From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44120) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1altkI-0006uC-Bh for qemu-devel@nongnu.org; Fri, 01 Apr 2016 03:43:55 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1altkF-0004gT-6e for qemu-devel@nongnu.org; Fri, 01 Apr 2016 03:43:54 -0400 Received: from mail-wm0-x242.google.com ([2a00:1450:400c:c09::242]:36460) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1altkE-0004gF-VQ for qemu-devel@nongnu.org; Fri, 01 Apr 2016 03:43:51 -0400 Received: by mail-wm0-x242.google.com with SMTP id 20so2455048wmh.3 for ; Fri, 01 Apr 2016 00:43:50 -0700 (PDT) Sender: Paolo Bonzini References: <56FD7B7E.4060004@redhat.com> <64B326DA-CDF4-4537-B38A-46E7B57C319C@alex.org.uk> <56FD8069.7020101@redhat.com> From: Paolo Bonzini Message-ID: <56FE26B2.6090903@redhat.com> Date: Fri, 1 Apr 2016 09:43:46 +0200 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [Nbd] Is NBD_CMD_FLAG_FUA valid during NBD_CMD_FLUSH? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alex Bligh , Eric Blake Cc: "nbd-general@lists.sourceforge.net" , "qemu-devel@nongnu.org" On 31/03/2016 22:17, Alex Bligh wrote: >> > In qemu, read+FUA just triggers blk_co_flush() prior to reading; but >> > that's the same function it calls for write+FUA. > That's harmless, but unnecessary in the sense that current documented > behaviour doesn't require it. Perhaps it should? > > I suppose TRIM etc. should support FUA too? TRIM is an advisory operation, so it doesn't make sense to force access to the medium. The closest you could get would be to add FUA to WRITE_ZEROES. But since WRITE_ZEROES is not a particularly common operation there isn't much to gain compared to FLUSHing after the write has completed; in fact SCSI doesn't have a FUA bit on its WRITE SAME command. Paolo