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75f187aba5
Currently, the NBD device does not accept flush requests from the Linux block layer. If the NBD server opened the target with neither O_SYNC nor O_DSYNC, however, the device will be effectively backed by a writeback cache. Without issuing flushes properly, operation of the NBD device will not be safe against power losses. The NBD protocol has support for both a cache flush command and a FUA command flag; the server will also pass a flag to note its support for these features. This patch adds support for the cache flush command and flag. In the kernel, we receive the flags via the NBD_SET_FLAGS ioctl, and map NBD_FLAG_SEND_FLUSH to the argument of blk_queue_flush. When the flag is active the block layer will send REQ_FLUSH requests, which we translate to NBD_CMD_FLUSH commands. FUA support is not included in this patch because all free software servers implement it with a full fdatasync; thus it has no advantage over supporting flush only. Because I [Paolo] cannot really benchmark it in a realistic scenario, I cannot tell if it is a good idea or not. It is also not clear if it is valid for an NBD server to support FUA but not flush. The Linux block layer gives a warning for this combination, the NBD protocol documentation says nothing about it. The patch also fixes a small problem in the handling of flags: nbd->flags must be cleared at the end of NBD_DO_IT, but the driver was not doing that. The bug manifests itself as follows. Suppose you two different client/server pairs to start the NBD device. Suppose also that the first client supports NBD_SET_FLAGS, and the first server sends NBD_FLAG_SEND_FLUSH; the second pair instead does neither of these two things. Before this patch, the second invocation of NBD_DO_IT will use a stale value of nbd->flags, and the second server will issue an error every time it receives an NBD_CMD_FLUSH command. This bug is pre-existing, but it becomes much more important after this patch; flush failures make the device pretty much unusable, unlike Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Alex Bligh <alex@alex.org.uk> Acked-by: Paul Clements <Paul.Clements@steeleye.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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