From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wei Liu Subject: Re: [PATCH v7 12/18] tools/libx{l, c}: add back channel to libxc Date: Thu, 4 Feb 2016 09:25:41 +0000 Message-ID: <20160204092541.GH23178@citrix.com> References: <1454045254-3711-1-git-send-email-wency@cn.fujitsu.com> <1454045254-3711-13-git-send-email-wency@cn.fujitsu.com> <20160203194047.GB23178@citrix.com> <56B2E16E.5040401@cn.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <56B2E16E.5040401@cn.fujitsu.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Wen Congyang Cc: Lars Kurth , Changlong Xie , Wei Liu , Ian Campbell , Andrew Cooper , Jiang Yunhong , Ian Jackson , xen devel , Dong Eddie , Gui Jianfeng , Shriram Rajagopalan , Yang Hongyang List-Id: xen-devel@lists.xenproject.org On Thu, Feb 04, 2016 at 01:28:14PM +0800, Wen Congyang wrote: > On 02/04/2016 03:40 AM, Wei Liu wrote: > > On Fri, Jan 29, 2016 at 01:27:28PM +0800, Wen Congyang wrote: > >> In COLO mode, both VMs are running, and are considered in sync if the > >> visible network traffic is identical. After some time, they fall out of > >> sync. > >> > >> At this point, the two VMs have definitely diverged. Lets call the > >> primary dirty bitmap set A, while the secondary dirty bitmap set B. > >> > >> Sets A and B are different. > >> > >> Under normal migration, the page data for set A will be sent from the > >> primary to the secondary. > >> > >> However, the set difference B - A (the one in B but not in A, lets > >> call this C) is out-of-date on the secondary (with respect to the > >> primary) and will not be sent by the primary (to secondary), as it > >> was not memory dirtied by the primary. The secondary needs C page data > >> to reconstruct an exact copy of the primary at the checkpoint. > >> > >> The secondary cannot calculate C as it doesn't know A. Instead, the > >> secondary must send B to the primary, at which point the primary > >> calculates the union of A and B (lets call this D) which is all the > >> pages dirtied by both the primary and the secondary, and sends all page > >> data covered by D. > >> > >> In the general case, D is a superset of both A and B. Without the > >> backchannel dirty bitmap, a COLO checkpoint can't reconstruct a valid > >> copy of the primary. > >> > >> We transfer the dirty bitmap on libxc side, so we need to introduce back > >> channel to libxc. > >> > >> Note: it is different from the paper. We change the original design to > >> the current one, according to our following concerns: > >> 1. The original design needs extra memory on Secondary host. When there's > >> multiple backups on one host, the memory cost is high. > >> 2. The memory cache code will be another 1k+, it will make the review > >> more time consuming. > >> > >> Note: the back channel will be used in the patch > > > > "will not be used" ? > > > > I don't see any read / write to the newly introduced fd. > > It is used in COLO series. > > Some patches in this series just introduce an API. Thess APIs will be used > in COLO series. Do you mean that these patches should be put in COLO series? > If so, I will check all patches. > Fine by me. Wei.