Re: Warm and Live Migration Implementation for bhyve
Date: Mon, 17 Jul 2023 17:08:27 UTC
I'm curious why the stream send bits are rolled into bhyve as opposed to using netcat/ssh to do the network transfer? sort of how one would do a zfs send/recv between hosts On Monday, July 17, 2023, Elena Mihailescu <elenamihailescu22@gmail.com> wrote: > Hi Corvin, > > On Mon, 3 Jul 2023 at 09:35, Corvin Köhne <corvink@freebsd.org> wrote: > > > > On Tue, 2023-06-27 at 16:35 +0300, Elena Mihailescu wrote: > > > Hi Corvin, > > > > > > Thank you for the questions! I'll respond to them inline. > > > > > > On Mon, 26 Jun 2023 at 10:16, Corvin Köhne <corvink@freebsd.org> > > > wrote: > > > > > > > > Hi Elena, > > > > > > > > thanks for posting this proposal here. > > > > > > > > Some open questions from my side: > > > > > > > > 1. How is the data send to the target? Does the host send a > > > > complete > > > > dump and the target parses it? Or does the target request data one > > > > by > > > > one und the host sends it as response? > > > > > > > It's not a dump of the guest's state, it's transmitted in steps. > > > However, some parts may be migrated as a chunk (e.g., the emulated > > > devices' state is transmitted as the buffer generated from the > > > snapshot functions). > > > > > > > How does the receiver know which chunk relates to which device? It > > would be nice if you can start bhyve on the receiver side without > > parameters e.g. `bhyve --receive=127.0.0.1:1234`. Therefore, the > > protocol has to carry some information about the device configuration. > > > > Regarding your first question, we send a chunk of data (a buffer) with > the state: we resume the data in the same order we saved it. It relies > on save/restore. We currently do not support migrating between > different versions of suspend&resume/migration. > > It would be nice to have something like `bhyve > --receive=127.0.0.1:1234`, but I don't think it is possible at this > point mainly because of the following two reasons: > - the guest image must be shared (e.g., via NFS) between the source > and destination hosts. If the mounting points differ between the two, > opening the disk at the destination will fail (also, we must suppose > that the user used an absolute path since a relative one won't work) > - if the VM uses a network adapter, we must specify the tap interface > on the destination host (e.g., if on the source host the VM uses > `tap0`, on the destination host, `tap0` may not exist or may be used > by other VMs). > > > > > > > I'll try to describe a bit the protocol we have implemented for > > > migration, maybe it can partially respond to the second and third > > > questions. > > > > > > The destination host waits for the source host to connect (through a > > > socket). > > > After that, the source sends its system specifications (hw_machine, > > > hw_model, hw_pagesize). If the source and destination hosts have > > > identical hardware configurations, the migration can take place. > > > > > > Then, if we have live migration, we migrate the memory in rounds > > > (i.e., we get a list of the pages that have the dirty bit set, send > > > it > > > to the destination to know what pages will be received, then send the > > > pages through the socket; this process is repeated until the last > > > round). > > > > > > Next, we stop the guest's vcpus, send the remaining memory (for live > > > migration) or the guest's memory from vmctx->baseaddr for warm > > > migration. Then, based on the suspend/resume feature, we get the > > > state > > > of the virtualized devices (the ones from the kernel space) and send > > > this buffer to the destination. We repeat this for the emulated > > > devices as well (the ones from the userspace). > > > > > > On the receiver host, we get the memory pages and set them to their > > > according position in the guest's memory, use the restore functions > > > for the state of the devices and start the guest's execution. > > > > > > Excluding the guest's memory transfer, the rest is based on the > > > suspend/resume feature. We snapshot the guest's state, but instead of > > > saving the data locally, we send it via network to the destination. > > > On > > > the destination host, we start a new virtual machine, but instead of > > > reading/getting the state from the disk (i.e., the snapshot files) we > > > get this state via the network from the source host. > > > > > > If the destination can properly resume the guest activity, it will > > > send an "OK" to the source host so it can destroy/remove the guest > > > from its end. > > > > > > Both warm and live migration are based on "cold migration". Cold > > > migration means we suspend the guest on the source host, and restore > > > the guest on the destination host from the snapshot files. Warm > > > migration only does this using a socket, while live migration changes > > > the way the memory is migrated. > > > > > > > 2. What happens if we add a new data section? > > > > > > > What are you referring to with a new data section? Is this question > > > related to the third one? If so, see my answer below. > > > > > > > 3. What happens if the bhyve version differs on host and target > > > > machine? > > > > > > The two hosts must be identical for migration, that's why we have the > > > part where we check the specifications between the two migration > > > hosts. They are expected to have the same version of bhyve and > > > FreeBSD. We will add an additional check in the check specs part to > > > see if we have the same FreeBSD build. > > > > > > As long as the changes in the virtual memory subsystem won't affect > > > bhyve (and how the virtual machine sees/uses the memory), the > > > migration constraints should only be related to suspend/resume. The > > > state of the virtual devices is handled by the snapshot system, so if > > > it is able to accommodate changes in the data structures, the > > > migration process will not be affected. > > > > > > Thank you, > > > Elena > > > > > > > > > > > > > > > -- > > > > Kind regards, > > > > Corvin > > > > > > > > On Fri, 2023-06-23 at 13:00 +0300, Elena Mihailescu wrote: > > > > > Hello, > > > > > > > > > > This mail presents the migration feature we have implemented for > > > > > bhyve. Any feedback from the community is much