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authorKevin Wolf <kwolf@redhat.com>2018-10-05 18:57:40 +0200
committerKevin Wolf <kwolf@redhat.com>2018-11-05 15:09:55 +0100
commite35bdc123a4ace9f4d3fccaaf88907014e2438cd (patch)
treef719625423098fd2fdb5aff5d451fbc5a0d0fa62 /include
parenteeae6a596b0efc092f5101c67683053e245e6250 (diff)
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>
Diffstat (limited to 'include')
-rw-r--r--include/block/block.h2
1 files changed, 2 insertions, 0 deletions
diff --git a/include/block/block.h b/include/block/block.h
index b189cf422e..580b3716c3 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -115,6 +115,7 @@ typedef struct HDGeometry {
select an appropriate protocol driver,
ignoring the format layer */
#define BDRV_O_NO_IO 0x10000 /* don't initialize for I/O */
+#define BDRV_O_AUTO_RDONLY 0x20000 /* degrade to read-only if opening read-write fails */
#define BDRV_O_CACHE_MASK (BDRV_O_NOCACHE | BDRV_O_NO_FLUSH)
@@ -125,6 +126,7 @@ typedef struct HDGeometry {
#define BDRV_OPT_CACHE_DIRECT "cache.direct"
#define BDRV_OPT_CACHE_NO_FLUSH "cache.no-flush"
#define BDRV_OPT_READ_ONLY "read-only"
+#define BDRV_OPT_AUTO_READ_ONLY "auto-read-only"
#define BDRV_OPT_DISCARD "discard"
#define BDRV_OPT_FORCE_SHARE "force-share"