Hmmm, maybe, but rsync would take a lot of time to find the changes. the actual blocks of a snap _are_ the changes, that's why SnapMirror is very efficient. And, I don't see how rsync will retain the snap's between both sites. It would be great if a tool like rsync could have access to the changed blocks in a snap. Don't know if btrfs exposes these somehow. Fred On Tue, Aug 31, 2010 at 12:56 AM, K. Richard Pixley <rich@xxxxxxxx> wrote: > If you can put the db into a consistent state, then rsync will do this. > Rsync does changed block transfers. > > --rich > > On 8/30/10 14:15 , Fred van Zwieten wrote: >> >> I just glanced over the DRBD/LVM combi, but I don't see it being >> functionally equal to SnapMirror. Let me (try to) explain how >> snapmirror works: >> >> On system A there is a volume (vol1). We let this vol1(A) replicate >> thru SnapMirror to vol1(B). This is done by creating a snap vol1sx(A) >> and replicate all changed blocks between this snapshot (x) and the >> previous snapshot (x-1). The first time, there is no x-1 and the whole >> volume will be replicated, but after this initial "full copy", only >> the changed blocks between the two snapshot's are being replicated to >> system B. This is also called snap based replication. Why we want >> this? Easy. To support consistent DB snap's. The proces works by first >> putting the DB in a consistent mode (depends on DB implementation), >> create a snapshot, let the DB continue, replicate the changes. This >> way a DB consistent state will be replicated. The cool thing about the >> NetApp implementation is that on system B the snap's (x, x-1, x-2, >> etc) are also available. When there is trouble, you can choose to >> online the DB on system B on any of the snap's, or, even cooler, to >> replicate one of those snap's back to system A, doing a block based >> rollback at the filesystem level. >> >> Fred >> >> On Mon, Aug 30, 2010 at 7:55 PM, K. Richard Pixley<rich@xxxxxxxx> wrote: >>> >>> On 20100830 10:07, Fred van Zwieten wrote: >>>> >>>> Hi there, >>>> >>>> I would like to know if there is something functionally equivalent to >>>> NetApp's SnapMirror in the works or planning? It would require block >>>> level access to a snap and the ability to rebuild (subvolumes >>>> including it's) snap's on another machine. >>>> >>>> If not, what would be the best way to build something more or less >>>> equivalent using existing tools? rsync-ing a snap seems the same, but >>>> it isn't. First of all it 's file based, not very nice for DB's, and >>>> you don't get the snap's on "the other side" the same. >>>> >>>> Fred >>> >>> I think drbd does precisely what you want. >>> >>> It's not useful for fault tolerance, nor for load balancing, but it will >>> produce a remote block copy that can be used as a sort of "hot backup". >>> >>> You can also do something very similar by combining LVM, (the logical >>> volume >>> manager), with LVM snapshots and NBD, (the network block device) by >>> mirroring to an NBD device. >>> >>> Neither of these approaches can tolerate the remote file system being >>> "live" >>> until and unless it takes over for the primary. But either can maintain >>> a >>> dynamic remote block device. >>> >>> --rich >>> > -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
