On Sat, Jun 11, 2011 at 10:35 AM, Joe Thornber <thornber@xxxxxxxxxx> wrote: > 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. > I meant _readonly_ snapshots of a _writable_ _external_ origin, which is what ext4 snapshots provides. All snapshots are chained on a list that points to the origin and only the latest (active) snapshot metadata get updated on origin writes. When older snapshots lookup return -ENODATA, you go up the list to the newer snapshot and up to the origin. Those _incremental_ snapshots cannot be _writable_, because older snapshots may implicitly share blocks with newer snapshots, but it should be possible to make _writable_ clones of these snapshots. Not sure what the implications are for deleting snapshots, because I am not familiar with all the implementation details of multisnap. > 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-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
