From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:44730) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UznNP-00033q-MT for qemu-devel@nongnu.org; Thu, 18 Jul 2013 08:32:12 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UznNO-0002Du-JQ for qemu-devel@nongnu.org; Thu, 18 Jul 2013 08:32:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:28648) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UznNO-0002Cz-Au for qemu-devel@nongnu.org; Thu, 18 Jul 2013 08:32:06 -0400 Message-ID: <51E7E035.3010702@redhat.com> Date: Thu, 18 Jul 2013 14:31:49 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <1373885375-13601-5-git-send-email-pl@kamp.de> <20130717084648.GD2458@dhcp-200-207.str.redhat.com> <51E66ACD.70706@redhat.com> <20130717102551.GF2458@dhcp-200-207.str.redhat.com> <51E6C5FC.1030304@redhat.com> <7C1EEB41-E2B3-4186-9188-379F02E76FF9@kamp.de> <51E6CE81.6000400@redhat.com> <36C25446-54C7-4D1F-9D8D-E8A3991489BD@kamp.de> <20130718092316.GG3582@dhcp-200-207.str.redhat.com> <51E7C260.50404@redhat.com> <51E7C707.7010101@kamp.de> <51E7C9C4.5010202@redhat.com> <51E7CBC8.1010804@kamp.de> In-Reply-To: <51E7CBC8.1010804@kamp.de> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Lieven Cc: Kevin Wolf , Stefan Hajnoczi , qemu-devel , ronnie sahlberg Il 18/07/2013 13:04, Peter Lieven ha scritto: >> But if you set BDRV_DISCARD_WRITE_ZEROES, then you always need a >> fallback to bdrv_write_zeroes. Why not just call bdrv_write_zeroes to >> begin with? That's why extending bdrv_write_zeroes is preferable. > In this case wo do not need a flag to the function at all. If the > driver sets bdi->write_zeroes_w_discard = 1 then bdrv_write_zeroes > can use bdrv_discard to write zeroes and the driver has to > ensure that all is zero afterwards. Peter, you removed exactly the part of the email where I explained the wrong part of your reasoning: you cannot do that [discard in bdrv_write_zeroes] unconditionally. Some operations can use it, some cannot. Think of SCSI disk emulation: it must not discard is WRITE SAME is sent without the UNMAP bit! > If the driver would have a better method of writing zeroes than > discard it simply should not set bdi->write_zeroes_w_discard = 1. If the driver had a better method of writing zeroes than discard, it simply should ignore the BDRV_MAY_UNMAP (or BDRV_MAY_DISCARD) flag in its bdrv_write_zeros implementation. With Kevin's proposal, there is no reason to add a flag to bdi. (Sorry it took a few iterations to get it right). Paolo