Re: [Qemu-devel] Adding Disk-Level Introspection to QEMU

[Paolo Bonzini]

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