Il 24/04/2013 10:37, Stefan Hajnoczi ha scritto: >> > Has there been any performance analysis of drive-mirror (impact on >> > executing guest)?
What Stefan wrote is about block-backup. drive-mirror has a limited impact on guest performance, but it doesn't pass the writes through to the channel. Instead, it uses a dirty bitmap that it periodically scans to copy new data to the destination. > 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. > > 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. Right. However, when block-backup gets in, I will try to change drive-mirror to use an "active" method. I don't have a timeframe for this, though. Paolo
