On Tue, Nov 18, 2014 at 1:43 PM, hp cre <hpcre1@xxxxxxxxx> wrote: > Ok thanks Greg. > But what openstack does, AFAIU, is use rbd devices directly, one for each > Vm instance, right? And that's how it supports live migrations on KVM, > etc.. Right? Openstack and similar cloud frameworks don't need to create vm > instances on filesystems, am I correct? Right; these systems are doing the cache coherency (by duplicating all the memory, including that of ext4/whatever) so that they work. -Greg > > On 18 Nov 2014 23:33, "Gregory Farnum" <greg@xxxxxxxxxxx> wrote: >> >> On Tue, Nov 18, 2014 at 1:26 PM, hp cre <hpcre1@xxxxxxxxx> wrote: >> > Hello everyone, >> > >> > I'm new to ceph but been working with proprietary clustered filesystem >> > for >> > quite some time. >> > >> > I almost understand how ceph works, but have a couple of questions >> > which >> > have been asked before here, but i didn't understand the answer. >> > >> > In the closed source world, we use clustered filesystems like Veritas >> > clustered filesystem to mount a shared block device (using San) to more >> > than >> > one compute node concurrently for shared read/write. >> > >> > What I can't seem to get a solid and clear answer for its this.. >> > How can I use ceph to do the same thing? Can RADOS guarantee coherency >> > and >> > integrity of my data if I use an rbd device with any filesystem on top >> > of >> > it? Or must I still use a cluster aware filesystem such as vxfs or >> > ocfs? >> >> RBD behaves just like a regular disk if you mount it to multiple nodes >> at once (although you need to disable the client caching). This means >> that the disk accesses will be coherent, but using ext4 on top of it >> won't work because ext4 assumes it is the only accessor — you have to >> use a cluster-aware FS like ocfs2. A SAN would have the same problem >> here, so I'm not sure why you think it works with them... >> >> >> > And is CephFS going to some this problem? Or does it not have support >> > for >> > concurrent read/write access among all now mounting it? >> >> CephFS definitely does support concurrent access to the same data. >> >> > And, does iscsi targets over rbd devices behave the same? >> >> Uh, yes, iSCSI over rbd will be the same as regular RBD in this >> regard, modulo anything the iSCSI gateway might be set up to do. >> -Greg _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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