[Qemu-devel] [RFC 0/6] block: generic copy-on-read

[Stefan Hajnoczi]

The new -drive copy-on-read=on|off feature populates the image file with data
from the backing file on read.  This is useful when accessing images backed
over a slow medium (e.g. http over internet).  All read data will be stored in
the local image file so it does not need to be fetched again in the future.

This series is a prerequisite for the image streaming feature, which uses
copy-on-read to populate the image file in the background while the VM is
running.  However, the copy-on-read feature is useful on its own.

Copy-on-read is implemented by checking whether or not data is allocated in the
image file before reading it.  If data is not allocated then it needs to be
read and written back to the image file.

The tricky bit is avoiding races with other I/O requests.  These patches add
request tracking to BlockDriverState so that the list of pending requests is
available.  Copy-on-read prevents races by serializing overlapping requests.

Finally, there is a performance impact when enabling this feature since an
additional write is performed.  Serializing overlapping requests also means
that I/O patterns where multiple requests access the same cluster will see a
loss in parallelism.  Perhaps we can be smarter about preventing corruption in
the future and win back some performance.

Git repo:
http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/generic-copy-on-read

Stefan Hajnoczi (6):
  block: add request tracking
  block: add bdrv_set_copy_on_read()
  block: wait for overlapping requests
  block: request overlap detection
  block: core copy-on-read logic
  block: add -drive copy-on-read=on|off

 block.c         |  243 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 block.h         |    3 +
 block_int.h     |    9 ++
 blockdev.c      |    6 ++
 hmp-commands.hx |    5 +-
 qemu-config.c   |    4 +
 qemu-options.hx |    9 ++-
 trace-events    |    1 +
 8 files changed, 276 insertions(+), 4 deletions(-)

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1.7.6.3