appreciated. > > > > > > > > > > We have opened a stack of reviews on Phabricator > > > > > (https://reviews.freebsd.org/D34717) that is meant to split the > > > > > code > > > > > in smaller parts so it can be more easily reviewed. A brief > > > > > history > > > > > of > > > > > the implementation can be found at the bottom of this email. > > > > > > > > > > The migration mechanism we propose needs two main components in > > > > > order > > > > > to move a virtual machine from one host to another: > > > > > 1. the guest's state (vCPUs, emulated and virtualized devices) > > > > > 2. the guest's memory > > > > > > > > > > For the first part, we rely on the suspend/resume feature. We > > > > > call > > > > > the > > > > > same functions as the ones used by suspend/resume, but instead of > > > > > saving the data in files, we send it via the network. > > > > > > > > > > The most time consuming aspect of migration is transmitting guest > > > > > memory. The UPB team has implemented two options to accomplish > > > > > this: > > > > > 1. Warm Migration: The guest execution is suspended on the source > > > > > host > > > > > while the memory is sent to the destination host. This method is > > > > > less > > > > > complex but may cause extended downtime. > > > > > 2. Live Migration: The guest continues to execute on the source > > > > > host > > > > > while the memory is transmitted to the destination host. This > > > > > method > > > > > is more complex but offers reduced downtime. > > > > > > > > > > The proposed live migration procedure (pre-copy live migration) > > > > > migrates the memory in rounds: > > > > > 1. In the initial round, we migrate all the guest memory (all > > > > > pages > > > > > that are allocated) > > > > > 2. In the subsequent rounds, we migrate only the pages that were > > > > > modified since the previous round started > > > > > 3. In the final round, we suspend the guest, migrate the > > > > > remaining > > > > > pages that were modified from the previous round and the guest's > > > > > internal state (vCPU, emulated and virtualized devices). > > > > > > > > > > To detect the pages that were modified between rounds, we propose > > > > > an > > > > > additional dirty bit (virtualization dirty bit) for each memory > > > > > page. > > > > > This bit would be set every time the page's dirty bit is set. > > > > > However, > > > > > this virtualization dirty bit is reset only when the page is > > > > > migrated. > > > > > > > > > > The proposed implementation is split in two parts: > > > > > 1. The first one, the warm migration, is just a wrapper on the > > > > > suspend/resume feature which, instead of saving the suspended > > > > > state > > > > > on > > > > > disk, sends it via the network to the destination > > > > > 2. The second part, the live migration, uses the layer previously > > > > > presented, but sends the guest's memory in rounds, as described > > > > > above. > > > > > > > > > > The migration process works as follows: > > > > > 1. we identify: > > > > > - VM_NAME - the name of the virtual machine which will be > > > > > migrated > > > > > - SRC_IP - the IP address of the source host > > > > > - DST_IP - the IP address of the destination host (default is > > > > > 24983) > > > > > - DST_PORT - the port we want to use for migration > > > > > 2. we start a virtual machine on the destination host that will > > > > > wait > > > > > for a migration. Here, we must specify SRC_IP (and the port we > > > > > want > > > > > to > > > > > open for migration, default is 24983). > > > > > e.g.: bhyve ... -R SRC_IP:24983 guest_vm_dst > > > > > 3. using bhyvectl on the source host, we start the migration > > > > > process. > > > > > e.g.: bhyvectl --migrate=DST_IP:24983 --vm=guest_vm > > > > > > > > > > A full tutorial on this can be found here: > > > > > https://github.com/FreeBSD-UPB/freebsd-src/wiki/Virtual- > Machine-Migration-using-bhyve > > > > > > > > > > For sending the migration request to a virtual machine, we use > > > > > the > > > > > same thread/socket that is used for suspend. > > > > > For receiving a migration request, we used a similar approach to > > > > > the > > > > > resume process. > > > > > > > > > > As some of you may remember seeing similar emails from our part > > > > > on > > > > > the > > > > > freebsd-virtualization list, I'll present a brief history of this > > > > > project: > > > > > The first part of the project was the suspend/resume > > > > > implementation > > > > > which landed in bhyve in 2020, under the BHYVE_SNAPSHOT guard > > > > > (https://reviews.freebsd.org/D19495). > > > > > After that, we focused on two tracks: > > > > > 1. adding various suspend/resume features (multiple device > > > > > support - > > > > > https://reviews.freebsd.org/D26387, CAPSICUM support - > > > > > https://reviews.freebsd.org/D30471, having an uniform file format > > > > > - > > > > > at > > > > > that time, during the bhyve bi-weekly calls, we concluded that > > > > > the > > > > > JSON format was the most suitable at that time - > > > > > https://reviews.freebsd.org/D29262) so we can remove the #ifdef > > > > > BHYVE_SNAPSHOT guard. > > > > > 2. implementing the migration feature for bhyve. Since this one > > > > > relies > > > > > on the save/restore, but does not modify its behaviour, we > > > > > considered > > > > > we can go in parallel with both tracks. > > > > > We had various presentations in the FreeBSD Community on these > > > > > topics: > > > > > AsiaBSDCon2018, AsiaBSDCon2019, BSDCan2019, BSDCan2020, > > > > > AsiaBSDCon2023. > > > > > > > > > > The first patches for warm and live migration were opened in > > > > > 2021: > > > > > https://reviews.freebsd.org/D28270, > > > > > https://reviews.freebsd.org/D30954. However, the general feedback > > > > > on > > > > > these was that the patches are too big to be reviewed, so we > > > > > should > > > > > split them in smaller chunks (this was also true for some of the > > > > > suspend/resume improvements). Thus, we split them into smaller > > > > > parts. > > > > > Also, as things changed in bhyve (i.e., capsicum support for > > > > > suspend/resume was added this year), we rebased and updated our > > > > > reviews. > > > > > > > > > > Thank you, > > > > > Elena > > > > > > > > > > > > > -- > > Kind regards, > > Corvin > > Thanks, > Elena > >