <hch at infradead.org> 
À: "Liu Yuan" <namei.unix at gmail.com> 
Cc: sheepdog-users at lists.wpkg.org 
Envoyé: Lundi 2 Juillet 2012 09:18:57 
Objet: Re: [sheepdog-users] Sheepdog image fills only one node 

On Sat, Jun 30, 2012 at 12:53:13AM +0800, Liu Yuan wrote: 
> On 06/29/2012 11:05 PM, Stefan Priebe - Profihost AG wrote: 
> > # kvm ... -drive 
> > file=sheepdog:10.0.255.100:7000:testsheep,if=none,id=drive-virtio0,cache=writeback,aio=native 
> > -device virtio-blk-pci,drive=drive-virtio0,id=virtio0 
> 
> cache=writeback enables object cache. See wiki about object cache, which 
> only flush dirty bits to the cluster. 

This is just another reason why enabling the object cache by defaul is 
wrong. In my opinion making sure it is not enabled by defauly is a high 
priority for the 0.4.0 release. 

Reasons: 

- While qemu still defaults to cache=writethrough all management tools 
that people actually use (most importantly libvirt) change that to 
cache=none 
- With cache=none the new sheepdog version will get semantics that 
people absolutely do not expect from a distributed block storage 
system: 
(1) data is not striped over different nodes for the actual 
write, thus not getting any scale out for big streaming 
writes 
(2) data is not written back to the cluster until a cache 
flush happens, thus causing havoc with restarting a 
VM on a different node when one node crashes 

That beeing said I really like the object cache for some specific 
workloads, mostly in complete read-only mode for VDI COW base images, 
and even in write mode for cloud deployments like as a replacement 
for the default openstack semantics where images get downloaded to 
a local host and exectuted there. But to make them useful for these 
use cases the cache needs to default to off, and there needs to be a 
sheep-side configuration to enable it for each VDI. A good reason for 
that is for example when we to enable it for the base image but not 
the overlay which isn't even possible from qemu even if we wanted to 
go through all the hoops instead of making things work out of the box. 

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