From patchwork Tue Apr 5 12:07:45 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Alex Bligh X-Patchwork-Id: 8750691 Return-Path: X-Original-To: patchwork-qemu-devel@patchwork.kernel.org Delivered-To: patchwork-parsemail@patchwork2.web.kernel.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.136]) by patchwork2.web.kernel.org (Postfix) with ESMTP id 3F367C0553 for ; Tue, 5 Apr 2016 12:08:02 +0000 (UTC) Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 2972D20204 for ; Tue, 5 Apr 2016 12:08:01 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id F35FF20117 for ; Tue, 5 Apr 2016 12:07:59 +0000 (UTC) Received: from localhost ([::1]:36730 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1anPm2-0001ig-TP for patchwork-qemu-devel@patchwork.kernel.org; Tue, 05 Apr 2016 08:07:58 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45739) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1anPlv-0001iD-75 for qemu-devel@nongnu.org; Tue, 05 Apr 2016 08:07:52 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1anPls-0005Xy-Cv for qemu-devel@nongnu.org; Tue, 05 Apr 2016 08:07:51 -0400 Received: from mail.avalus.com ([2001:41c8:10:1dd::10]:52270) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1anPls-0005Xo-42 for qemu-devel@nongnu.org; Tue, 05 Apr 2016 08:07:48 -0400 Received: by mail.avalus.com (Postfix) with ESMTPSA id 22999C560A7; Tue, 5 Apr 2016 13:07:47 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=alex.org.uk; s=mail; t=1459858067; bh=VHgsJd9yu4Xz2tIPjf/oyuPc5GtxTBISt/HRhsfFiCs=; h=From:To:Cc:Subject:Date; b=3LgMQy/CK61ZfxIqlcao4qvwOcWBDH8i7I8VFfE7f9bMBzj5xt5zRuKE+52+l6IkA 2vZ+C9949qAi45Wgy00teAZ8haXKA03pBf/683gDbgugMHw8UHYDzwjRWxF4M+BRR1 vILutQTxF1UcmvghSzSpYnhSCPyj66ddtFNNaKcQ= From: Alex Bligh To: Eric Blake , Wouter Verhelst Date: Tue, 5 Apr 2016 13:07:45 +0100 Message-Id: <1459858065-13348-1-git-send-email-alex@alex.org.uk> X-Mailer: git-send-email 1.9.1 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x X-Received-From: 2001:41c8:10:1dd::10 Cc: "nbd-general@lists.sourceforge.net" , "qemu-devel@nongnu.org" , Alex Bligh Subject: [Qemu-devel] [PATCHv3] Improve the documentation of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA. X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org X-Spam-Status: No, score=-6.8 required=5.0 tests=BAYES_00,DKIM_SIGNED, RCVD_IN_DNSWL_HI, T_DKIM_INVALID, UNPARSEABLE_RELAY autolearn=ham version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Improve the documentation of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA. Specifically the latter may be set on any command, and its semantics on commands other than NBD_CMD_WRITE need explaining. Further, explain how these relate to reordering of commands. Signed-off-by: Alex Bligh --- doc/proto.md | 51 +++++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 41 insertions(+), 10 deletions(-) ARRRGH - this time with a proper commit header. Changes since version 2: * Rebase on master * Remove bogus 'SHOULD' for FLUSH in relation to writes that are in flight but not yet completed * After consultation with lkml etc., document that FUA on things that do not write does nothing * Document that sending FUA for commands that do nothing is permissible, but 'SHOULD NOT' be done; an existing client does this. * Document that FUA on TRIM should do something after all, per Kevin Wolf's comment I'm hoping this is now complete. diff --git a/doc/proto.md b/doc/proto.md index 35a3266..d7d9939 100644 --- a/doc/proto.md +++ b/doc/proto.md @@ -217,6 +217,33 @@ handle as was sent by the client in the corresponding request. In this way, the client can correlate which request is receiving a response. +#### Ordering of messages and writes + +The server MAY process commands out of order, and MAY reply out of +order, save that: + +* All write commands (that includes both `NBD_CMD_WRITE` and + `NBD_CMD_TRIM`) that the server completes (i.e. replies to) + prior to processing to a `NBD_CMD_FLUSH` MUST be written to non-volatile + storage prior to replying to that `NBD_CMD_FLUSH`. This + paragraph only applies if `NBD_FLAG_SEND_FLUSH` is set within + the transmission flags, as otherwise `NBD_CMD_FLUSH` will never + be sent by the client to the server. + +* A server MUST NOT reply to a command that has `NBD_CMD_FLAG_FUA` set + in its command flags until the data (if any) written by that command + is persisted to non-volatile storage. This only applies if + `NBD_FLAG_SEND_FLUSH` is set within the transmission flags, as otherwise + `NBD_CMD_FLAG_FUA` will not be set on any commands sent to the server + by the client. + +`NBD_CMD_FLUSH` is modelled on the Linux kernel empty bio with +`REQ_FLUSH` set. `NBD_CMD_FLAG_FUA` is modelled on the Linux +kernel bio with `REQ_FUA` set. In case of ambiguity in this +specification, the +[kernel documentation](https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt) +may be useful. + #### Request message The request message, sent by the client, looks as follows: @@ -483,10 +510,20 @@ affects a particular command. Clients MUST NOT set a command flag bit that is not documented for the particular command; and whether a flag is valid may depend on negotiation during the handshake phase. -- bit 0, `NBD_CMD_FLAG_FUA`; valid during `NBD_CMD_WRITE` and - `NBD_CMD_WRITE_ZEROES` commands. SHOULD be set to 1 if the client requires - "Force Unit Access" mode of operation. MUST NOT be set unless transmission - flags included `NBD_FLAG_SEND_FUA`. +- bit 0, `NBD_CMD_FLAG_FUA`; This flag is valid for all commands provided + `NBD_FLAG_SEND_FUA` has been negotiated, in which case the server MUST + accept all commands with this bit set (even by ignoring the bit). The + client SHOULD NOT set this bit unless the command has the potential of + writing data (current commands are `NBD_CMD_WRITE`, `NBD_CMD_WRITE_ZEROES` + and `NBD_CMD_TRIM`); existing clients are known to set this bit on + other commands; subject to that, provided `NBD_FLAG_SEND_FUA` is + negotiated, the client MAY set this bit as it wishes. If the server + receives a command with `NBD_CMD_FLAG_FUA` set it MUST NOT send its + reply to that command until all write operations (if any) associated with + that command command have been completed and persisted to non-volatile + storage. If the command does not in fact write data (for instance on an + `NBD_CMD_TRIM` which does is ignored), the server MAY ignore this bit + being set on such a command. - bit 1, `NBD_CMD_NO_HOLE`; defined by the experimental `WRITE_ZEROES` extension; see below. - bit 2, `NBD_CMD_FLAG_DF`; defined by the experimental `STRUCTURED_REPLY` @@ -535,12 +572,6 @@ The following request types exist: message. The server MAY send the reply message before the data has reached permanent storage. - If the `NBD_FLAG_SEND_FUA` flag ("Force Unit Access") was set in the - transmission flags field, the client MAY set the flag `NBD_CMD_FLAG_FUA` in - the command flags field. If this flag was set, the server MUST NOT send - the reply until it has ensured that the newly-written data has reached - permanent storage. - If an error occurs, the server SHOULD set the appropriate error code in the error field. The server MAY then close the connection.