6:23 a.m.
On Sat, Jul 06, 2019 at 22:56:03 -0500, Eric Blake wrote:
Now that various new API have been added or are coming soon, it is
worth a landing page that gives an overview of capturing various
pieces of guest state, and which APIs are best suited to which tasks.
Signed-off-by: Eric Blake <eblake(a)redhat.com>
Reviewed-by: John Ferlan <jferlan(a)redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange(a)redhat.com>
---
docs/docs.html.in | 5 +
docs/domainstatecapture.html.in | 314 ++++++++++++++++++++++++++++++++
docs/formatcheckpoint.html.in | 4 +-
docs/formatsnapshot.html.in | 2 +
4 files changed, 324 insertions(+), 1 deletion(-)
create mode 100644 docs/domainstatecapture.html.in
[...]
diff --git a/docs/domainstatecapture.html.in
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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE html>
+<html xmlns="http://www.w3.org/1999/xhtml">
+ <body>
+
+ <h1>Domain state capture using Libvirt</h1>
+
+ <ul id="toc"></ul>
+
+ <p>
+ In order to aid application developers to choose which
+ operations best suit their needs, this page compares the
+ different means for capturing state related to a domain managed
+ by libvirt.
+ </p>
+
+ <p>
+ The information here is primarily geared towards capturing the
+ state of an active domain. Capturing the state of an inactive
+ domain essentially amounts to copying the contents of guest
+ disks, followed by a fresh boot with disks restored to that
+ state.
Also the config may be relevant.
+ </p>
+
+ <h2><a id="definitions">State capture
trade-offs</a></h2>
+
+ <p>One of the features made possible with virtual machines is live
+ migration -- transferring all state related to the guest from
+ one host to another with minimal interruption to the guest's
+ activity. In this case, state includes domain memory (including
+ register and device contents), and domain storage (whether the
+ guest's view of the disks are backed by local storage on the
+ host, or by the hypervisor accessing shared storage over a
+ network). A clever observer will then note that if all state is
+ available for live migration, then there is nothing stopping a
+ user from saving some or all of that state at a given point of
+ time in order to be able to later rewind guest execution back to
+ the state it previously had. The astute reader will also realize
+ that state capture at any level requires that the data must be
+ stored and managed by some mechanism. This processing might fit
+ in a single file, or more likely require a chain of related
+ files, and may require synchronization with third-party tools
+ built around managing the amount of data resulting from
+ capturing the state of multiple guests that each use multiple
+ disks.
+ </p>
+
+ <p>
+ There are several libvirt APIs associated with capturing the
+ state of a guest, which can later be used to rewind that guest
+ to the conditions it was in earlier. The following is a list of
+ trade-offs and differences between the various facets that
+ affect capturing domain state for active domains:
+ </p>
+
+ <dl>
+ <dt>Duration</dt>
+ <dd>Capturing state can be a lengthy process, so while the
+ captured state ideally represents an atomic point in time
+ corresponding to something the guest was actually executing,
+ capturing state tends to focus on minimizing guest downtime
+ while performing the rest of the state capture in parallel
+ with guest execution. Some interfaces require up-front
+ preparation (the state captured is not complete until the API
+ ends, which may be some time after the command was first
+ started), while other interfaces track the state when the
+ command was first issued, regardless of the time spent in
+ capturing the rest of the state. Also, time spent in state
+ capture may be longer than the time required for live
+ migration, when state must be duplicated rather than shared.
+ </dd>
+
+ <dt>Amount of state</dt>
+ <dd>For an online guest, there is a choice between capturing the
+ guest's memory (all that is needed during live migration when
+ the storage is already shared between source and destination),
+ the guest's disk state (all that is needed if there are no
+ pending guest I/O transactions that would be lost without the
+ corresponding memory state), or both together. Reverting to
+ partial state may still be viable, but typically, booting from
+ captured disk state without corresponding memory is comparable
+ to rebooting a machine that had power cut before I/O could be
+ flushed. Guests may need to use proper journaling methods to
+ avoid problems when booting from partial state.
+ </dd>
+
+ <dt>Quiescing of data</dt>
+ <dd>Even if a guest has no pending I/O, capturing disk state may
+ catch the guest at a time when the contents of the disk are
+ inconsistent. Cooperating with the guest to perform data
+ quiescing is an optional step to ensure that captured disk
+ state is fully consistent without requiring additional memory
+ state, rather than just crash-consistent. But guest
+ cooperation may also have time constraints, where the guest
+ can rightfully panic if there is too much downtime while I/O
+ is frozen.
