From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: [PATCH v6 12/18] tools/libx{l, c}: add back channel to libxc Date: Mon, 25 Jan 2016 14:41:47 -0500 Message-ID: <20160125194147.GR14977@char.us.oracle.com> References: <1451442548-26974-1-git-send-email-wency@cn.fujitsu.com> <1451442548-26974-13-git-send-email-wency@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: <1451442548-26974-13-git-send-email-wency@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 Wed, Dec 30, 2015 at 10:29:02AM +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 form the s/form/from/ > primary to the secondary. > > However, the set difference B - A (lets call this C) is out-of-date on 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, as it was not memory dirtied by the primary. The secondary s/primary/primary (to secondary)/ > needs the page data for C to reconstruct an exact copy of the primary at s/the page data/C page data/ > 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. You could invert this - the primary could send A to secondary? I presume this non-optimal as the 'A' set is much much bigger than 'C' set? It may be good to include this in the commit description. > > 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. Well, that 2) is a very good reason :-) > > Signed-off-by: Yang Hongyang > commit message: ? Huh? > Signed-off-by: Andrew Cooper > CC: Ian Campbell > CC: Ian Jackson > CC: Wei Liu .. snip.. > index 05159bb..d4dc501 100644 > --- a/tools/libxc/xc_sr_restore.c > +++ b/tools/libxc/xc_sr_restore.c > @@ -722,7 +722,7 @@ int xc_domain_restore(xc_interface *xch, int io_fd, uint32_t dom, > unsigned long *console_gfn, domid_t console_domid, > unsigned int hvm, unsigned int pae, int superpages, > int checkpointed_stream, > - struct restore_callbacks *callbacks) > + struct restore_callbacks *callbacks, int back_fd) > { > struct xc_sr_context ctx = > { > diff --git a/tools/libxc/xc_sr_save.c b/tools/libxc/xc_sr_save.c > index 8ffd71d..a49d083 100644 > --- a/tools/libxc/xc_sr_save.c > +++ b/tools/libxc/xc_sr_save.c > @@ -824,7 +824,7 @@ static int save(struct xc_sr_context *ctx, uint16_t guest_type) > int xc_domain_save(xc_interface *xch, int io_fd, uint32_t dom, > uint32_t max_iters, uint32_t max_factor, uint32_t flags, > struct save_callbacks* callbacks, int hvm, > - int checkpointed_stream) > + int checkpointed_stream, int back_fd) > { > struct xc_sr_context ctx = > { But where is the code? Or is that suppose to be done in another patch? If so you may want to mention that in the commit description?