From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55577) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dIZNo-00033O-Ie for qemu-devel@nongnu.org; Wed, 07 Jun 2017 07:44:17 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dIZNj-0003a0-M8 for qemu-devel@nongnu.org; Wed, 07 Jun 2017 07:44:16 -0400 Date: Wed, 7 Jun 2017 13:44:06 +0200 From: Alberto Garcia Message-ID: <20170607114406.GA24076@igalia.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH 0/7] qcow2: Reduce the number of I/O ops when doing COW List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: qemu-block@nongnu.org, Kevin Wolf , Max Reitz , Eric Blake , Stefan Hajnoczi , "Denis V . Lunev" ping On Tue, May 23, 2017 at 01:22:55PM +0200, Alberto Garcia wrote: > Hi all, > > here's a patch series that rewrites the copy-on-write code in the > qcow2 driver to reduce the number of I/O operations. > > The situation is that when a guest sends a write request and QEMU > needs to allocate new cluster(s) in a qcow2 file, the unwritten > regions of the new cluster(s) need to be filled with the existing data > (e.g. from the backing image) or with zeroes. > > The whole process can require up to 5 I/O operations: > > 1) Write the data from the actual write request. > 2) Read the existing data located before the guest data. > 3) Write that data to the new clusters. > 4) Read the existing data located after the guest data. > 5) Write that data to the new clusters. > > This series reduces that to only two operations: > > 1) Read the existing data from the original clusters > 2) Write the updated data (=original + guest request) to the new clusters > > Step (1) implies that there's data that will be read but will be > immediately discarded (because it's overwritten by the guest > request). I haven't really detected any big performance problems > because of that, but I decided to be conservative and my code includes > a simple heuristic that keeps the old behavior if the amount of data > to be discarded is higher than 16KB. > > I've been testing this series in several scenarios, with different > cluster sizes (32K, 64K, 1MB) and request sizes (from 4 up to 512KB), > and both with an SSD and a rotating HDD. The results vary depending on > the case, with an average increase of 60% in the number of IOPS in the > HDD case, and 15% in the SSD case. In some cases there are really no > big differences and the results are similar before and after this > patch. > > Further work for the future includes detecting when the data that > needs to be written consists on zeroes (i.e. allocating a new cluster > with no backing image) and optimizing that case, but let's start with > this. > > Regards, > > Berto > > Alberto Garcia (7): > qcow2: Remove unused Error in do_perform_cow() > qcow2: Use unsigned int for both members of Qcow2COWRegion > qcow2: Make perform_cow() call do_perform_cow() twice > qcow2: Split do_perform_cow() into _read(), _encrypt() and _write() > qcow2: Allow reading both COW regions with only one request > qcow2: Pass a QEMUIOVector to do_perform_cow_{read,write}() > qcow2: Merge the writing of the COW regions with the guest data > > block/qcow2-cluster.c | 188 +++++++++++++++++++++++++++++++++++++------------- > block/qcow2.c | 58 +++++++++++++--- > block/qcow2.h | 11 ++- > 3 files changed, 197 insertions(+), 60 deletions(-) > > -- > 2.11.0