From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:36146) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Vb8AB-0006qw-VA for qemu-devel@nongnu.org; Tue, 29 Oct 2013 08:12:53 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Vb8A5-0005OX-Kh for qemu-devel@nongnu.org; Tue, 29 Oct 2013 08:12:47 -0400 Received: from mx1.redhat.com ([209.132.183.28]:5328) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Vb8A5-0005OJ-Af for qemu-devel@nongnu.org; Tue, 29 Oct 2013 08:12:41 -0400 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id r9TCCe6P024393 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Tue, 29 Oct 2013 08:12:40 -0400 Date: Tue, 29 Oct 2013 13:12:38 +0100 From: Kevin Wolf Message-ID: <20131029121238.GD4690@dhcp-200-207.str.redhat.com> References: <1383046524-17801-1-git-send-email-kwolf@redhat.com> <526FA3D4.1060600@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <526FA3D4.1060600@redhat.com> Subject: Re: [Qemu-devel] [PATCH] block: Avoid unecessary drv->bdrv_getlength() calls List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: asias@redhat.com, qemu-devel@nongnu.org, stefanha@redhat.com, armbru@redhat.com Am 29.10.2013 um 13:02 hat Paolo Bonzini geschrieben: > Il 29/10/2013 12:35, Kevin Wolf ha scritto: > > The block layer generally keeps the size of an image cached in > > bs->total_sectors so that it doesn't have to perform expensive > > operations to get the size whenever it needs it. > > > > This doesn't work however when using a backend that can change its size > > without qemu being aware of it, i.e. passthrough of removable media like > > CD-ROMs or floppy disks. For this reason, the caching is disabled when a > > removable device is used. > > > > It is obvious that checking whether the _guest_ device has removable > > media isn't the right thing to do when we want to know whether the size > > of the host backend can change. To make things worse, non-top-level > > BlockDriverStates never have any device attached, which makes qemu > > assume they are removable, so drv->bdrv_getlength() is always called on > > the protocol layer. In the case of raw-posix, this causes unnecessary > > lseek() system calls, which turned out to be rather expensive. > > > > This patch completely changes the logic and disables bs->total_sectors > > caching only for certain block driver types, for which a size change is > > expected: host_cdrom and host_floppy; also the raw format in case it > > sits on top of one of these protocols, but in the common case the nested > > bdrv_getlength() call on the protocol driver will use the cache again > > and avoid an expensive drv->bdrv_getlength() call. > > > > Signed-off-by: Kevin Wolf > raw-win32.c probably needs to have a .has_variable_length=true in > bdrv_host_device. Apart from that, > > Reviewed-by: Paolo Bonzini Thanks, good catch. I've added this now. Kevin