On Sat, Jun 11, 2011 at 08:41:38AM +0300, Amir G. wrote: > On Fri, Jun 10, 2011 at 1:11 PM, Joe Thornber <thornber@xxxxxxxxxx> wrote: > > On Fri, Jun 10, 2011 at 11:01:41AM +0200, Lukas Czerner wrote: > >> On Fri, 10 Jun 2011, Amir G. wrote: > >> > >> > CC'ing lvm-devel and fsdevel > >> > > >> > > >> > On Wed, Jun 8, 2011 at 9:26 PM, Amir G. <amir73il@xxxxxxxxxxxxxxxxxxxxx> wrote: > >> > For the sake of letting everyone understand the differences and trade > >> > offs between > >> > LVM and ext4 snapshots, so ext4 snapshots can get a fair trial, I need > >> > to ask you > >> > some questions about the implementation, which I could not figure out by myself > >> > from reading the documents. > > > > First up let me say that I'm not intending to support writeable > > _external_ origins with multisnap. This will come as a suprise to > > many people, but I don't think we can resolve the dual requirements to > > efficiently update many, many snapshots when a write occurs _and_ make > > those snapshots quick to delete (when you're encouraging people to > > take lots of snapshots performance of delete becomes a real issue). > > > > If I understand this article correctly: > http://people.redhat.com/mpatocka/papers/shared-snapshots.pdf > It says that _external_ origin write updates can be efficient to readonly > (or not written) snapshots. > > Could you not support readonly snapshots of an _external_ origin? Yes, that is the intention, and very little work to add. We just do something different if the metadata lookup returns -ENODATA. Above I said I didn't intend to support _writeable_ external snaps. Readable ones are a must, for instance for supporting virtual machine base images. > You could even support writable snapshots, that will degrade write > performance to origin temporarily. > It can be useful, if one wants to "try-out" mounting a temporary > writable snapshot, when the origin is not even mounted. > After the "try-out", the temporary snapshot can be deleted > and origin write performance would go back to normal. Not sure what you're getting at here. All snapshots are writeable. Of course you can take a snapshot of an external origin and then use this as your temporary origin for experiments. If the origin is itself a dm device then LVM can shuffle tables around to make this transparent. The user may want to commit to their experiment at a later time by merging back to the external origin. This involves copying, but no more than a copy-on-write scheme. Arguably it's better to do the copy only once we know they want to commit to it. - Joe -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
