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HTML
304 lines
16 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html>
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>Domain state capture using Libvirt</h1>
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<ul id="toc"></ul>
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<p>
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In order to aid application developers to choose which
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operations best suit their needs, this page compares the
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different means for capturing state related to a domain managed
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by libvirt.
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</p>
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<p>
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The information here is primarily geared towards capturing the
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state of an active domain. Capturing the state of an inactive
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domain essentially amounts to copying the contents of guest
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disks, followed by a fresh boot of the same domain configuration
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with disks restored back to that saved state.
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</p>
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<h2><a id="definitions">State capture trade-offs</a></h2>
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<p>One of the features made possible with virtual machines is live
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migration -- transferring all state related to the guest from
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one host to another with minimal interruption to the guest's
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activity. In this case, state includes domain memory (including
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register and device contents), and domain storage (whether the
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guest's view of the disks are backed by local storage on the
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host, or by the hypervisor accessing shared storage over a
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network). A clever observer will then note that if all state is
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available for live migration, then there is nothing stopping a
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user from saving some or all of that state at a given point of
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time in order to be able to later rewind guest execution back to
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the state it previously had. The astute reader will also realize
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that state capture at any level requires that the data must be
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stored and managed by some mechanism. This processing might fit
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in a single file, or more likely require a chain of related
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files, and may require synchronization with third-party tools
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built around managing the amount of data resulting from
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capturing the state of multiple guests that each use multiple
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disks.
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</p>
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<p>
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There are several libvirt APIs associated with capturing the
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state of a guest, which can later be used to rewind that guest
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to the conditions it was in earlier. The following is a list of
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trade-offs and differences between the various facets that
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affect capturing domain state for active domains:
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</p>
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<dl>
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<dt>Duration</dt>
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<dd>Capturing state can be a lengthy process, so while the
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captured state ideally represents an atomic point in time
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corresponding to something the guest was actually executing,
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capturing state tends to focus on minimizing guest downtime
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while performing the rest of the state capture in parallel
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with guest execution. Some interfaces require up-front
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preparation (the state captured is not complete until the API
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ends, which may be some time after the command was first
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started), while other interfaces track the state when the
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command was first issued, regardless of the time spent in
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capturing the rest of the state. Also, time spent in state
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capture may be longer than the time required for live
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migration, when state must be duplicated rather than shared.
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</dd>
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<dt>Amount of state</dt>
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<dd>For an online guest, there is a choice between capturing the
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guest's memory (all that is needed during live migration when
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the storage is already shared between source and destination),
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the guest's disk state (all that is needed if there are no
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pending guest I/O transactions that would be lost without the
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corresponding memory state), or both together. Reverting to
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partial state may still be viable, but typically, booting from
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captured disk state without corresponding memory is comparable
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to rebooting a machine that had power cut before I/O could be
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flushed. Guests may need to use proper journaling methods to
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avoid problems when booting from partial state.
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</dd>
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<dt>Quiescing of data</dt>
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<dd>Even if a guest has no pending I/O, capturing disk state may
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catch the guest at a time when the contents of the disk are
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inconsistent. Cooperating with the guest to perform data
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quiescing is an optional step to ensure that captured disk
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state is fully consistent without requiring additional memory
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state, rather than just crash-consistent. But guest
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cooperation may also have time constraints, where the guest
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can rightfully panic if there is too much downtime while I/O
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is frozen.
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</dd>
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<dt>Quantity of files</dt>
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<dd>When capturing state, some approaches store all state within
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the same file (internal), while others expand a chain of
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related files that must be used together (external), for more
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files that a management application must track.
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</dd>
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<dt>Impact to guest definition</dt>
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<dd>Capturing state may require temporary changes to the guest
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definition, such as associating new files into the domain
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definition. While state capture should never impact the
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running guest, a change to the domain's active XML may have
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impact on other host operations being performed on the domain.
