From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59339) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UzoHM-0003c8-BY for qemu-devel@nongnu.org; Thu, 18 Jul 2013 09:30:02 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UzoHI-0004wZ-HG for qemu-devel@nongnu.org; Thu, 18 Jul 2013 09:29:56 -0400 Received: from mx.ipv6.kamp.de ([2a02:248:0:51::16]:41817 helo=mx01.kamp.de) by eggs.gnu.org with smtp (Exim 4.71) (envelope-from ) id 1UzoHI-0004wK-5y for qemu-devel@nongnu.org; Thu, 18 Jul 2013 09:29:52 -0400 Message-ID: <51E7EDD0.6050001@kamp.de> Date: Thu, 18 Jul 2013 15:29:52 +0200 From: Peter Lieven 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> <51E7E035.3010702@redhat.com> In-Reply-To: <51E7E035.3010702@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed 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: Paolo Bonzini Cc: Kevin Wolf , Stefan Hajnoczi , qemu-devel , ronnie sahlberg On 18.07.2013 14:31, Paolo Bonzini wrote: > 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. ok, but this would require an individual patch in every driver, wouldn't it. i am ok with that. the BDRV_MAY_DISCARD flag is at the end a hint if the driver can optimize writing zeroes by a discard or if real zeroes should be written e.g. to sanitize the device? talking for iscsi: bdrv->discard can remain to use UNMAP and silently fail if lbpu == 0. bdrv->write_zeroes will use writesame16 and set the unmap flag only if BDRV_MAY_DISCARD == 1 and BDRV_O_UNMAP == 1 and lbprz == 1. in case lbpws == 0 it will return -ENOSUP. Correct? Peter