On Wed, Apr 24, 2013 at 4:37 AM, Stefan Hajnoczi <stefa...@gmail.com> wrote:
> > Has there been any performance analysis of drive-mirror (impact on > executing guest)? > > It slows down guest I/O for a couple of reasons: > > 1. Writes now require a read from the original device followed by a > write to the target device. Only after this completes is the write > allowed to proceed. > > 2. Overlapping read/write requests are serialized to maintain > consistency between the guests I/Os and the block-backup I/Os. > Makes sense, #2 is what I want/need (I don't care about the original data). > But on second thought, I don't think block-backup fits the bill. You > don't care about the original data, you care about what new data the > guest is writing. > Precisely. I crawl and index original data before we start getting the live stream of new data/writes. > I think what you really want is a "tap" block driver which mirrors > writes to a target device (typically a NBD volume). You can model this > on blkverify or check out Benoit Canet's quorum patches. > Something like this, or live replication via drive-mirror which implements #2. -- Wolf