+ </dd>
+
+ <dt>Quantity of files</dt>
+ <dd>When capturing state, some approaches store all state within
+ the same file (internal), while others expand a chain of
+ related files that must be used together (external), for more
+ files that a management application must track.
+ </dd>
+
+ <dt>Impact to guest definition</dt>
+ <dd>Capturing state may require temporary changes to the guest
+ definition, such as associating new files into the domain
+ definition. While state capture should never impact the
+ running guest, a change to the domain's active XML may have
+ impact on other host operations being performed on the domain.
+ </dd>
+
+ <dt>Third-party integration</dt>
+ <dd>When capturing state, there are tradeoffs to how much of the
+ process must be done directly by the hypervisor, and how much
+ can be off-loaded to third-party software. Since capturing
+ state is not instantaneous, it is essential that any
+ third-party integration see consistent data even if the
+ running guest continues to modify that data after the point in
+ time of the capture.</dd>
+
+ <dt>Full vs. incremental</dt>
+ <dd>When periodically repeating the action of state capture, it
+ is useful to minimize the amount of state that must be
+ captured by exploiting the relation to a previous capture,
+ such as focusing only on the portions of the disk that the
+ guest has modified in the meantime. Some approaches are able
+ to take advantage of checkpoints to provide an incremental
+ backup, while others are only capable of a full backup even if
+ that means re-capturing unchanged portions of the disk.</dd>
+
+ <dt>Local vs. remote</dt>
+ <dd>Domains that completely use remote storage may only need
+ some mechanism to keep track of guest memory state while using
+ external means to manage storage. Still, hypervisor and guest
+ cooperation to ensure points in time when no I/O is in flight
+ across the network can be important for properly capturing
+ disk state.</dd>
+
+ <dt>Network latency</dt>
+ <dd>Whether it's domain storage or saving domain state into
+ remote storage, network latency has an impact on snapshot
+ data. Having dedicated network capacity, bandwidth, or quality
+ of service levels may play a role, as well as planning for how
+ much of the backup process needs to be local.</dd>
+ </dl>
+
+ <p>
+ An example of the various facets in action is migration of a
+ running guest. In order for the guest to be able to resume on
+ the destination at the same place it left off at the source, the
+ hypervisor has to get to a point where execution on the source
+ is stopped, the last remaining changes occurring since the
+ migration started are then transferred, and the guest is started
+ on the target. The management software thus must keep track of
+ the starting point and any changes since the starting
+ point. These last changes are often referred to as dirty page
+ tracking or dirty disk block bitmaps. At some point in time
+ during the migration, the management software must freeze the
+ source guest, transfer the dirty data, and then start the guest
+ on the target. This period of time must be minimal. To minimize
+ overall migration time, one is advised to use a dedicated
+ network connection with a high quality of service. Alternatively
+ saving the current state of the running guest can just be a
+ point in time type operation which doesn't require updating the
+ "last vestiges" of state prior to writing out the saved state
+ file. The state file is the point in time of whatever is current
+ and may contain incomplete data which if used to restart the
+ guest could cause confusion or problems because some operation
+ wasn't completed depending upon where in time the operation was
+ commenced.
+ </p>
+
+ <h2><a id="apis">State capture APIs</a></h2>
+ <p>With those definitions, the following libvirt APIs related to
+ state capture have these properties:</p>
+ <dl>
+ <dt><a
href="html/libvirt-libvit-domain.html#virDomainManagedSave"><code>virDomainManagedSave</code></a></dt>
+ <dd>This API saves guest memory, with libvirt managing all of
+ the saved state, then stops the guest. While stopped, the
+ disks can be copied by a third party. However, since any
+ subsequent restart of the guest by libvirt API will restore
+ the memory state (which typically only works if the disk state
+ is unchanged in the meantime), and since it is not possible to
+ get at the memory state that libvirt is managing, this is not
+ viable as a means for rolling back to earlier saved states,
+ but is rather more suited to situations such as suspending a
+ guest prior to rebooting the host in order to resume the guest
+ when the host is back up. This API also has a drawback of
+ potentially long guest downtime, and therefore does not lend
+ itself well to live backups.</dd>
+
+ <dt><a
href="html/libvirt-libvit-domain.html#virDomainSave"><code>virDomainSave</code></a></dt>
+ <dd>This API is similar to virDomainManagedSave(), but moves the
+ burden on managing the stored memory state to the user. As
+ such, the user can now couple saved state with copies of the
+ disks to perform a revert to an arbitrary earlier saved state.
