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

[Wolfgang Richter]

On Wed, Apr 24, 2013 at 4:37 AM, Stefan Hajnoczi <address@hidden> 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