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</dd>
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<dt>Third-party integration</dt>
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<dd>When capturing state, there are tradeoffs to how much of the
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process must be done directly by the hypervisor, and how much
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can be off-loaded to third-party software. Since capturing
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state is not instantaneous, it is essential that any
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third-party integration see consistent data even if the
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running guest continues to modify that data after the point in
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time of the capture.</dd>
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<dt>Full vs. incremental</dt>
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<dd>When periodically repeating the action of state capture, it
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is useful to minimize the amount of state that must be
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captured by exploiting the relation to a previous capture,
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such as focusing only on the portions of the disk that the
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guest has modified in the meantime. Some approaches are able
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to take advantage of checkpoints to provide an incremental
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backup, while others are only capable of a full backup even if
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that means re-capturing unchanged portions of the disk.</dd>
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<dt>Local vs. remote</dt>
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<dd>Domains that completely use remote storage may only need
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some mechanism to keep track of guest memory state while using
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external means to manage storage. Still, hypervisor and guest
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cooperation to ensure points in time when no I/O is in flight
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across the network can be important for properly capturing
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disk state.</dd>
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<dt>Network latency</dt>
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<dd>Whether it's domain storage or saving domain state into
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remote storage, network latency has an impact on snapshot
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data. Having dedicated network capacity, bandwidth, or quality
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of service levels may play a role, as well as planning for how
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much of the backup process needs to be local.</dd>
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</dl>
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<p>
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An example of the various facets in action is migration of a
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running guest. In order for the guest to be able to resume on
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the destination at the same place it left off at the source, the
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hypervisor has to get to a point where execution on the source
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is stopped, the last remaining changes occurring since the
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migration started are then transferred, and the guest is started
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on the target. The management software thus must keep track of
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the starting point and any changes since the starting
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point. These last changes are often referred to as dirty page
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tracking or dirty disk block bitmaps. At some point in time
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during the migration, the management software must freeze the
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source guest, transfer the dirty data, and then start the guest
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on the target. This period of time must be minimal. To minimize
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overall migration time, one is advised to use a dedicated
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network connection with a high quality of service. Alternatively
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saving the current state of the running guest can just be a
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point in time type operation which doesn't require updating the
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"last vestiges" of state prior to writing out the saved state
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file. The state file is the point in time of whatever is current
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and may contain incomplete data which if used to restart the
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guest could cause confusion or problems because some operation
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wasn't completed depending upon where in time the operation was
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commenced.
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</p>
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<h2><a id="apis">State capture APIs</a></h2>
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<p>With those definitions, the following libvirt APIs related to
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state capture have these properties:</p>
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<dl>
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<dt><a href="html/libvirt-libvirt-domain.html#virDomainManagedSave"><code>virDomainManagedSave</code></a></dt>
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<dd>This API saves guest memory, with libvirt managing all of
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the saved state, then stops the guest. While stopped, the
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disks can be copied by a third party. However, since any
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subsequent restart of the guest by libvirt API will restore
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the memory state (which typically only works if the disk state
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is unchanged in the meantime), and since it is not possible to
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get at the memory state that libvirt is managing, this is not
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viable as a means for rolling back to earlier saved states,
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but is rather more suited to situations such as suspending a
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guest prior to rebooting the host in order to resume the guest
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when the host is back up. This API also has a drawback of
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potentially long guest downtime, and therefore does not lend
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itself well to live backups.</dd>
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<dt><a href="html/libvirt-libvirt-domain.html#virDomainSave"><code>virDomainSave</code></a></dt>
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<dd>This API is similar to virDomainManagedSave(), but moves the
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burden on managing the stored memory state to the user. As
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such, the user can now couple saved state with copies of the
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disks to perform a revert to an arbitrary earlier saved state.
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However, changing who manages the memory state does not change
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the drawback of potentially long guest downtime when capturing
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state.</dd>
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<dt><a href="html/libvirt-libvirt-domain-snapshot.html#virDomainSnapshotCreateXML"><code>virDomainSnapshotCreateXML</code></a></dt>
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<dd>This API wraps several approaches for capturing guest state,
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with a general premise of creating a snapshot (where the
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current guest resources are frozen in time and a new wrapper
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layer is opened for tracking subsequent guest changes). It
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can operate on both offline and running guests, can choose
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whether to capture the state of memory, disk, or both when
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used on a running guest, and can choose between internal and
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external storage for captured state. However, it is geared
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towards post-event captures (when capturing both memory and
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disk state, the disk state is not captured until all memory
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state has been collected first). Using QEMU as the
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hypervisor, internal snapshots currently have lengthy downtime
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that is incompatible with freezing guest I/O, but external
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snapshots are quick when memory contents are not also saved.