+ However, changing who manages the memory state does not change
+ the drawback of potentially long guest downtime when capturing
+ state.</dd>
+
+ <dt><a
href="html/libvirt-libvit-domain-snapshot.html#virDomainSnapshotCreateXML"><code>virDomainSnapshotCreateXML</code></a></dt>
+ <dd>This API wraps several approaches for capturing guest state,
+ with a general premise of creating a snapshot (where the
+ current guest resources are frozen in time and a new wrapper
+ layer is opened for tracking subsequent guest changes). It
+ can operate on both offline and running guests, can choose
+ whether to capture the state of memory, disk, or both when
+ used on a running guest, and can choose between internal and
+ external storage for captured state. However, it is geared
+ towards post-event captures (when capturing both memory and
+ disk state, the disk state is not captured until all memory
+ state has been collected first). Using QEMU as the
+ hypervisor, internal snapshots currently have lengthy downtime
+ that is incompatible with freezing guest I/O, but external
+ snapshots are quick. Since creating an external snapshot
This only applies for external snapshots without memory! Including
memory involves a migration with a potentially lenghty downtime.
+ changes which disk image resource is in use by the guest,
this
+ API can be coupled with <a
href="html/libvirt-libvirt-domain.html#virDomainBlockCommit"><code>virDomainBlockCommit()</code></a>
to
+ restore things back to the guest using its original disk
+ image, where a third-party tool can read the backing file
+ prior to the live commit. See also
+ the <a href="formatsnapshot.html">XML details</a> used
with
+ this command.</dd>
+
+ <dt><a
href="html/libvirt-libvit-domain.html#virDomainFSFreeze"><code>virDomainFSFreeze</code></a>,
<a
href="html/libvirt-libvit-domain.html#virDomainFSThaw"><code>virDomainFSThaw</code></a></dt>
+ <dd>This pair of APIs does not directly capture guest state, but
+ can be used to coordinate with a trusted live guest that state
+ capture is about to happen, and therefore guest I/O should be
+ quiesced so that the state capture is fully consistent, rather
+ than merely crash consistent. Some APIs are able to
+ automatically perform a freeze and thaw via a flags parameter,
+ rather than having to make separate calls to these
+ functions. Also, note that freezing guest I/O is only possible
+ with trusted guests running a guest agent, and that some
+ guests place maximum time limits on how long I/O can be
+ frozen.</dd>
+
+ <dt><a
href="html/libvirt-libvit-domain.html#virDomainBlockCopy"><code>virDomainBlockCopy</code></a></dt>
+ <dd>This API wraps approaches for capturing the disk state of a
+ running guest, but does not track accompanying guest memory
+ state, and can only operate on one block device per job. To
+ get a consistent copy of multiple disks, multiple jobs must be
+ run in parallel, then the domain must be paused before ending
+ all of the jobs. The capture is consistent only at the end of
+ the operation with a choice for future guest changes to either
+ pivot to the new file or to resume to just using the original
+ file. The resulting backup file is thus the other file no
+ longer in use by the guest.</dd>
I already said that the abuse of block copy should not be documented at
all here.
+
+ <dt><a
href="html/libvirt-libvit-domain-checkpoint.html#virDomainCheckpointCreateXML"><code>virDomainCheckpointCreateXML</code></a></dt>
+ <dd>This API does not actually capture guest state, rather it
+ makes it possible to track which portions of guest disks have
+ changed between a checkpoint and the current live execution of
+ the guest. However, while it is possible use this API to
+ create checkpoints in isolation, it is more typical to create
+ a checkpoint as a side-effect of starting a new incremental
+ backup with <code>virDomainBackupBegin()</code> or at the
+ creation of an external snapshot
+ with <code>virDomainSnapshotCreateXML2()</code>, since a
+ second incremental backup is most useful when using the
+ checkpoint created during the first. See also
+ the <a href="formatcheckpoint.html">XML details</a> used
with
+ this command.</dd>
+
+ <dt><a
href="html/libvirt-libvit-domain.html#virDomainBackupBegin"><code>virDomainBackupBegin</code></a>,
<a
href="html/libvirt-libvit-domain.html#virDomainBackupEnd"><code>virDomainBackupEnd</code></a></dt>
+ <dd>This API wraps approaches for capturing the state of disks
+ of a running guest, but does not track accompanying guest
+ memory state. The capture is consistent to the start of the
+ operation, where the captured state is stored independently
+ from the disk image in use with the guest and where it can be
+ easily integrated with a third-party for capturing the disk
+ state. Since the backup operation is stored externally from
+ the guest resources, there is no need to commit data back in
+ at the completion of the operation. When coupled with
+ checkpoints, this can be used to capture incremental backups
+ instead of full.</dd>
[...]
All in all my only complaint is that this reads as an informercial for
backups rather than a summarization of all the approaches.
ACK if and only if you remove the documentation of blockCopy abuse.