On Mon, 26 Sep 2016, Michele Brodoloni wrote: > Hi Eric, > thank you for your reply. > > The shared storage will have LVM on top and each node will have RW > access only to its own LVs (actually, these will be the virtual disks of > the vms running on each node), this is meant to say that each node is > likely going to access different data -- unless a vm is migrated between > nodes (??). Does this change anything? You will still have cache coherency issues between nodes. This is possible if you never migrate and keep per-node volumes isolated, but you would need to detach the cache on both sides before migrating (if writeback) or at least on the destination node (if write-through) and re-attach after migration. If you do need to migrate (or if a node fails and you must boot from another), that could be rather inconvenient and may affect other VMs as well. > Unfortunately, exporting luns via iSCSI/AoE is not an option since I'm > going to use a DELL VRTX and the storage is provided by a DAS, where > there will be both rotational and solid-state disks. It sounds like you need some kind of distributed coherency layer atop of your backend storage. -Eric > > Regards, > Michele > > > > > Il giorno 26/set/2016, alle ore 00:47, Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx> ha scritto: > > > >> On Sun, 25 Sep 2016, Michele wrote: > >> > >> Hello everybody, > >> > >> I have a KVM virtualization cluster (3 nodes) which shares the same > >> storage. It would be possible to attach a caching device on each node to > >> that storage? > >> > >> Let me explain better: I intend to create a raid-0 with two SSDs on each > >> node as WT caching devices, then initialize the shared storage as bcache > >> backing device, and then attach the caching device on each node to it. > >> > >> Is this going to work? > > > > No. Even in writethrough mode, remote writes of one node would hit the > > lower layer creating stale writethrough caches on the other nodes---that > > is unless you would like to implement distributed cache coherency using > > DLM. That would be quite interesting! > > > > You could share bcache0 to multiple nodes if you control its export via > > iSCSI or AoE and place a shared filesystem (gfs, ocfs, cLVM, etc) atop of > > the exported volume. I'm not sure if it has been tested, but it would be > > neat to get feadback on the subject. > > > > -- > > Eric Wheeler > > > > > >> > >> > >> Thanks, > >> Michele > >> > >> > >> -- > >> Newsy for iOS > >> Download on the App Store: > >> https://itunes.apple.com/app/newsy/id598711199 > >> > >> > >> > >> > >> -- > >> To unsubscribe from this list: send the line "unsubscribe linux-bcache" in > >> the body of a message to majordomo@xxxxxxxxxxxxxxx > >> More majordomo info at http://vger.kernel.org/majordomo-info.html > >> > -- > To unsubscribe from this list: send the line "unsubscribe linux-bcache" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
