qemu-e2k/include/block
Kevin Wolf e35bdc123a block: Add auto-read-only option
If a management application builds the block graph node by node, the
protocol layer doesn't inherit its read-only option from the format
layer any more, so it must be set explicitly.

Backing files should work on read-only storage, but at the same time, a
block job like commit should be able to reopen them read-write if they
are on read-write storage. However, without option inheritance, reopen
only changes the read-only option for the root node (typically the
format layer), but not the protocol layer, so reopening fails (the
format layer wants to get write permissions, but the protocol layer is
still read-only).

A simple workaround for the problem in the management tool would be to
open the protocol layer always read-write and to make only the format
layer read-only for backing files. However, sometimes the file is
actually stored on read-only storage and we don't know whether the image
can be opened read-write (for example, for NBD it depends on the server
we're trying to connect to). This adds an option that makes QEMU try to
open the image read-write, but allows it to degrade to a read-only mode
without returning an error.

The documentation for this option is consciously phrased in a way that
allows QEMU to switch to a better model eventually: Instead of trying
when the image is first opened, making the read-only flag dynamic and
changing it automatically whenever the first BLK_PERM_WRITE user is
attached or the last one is detached would be much more useful
behaviour.

Unfortunately, this more useful behaviour is also a lot harder to
implement, and libvirt needs a solution now before it can switch to
-blockdev, so let's start with this easier approach for now.

Instead of adding a new auto-read-only option, turning the existing
read-only into an enum (with a bool alternate for compatibility) was
considered, but it complicated the implementation to the point that it
didn't seem to be worth it.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
2018-11-05 15:09:55 +01:00
..
accounting.h block/accounting: introduce latency histogram 2018-03-19 14:58:37 -05:00
aio-wait.h block: Use a single global AioWait 2018-09-25 15:50:15 +02:00
aio.h qemu-timer: introduce timer attributes 2018-10-19 13:44:03 +02:00
block_backup.h backup: Switch block_backup.h to byte-based 2017-07-10 13:18:06 +02:00
block_int.h dirty-bitmap: rename bdrv_undo_clear_dirty_bitmap 2018-10-29 16:23:14 -04:00
block.h block: Add auto-read-only option 2018-11-05 15:09:55 +01:00
blockjob_int.h block: Really pause block jobs on drain 2018-06-18 15:03:25 +02:00
blockjob.h block: Use a single global AioWait 2018-09-25 15:50:15 +02:00
dirty-bitmap.h dirty-bitmaps: clean-up bitmaps loading and migration logic 2018-10-29 16:23:17 -04:00
nbd.h nbd: fix NBD_FLAG_SEND_CACHE value 2018-10-04 09:08:56 -05:00
nvme.h block: Move NVMe constants to a separate header 2018-02-08 09:22:03 +08:00
qapi.h Drop superfluous includes of qapi-types.h and test-qapi-types.h 2018-02-09 05:05:11 +01:00
qdict.h block: Factor out qobject_input_visitor_new_flat_confused() 2018-06-15 14:49:44 +02:00
raw-aio.h file-posix: Make .bdrv_co_truncate asynchronous 2018-06-29 14:20:56 +02:00
snapshot.h Move include qemu/option.h from qemu-common.h to actual users 2018-02-09 13:52:16 +01:00
thread-pool.h Clean up decorations and whitespace around header guards 2016-07-12 16:20:46 +02:00
throttle-groups.h block: add throttle block filter driver 2017-09-06 10:12:02 +02:00
write-threshold.h Use scripts/clean-includes to drop redundant qemu/typedefs.h 2016-03-22 22:20:16 +01:00