Il 13/02/2013 01:03, Alex Bligh ha scritto: >> > Obviously the changelog was inadequate. Please send along a new one >> > which fully describes the reasons for this change. > To be clear I have no complaints about the rest of the patch being > merged. Supporting FLUSH but not FUA is far better than supporting > neither. I just didn't understand dropping FUA given the semantics > of nbd is in essence 'linux bios over tcp'.
Not really bios, since it expects FLUSH requests to include no I/O, but yes the semantics of NBD (and virtio-blk) are quite close to those of the Linux block layer. But as far as I can test with free servers, the FUA bits have no advantage over flush. Also, I wasn't sure if SEND_FUA without SEND_FLUSH is valid, and if so how to handle this combination (treat it as writethrough and add FUA to all requests? warn and do nothing?). Andrew, here is a better commit message: ------------ 8< --------------- From: Alex Bligh <a...@alex.org.uk> Subject: nbd: support FLUSH requests 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 discard failures. Signed-off-by: Alex Bligh <a...@alex.org.uk> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> Cc: Paul Clements <paul.cleme...@steeleye.com> Signed-off-by: Andrew Morton <a...@linux-foundation.org> --- Paolo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
