On Tue, Jul 5, 2011 at 7:18 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > On Tue, Jul 05, 2011 at 04:37:08PM +0100, Stefan Hajnoczi wrote: >> On Tue, Jul 5, 2011 at 3:32 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: >> > On Tue, Jul 05, 2011 at 04:39:06PM +0300, Dor Laor wrote: >> >> On 07/05/2011 03:58 PM, Marcelo Tosatti wrote: >> >> >On Tue, Jul 05, 2011 at 01:40:08PM +0100, Stefan Hajnoczi wrote: >> >> >>On Tue, Jul 5, 2011 at 9:01 AM, Dor Laor<dlaor@xxxxxxxxxx> wrote: >> >> >>>I tried to re-arrange all of the requirements and use cases using this wiki >> >> >>>page: http://wiki.qemu.org/Features/LiveBlockMigration >> >> >>> >> >> >>>It would be the best to agree upon the most interesting use cases (while we >> >> >>>make sure we cover future ones) and agree to them. >> >> >>>The next step is to set the interface for all the various verbs since the >> >> >>>implementation seems to be converging. >> >> >> >> >> >>Live block copy was supposed to support snapshot merge. I think the >> >> >>current favored approach is to make the source image a backing file to >> >> >>the destination image and essentially do image streaming. >> >> >> >> >> >>Using this mechanism for snapshot merge is tricky. The COW file >> >> >>already uses the read-only snapshot base image. So now we cannot >> >> >>trivally copy the COW file contents back into the snapshot base image >> >> >>using live block copy. >> >> > >> >> >It never did. Live copy creates a new image were both snapshot and >> >> >"current" are copied to. >> >> > >> >> >This is similar with image streaming. >> >> >> >> Not sure I realize what's bad to do in-place merge: >> >> >> >> Let's suppose we have this COW chain: >> >> >> >> base <-- s1 <-- s2 >> >> >> >> Now a live snapshot is created over s2, s2 becomes RO and s3 is RW: >> >> >> >> base <-- s1 <-- s2 <-- s3 >> >> >> >> Now we've done with s2 (post backup) and like to merge s3 into s2. >> >> >> >> With your approach we use live copy of s3 into newSnap: >> >> >> >> base <-- s1 <-- s2 <-- s3 >> >> base <-- s1 <-- newSnap >> >> >> >> When it is over s2 and s3 can be erased. >> >> The down side is the IOs for copying s2 data and the temporary >> >> storage. I guess temp storage is cheap but excessive IO are >> >> expensive. >> >> >> >> My approach was to collapse s3 into s2 and erase s3 eventually: >> >> >> >> before: base <-- s1 <-- s2 <-- s3 >> >> after: base <-- s1 <-- s2 >> >> >> >> If we use live block copy using mirror driver it should be safe as >> >> long as we keep the ordering of new writes into s3 during the >> >> execution. >> >> Even a failure in the the middle won't cause harm since the >> >> management will keep using s3 until it gets success event. >> > >> > Well, it is more complicated than simply streaming into a new >> > image. I'm not entirely sure it is necessary. The common case is: >> > >> > base -> sn-1 -> sn-2 -> ... -> sn-n >> > >> > When n reaches a limit, you do: >> > >> > base -> merge-1 >> > >> > You're potentially copying similar amount of data when merging back into >> > a single image (and you can't easily merge multiple snapshots). >> > >> > If the amount of data thats not in 'base' is large, you create >> > leave a new external file around: >> > >> > base -> merge-1 -> sn-1 -> sn-2 ... -> sn-n >> > to >> > base -> merge-1 -> merge-2 >> > >> >> > >> >> >>It seems like snapshot merge will require dedicated code that reads >> >> >>the allocated clusters from the COW file and writes them back into the >> >> >>base image. >> >> >> >> >> >>A very inefficient alternative would be to create a third image, the >> >> >>"merge" image file, which has the COW file as its backing file: >> >> >>snapshot (base) -> cow -> merge >> > >> > Remember there is a 'base' before snapshot, you don't copy the entire >> > image. >> >> One use case I have in mind is the Live Backup approach that Jagane >> has been developing. Here the backup solution only creates a snapshot >> for the period of time needed to read out the dirty blocks. Then the >> snapshot is deleted again and probably contains very little new data >> relative to the base image. The backup solution does this operation >> every day. >> >> This is the pathalogical case for any approach that copies the entire >> base into a new file. We could have avoided a lot of I/O by doing an >> in-place update. >> >> I want to make sure this works well. > > This use case does not fit the streaming scheme that has come up. Its a > completly different operation. > > IMO it should be implemented separately. Okay, not everything can fit into this one grand unified block copy/image streaming mechanism :). Stefan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