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Since creating an external snapshot changes which disk image
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resource is in use by the guest, this API can be coupled
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with <a href="html/libvirt-libvirt-domain.html#virDomainBlockCommit"><code>virDomainBlockCommit()</code></a>
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to restore things back to the guest using its original disk
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image, where a third-party tool can read the backing file
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prior to the live commit. See also
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the <a href="formatsnapshot.html">XML details</a> used with
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this command.</dd>
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<dt><a href="html/libvirt-libvirt-domain.html#virDomainFSFreeze"><code>virDomainFSFreeze</code></a>, <a href="html/libvirt-libvirt-domain.html#virDomainFSThaw"><code>virDomainFSThaw</code></a></dt>
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<dd>This pair of APIs does not directly capture guest state, but
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can be used to coordinate with a trusted live guest that state
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capture is about to happen, and therefore guest I/O should be
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quiesced so that the state capture is fully consistent, rather
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than merely crash consistent. Some APIs are able to
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automatically perform a freeze and thaw via a flags parameter,
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rather than having to make separate calls to these
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functions. Also, note that freezing guest I/O is only possible
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with trusted guests running a guest agent, and that some
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guests place maximum time limits on how long I/O can be
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frozen.</dd>
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<dt><a href="html/libvirt-libvirt-domain-checkpoint.html#virDomainCheckpointCreateXML"><code>virDomainCheckpointCreateXML</code></a></dt>
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<dd>This API does not actually capture guest state, rather it
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makes it possible to track which portions of guest disks have
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changed between a checkpoint and the current live execution of
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the guest. However, while it is possible use this API to
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create checkpoints in isolation, it is more typical to create
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a checkpoint as a side-effect of starting a new incremental
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backup with <code>virDomainBackupBegin()</code> or at the
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creation of an external snapshot
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with <code>virDomainSnapshotCreateXML2()</code>, since a
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second incremental backup is most useful when using the
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checkpoint created during the first. See also
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the <a href="formatcheckpoint.html">XML details</a> used with
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this command.</dd>
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<dt><a href="html/libvirt-libvirt-domain.html#virDomainBackupBegin"><code>virDomainBackupBegin</code></a>, <a href="html/libvirt-libvirt-domain.html#virDomainBackupEnd"><code>virDomainBackupEnd</code></a></dt>
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<dd>This API wraps approaches for capturing the state of disks
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of a running guest, but does not track accompanying guest
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memory state. The capture is consistent to the start of the
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operation, where the captured state is stored independently
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from the disk image in use with the guest and where it can be
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easily integrated with a third-party for capturing the disk
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state. Since the backup operation is stored externally from
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the guest resources, there is no need to commit data back in
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at the completion of the operation. When coupled with
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checkpoints, this can be used to capture incremental backups
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instead of full.</dd>
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</dl>
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<h2><a id="examples">Examples</a></h2>
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<p>The following two sequences both accomplish the task of
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capturing the disk state of a running guest, then wrapping
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things up so that the guest is still running with the same file
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as its disk image as before the sequence of operations began.
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The difference between the two sequences boils down to the
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impact of an unexpected interruption made at any point in the
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middle of the sequence: with such an interruption, the first
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example leaves the guest tied to a temporary wrapper file rather
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than the original disk, and requires manual clean up of the
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domain definition; while the second example has no impact to the
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domain definition.</p>
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<p>1. Backup via temporary snapshot
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<pre>
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virDomainFSFreeze()
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virDomainSnapshotCreateXML(VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY)
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virDomainFSThaw()
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third-party copy the backing file to backup storage # most time spent here
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virDomainBlockCommit(VIR_DOMAIN_BLOCK_COMMIT_ACTIVE) per disk
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wait for commit ready event per disk
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virDomainBlockJobAbort() per disk
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</pre></p>
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<p>2. Direct backup
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<pre>
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virDomainFSFreeze()
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virDomainBackupBegin()
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virDomainFSThaw()
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wait for push mode event, or pull data over NBD # most time spent here
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virDomainBackupEnd()
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</pre></p>
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</body>
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</html>
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