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@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@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/docs.html.in b/docs/docs.html.in
index 2d44e3ab2e..a00b131466 100644
--- a/docs/docs.html.in
+++ b/docs/docs.html.in
@@ -124,6 +124,11 @@
<dt><a href="secureusage.html">Secure usage</a></dt>
<dd>Secure usage of the libvirt APIs</dd>
+
+ <dt><a href="domainstatecapture.html">Domain state
+ capture</a></dt>
+ <dd>Comparison between different methods of capturing domain
+ state</dd>
</dl>
</div>
diff --git a/docs/domainstatecapture.html.in b/docs/domainstatecapture.html.in
new file mode 100644
index 0000000000..4f109180da
--- /dev/null
+++ b/docs/domainstatecapture.html.in
@@ -0,0 +1,314 @@
+<?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.
+ </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
+ 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>
+
+ <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>
+ </dl>
+
+ <h2><a id="examples">Examples</a></h2>
+ <p>The following two sequences both accomplish the task of
+ capturing the disk state of a running guest, then wrapping
+ things up so that the guest is still running with the same file
+ as its disk image as before the sequence of operations began.
+ The difference between the two sequences boils down to the
+ impact of an unexpected interruption made at any point in the
+ middle of the sequence: with such an interruption, the first
+ example leaves the guest tied to a temporary wrapper file rather
+ than the original disk, and requires manual clean up of the
+ domain definition; while the second example has no impact to the
+ domain definition.</p>
+
+ <p>1. Backup via temporary snapshot
+ <pre>
+virDomainFSFreeze()
+virDomainSnapshotCreateXML(VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY)
+virDomainFSThaw()
+third-party copy the backing file to backup storage # most time spent here
+virDomainBlockCommit(VIR_DOMAIN_BLOCK_COMMIT_ACTIVE) per disk
+wait for commit ready event per disk
+virDomainBlockJobAbort() per disk
+ </pre></p>
+
+ <p>2. Direct backup
+ <pre>
+virDomainFSFreeze()
+virDomainBackupBegin()
+virDomainFSThaw()
+wait for push mode event, or pull data over NBD # most time spent here
+virDomainBackupEnd()
+ </pre></p>
+
+ </body>
+</html>
diff --git a/docs/formatcheckpoint.html.in b/docs/formatcheckpoint.html.in
index ef5f8a826b..030f0d6af0 100644
--- a/docs/formatcheckpoint.html.in
+++ b/docs/formatcheckpoint.html.in
@@ -13,7 +13,9 @@
incremental backups. Right now, incremental backups are only
supported for the QEMU hypervisor when using qcow2 disks at the
active layer; if other disk formats are in use, capturing disk
- backups requires different libvirt APIs.
+ backups requires different libvirt APIs
+ (see <a href="domainstatecapture.html">domain state capture</a>
+ for a comparison between APIs).
</p>
<p>
Libvirt is able to facilitate incremental backups by tracking
diff --git a/docs/formatsnapshot.html.in b/docs/formatsnapshot.html.in
index 2bfb69cf49..ee9aa7817f 100644
--- a/docs/formatsnapshot.html.in
+++ b/docs/formatsnapshot.html.in
@@ -9,6 +9,8 @@
<h2><a id="SnapshotAttributes">Snapshot XML</a></h2>
<p>
+ Snapshots are one form
+ of <a href="domainstatecapture.html">domain state capture</a>.
There are several types of snapshots:
</p>
<dl>
--
2.20.1
--
libvir-list mailing list
libvir-list@redhat.com
https://www.redhat.com/mailman/listinfo/libvir-listOn 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@redhat.com> > Reviewed-by: John Ferlan <jferlan@redhat.com> > Reviewed-by: Daniel P. Berrangé <berrange@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 b/docs/domainstatecapture.html.in > new file mode 100644 > index 0000000000..4f109180da > --- /dev/null > +++ b/docs/domainstatecapture.html.in > @@ -0,0 +1,314 @@ > +<?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. -- libvir-list mailing list libvir-list@redhat.com https://www.redhat.com/mailman/listinfo/libvir-list
On 7/6/19 11:56 PM, 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@redhat.com> > Reviewed-by: John Ferlan <jferlan@redhat.com> > Reviewed-by: Daniel P. Berrangé <berrange@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 b/docs/domainstatecapture.html.in > + <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> s/libvit/libvirt/g in this file, there's other hits in this file too - Cole -- libvir-list mailing list libvir-list@redhat.com https://www.redhat.com/mailman/listinfo/libvir-list
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