block: Fix how mirror_run() frees its buffer

It allocates with qemu_blockalign(), therefore it must free with
qemu_vfree(), not g_free().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
mirror: perform COW if the cluster size is bigger than the granularity

When mirroring runs, the backing files for the target may not yet be
ready.  However, this means that a copy-on-write operation on the target
would fill the missing sectors with zeros.  Copy-on-write only happens
if the granularity of the dirty bitmap is smaller than the cluster size
(and only for clusters that are allocated in the source after the job
has started copying).  So far, the granularity was fixed to 1MB; to avoid
the problem we detected the situation and required the backing files to
be available in that case only.

However, we want to lower the granularity for efficiency, so we need
a better solution.  The solution is to always copy a whole cluster the
first time it is touched.  The code keeps a bitmap of clusters that
have already been allocated by the mirroring job, and only does "manual"
copy-on-write if the chunk being copied is zero in the bitmap.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
block: return count of dirty sectors, not chunks

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
block: allow customizing the granularity of the dirty bitmap

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
mirror: allow customizing the granularity

The desired granularity may be very different depending on the kind of
operation (e.g. continuous replication vs. collapse-to-raw) and whether
the VM is expected to perform lots of I/O while mirroring is in progress.

Allow the user to customize it, while providing a sane default so that
in general there will be no extra allocated space in the target compared
to the source.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
mirror: switch mirror_iteration to AIO

There is really no change in the behavior of the job here, since
there is still a maximum of one in-flight I/O operation between
the source and the target.  However, this patch already introduces
the AIO callbacks (which are unmodified in the next patch)
and some of the logic to count in-flight operations and only
complete the job when there is none.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
mirror: add buf-size argument to drive-mirror

This makes sense when the next commit starts using the extra buffer space
to perform many I/O operations asynchronously.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
mirror: support more than one in-flight AIO operation

With AIO support in place, we can start copying more than one chunk
in parallel.  This patch introduces the required infrastructure for
this: the buffer is split into multiple granularity-sized chunks,
and there is a free list to access them.

Because of copy-on-write, a single operation may already require
multiple chunks to be available on the free list.

In addition, two different iterations on the HBitmap may want to
copy the same cluster.  We avoid this by keeping a bitmap of in-flight
I/O operations, and blocking until the previous iteration completes.
This should be a pretty rare occurrence, though; as long as there is
no overlap the next iteration can start before the previous one finishes.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
mirror: support arbitrarily-sized iterations

Yet another optimization is to extend the mirroring iteration to include more
adjacent dirty blocks.  This limits the number of I/O operations and makes
mirroring efficient even with a small granularity.  Most of the infrastructure
is already in place; we only need to put a loop around the computation of
the origin and sector count of the iteration.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>