From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:51337) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cGk8Z-0006OI-LE for qemu-devel@nongnu.org; Tue, 13 Dec 2016 05:16:46 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cGk8Y-0000sP-LA for qemu-devel@nongnu.org; Tue, 13 Dec 2016 05:16:43 -0500 Date: Tue, 13 Dec 2016 18:16:35 +0800 From: Fam Zheng Message-ID: <20161213101635.GC2165@lemon> References: <468dbeb8-5b1b-669f-f7e5-0d24a4308da8@redhat.com> <756cfb05-61e1-7ee3-435b-93759438a52d@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <756cfb05-61e1-7ee3-435b-93759438a52d@redhat.com> Subject: Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Max Reitz Cc: Alberto Garcia , qemu-devel@nongnu.org, Kevin Wolf , qemu-block@nongnu.org On Tue, 12/13 09:02, Max Reitz wrote: > > Yes but the use case is that the qcow2 image is stored in a slow disk, > > so things will be faster if we avoid having to read it too often. > > > > But the data is there and it needs to be read, so we have three options: > > > > 1) Read it everytime we need it. It slows things down. > > 2) Keep (part of) it in memory. It can use a lot of memory. > > 3) Keep it in a faster disk. > > > > We're talking about 3) here, and this it not about creating new > > structures, but about changing the storage backend of the existing L2 > > cache (disk rather than RAM). > > I'm arguing that we already have an on-disk L2 structure and that is called > simply the L1-L2 structure in the qcow2 file. The cache only makes sense > because it is in RAM. I am looking at this problem from a different angle: Assume a qcow2 image created by a known version of QEMU with the option preallocation=falloc, it is theoretically possible to do a hacky optimization in qcow2 I/O path that, instead of looking up in L1/L2, we can calculate file offset for any given virtual address, by adding the file offset of LBA 0. If we make this optimization a header extention and add it to the spec, it is no longer a hack, and it should offer nearly the same performance as raw in all cases - if we say raw is doing "identical mapping" from LBA to file offset, this new operation mode of qcow2 file is basically doing a "shift by constant" mapping. Fam