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Brian Bennett <brian.bennett@joyent.com>, Todd Whiteman <todd.whiteman@joyent.com>
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RFD 34 Instance migration

Introduction

Migration of instances is a common feature among virtualization platforms. SmartDataCenter is a notable exception. Among public clouds, instance replacement is generally preferred to instance migration.

Nevertheless, at times it has been necessary to migrate instances within the Triton Cloud between compute nodes.

Types of Migration

There are three types of migration.

  • Live migration
  • Offline migration
  • Incremental offline migration

Live Migration

This is when an instance compute task is transferred between physical compute nodes. This is generally done using shared storage, requiring only the memory state and execution context to be synchronized leveraging instance "pausing" to briefly quiesce the instance before resuming on the destination compute node. Due to design choices (vis. the exclusion of shared storage) in SDC, and the nature of OS virtualization in SmartOS (containers, not virtual machines*) live migration is not considered for implementation at this time.

* Even KVM instances are qemu in a container.

Offline migration

Where an instance is stopped completely (i.e., the guest performs a shutdown procedure), the dataset backing the storage for the instance is synced, and the instance is booted on the destination compute node. Offline migration is the most straightforward type of migration and works in almost any circumstance, but has the disadvantage that the instance will be down for the entire time it takes to transfer the storage dataset.

Incremental offline migration

Occurs when the dataset backing the storage for the instance is synced while the instance is running. Once the dataset is within an acceptable delta on both the source and destination compute node the instance is stopped, a final sync is performed, and the instance is booted on the destination compute node. This allows for both dedicates storage and significantly reduced down time (though, still not as little as live migration).

The state of migration in SmartOS

ZFS has well known send/receive capabilities. The ability to snapshot a dataset and send incremental datasets makes syncing ZVOLs between compute nodes easy. Similarly, vmadm has undocumented send/receive subcommands that, in addition to performing zfs send, will send the zone metadata. Due to a design decision with the implementation of KVM zones, vmadm send does not support KVM. Migrations can fail when the dataset size is too close to the quota.

The state of migration in Triton

While vmadm does have the beginning of a migration feature, migrations are not supported in Triton. Out of necessity, some shell scripts have been written that are capable of migrating instances. This is a road fraught with danger, however. On several occasions changes to the Triton stack have rendered the migration script inoperable or worse, dangerous (i.e., incurring data loss).

A vision for the future

The following is a description of how instance migration might work.

Instance migration should be a first-class supported feature in Triton. This would be comprised of API calls that trigger a workflow to perform the migration unattended. Migrations could be performed on demand, or scheduled. AdminUI would also provide an interface for migrating an instance. Instance migration should use regular DAPI workflows for selecting the destination compute node, or could be specified by an operator to override DAPI selection.

Product messaging via CloudAPI and/or the user portal would notify customers of upcoming scheduled migration, and at their option choose to migrate ahead of the predefined schedule.

Design

The incremental offline migration will be the only migration type implemented (it effectively covers the complete offline migration type as well).

Migration will make use of existing provisioning and placement APIs (e.g. CNAPI) to ensure compatibility with the regular instance creation workflows. This is so the created instance will be placed on the correct server and that all of the same services are available (e.g. CNS, Volumes, Networks, etc...) without having to duplicate the creation of these services.

A migration can be performed immediately or be scheduled to run at a later time.

All migrations will store information in a migrations Moray bucket, that includes the details for the migration event (owner, instance, servers, state, etc...).

When necessary, the migrating instance will be marked with a special "migrating" vm state to ensure no other operations (e.g. "start", "stop", "reprovision") can occur on the instance whilst the migration is ongoing (including another migration attempt).

It will be possible to manually abort or pause a migration, though it may take some time before the migration process can be safely interrupted and before the source instance can return to it's prior state.

Migration phases

All migrations will run in three phases, the user (or operator) can set the migration to run these phases all at once "automatic", or they can be performed individually as needed "on-demand". The three phases are "begin", "sync" and "switch".

Q. Do we want to put a time limit on how long a migration can stay in the "sync" phase, or do we not care if/can the user will be billed for this?

Begin phase

This phase starts the migration process, creates a new migration database entry and provisions the target instance.

  1. VMAPI /vms/:uuid/migrate POST endpoint will start the migration process for this (source) instance by acquiring a ticket/lock on the instance, creating the migrations bucket entry, and then changing the instance state to "migrating".
  2. DAPI will be used to provision a new (target) instance with the same set of provisioning parameters (you can think of this as an instance reservation) on a different CN (the target CN). Keep the same uuid but flag it as 'do_not_inventory' to ensure that no systems know/use it yet. This step should install the necessary source images on the target CN and/or setup necessary supporting zones (e.g. NAT zone). See the provisioning section for more details on this step.

Sync phase

This phase synchronizes the zfs datasets of the source instance with the zfs datasets in the target instance (without stopping the instance).

  1. A workflow job will be created to oversee/retry the synchronization steps below.
  2. The workflow will make a CNAPI migration-sync request which will run a cn-agent task on the source and target CN. These tasks will set up a communication channel (TCP socket) on the admin network which the two CNs can use to perform the migration operation. The workflow will then watch (by connecting to the cn-agent TCP socket) the migration for progress and success/failure.
  3. If this is the first sync, send a full filesystem snapshot to the target CN. This will create a zfs snapshot and then zfs send that dataset to the waiting zfs receive on the target CN. Else, if this is not the first sync, send an incremental filesystem snapshot to the target CN.
  4. When in "automatic" mode, evaluate whether to re-sync (step 3 again) or perform the final switch. The migration process will repeat this sync phase (with expectedly smaller incremental filesystem updates each time) until certain criteria are met - then it will move on to the "switch" phase.

Switch phase

This phase stops the instance from running, synchronizes the zfs datasets of the source instance with the zfs datasets in the target instance, moves the NICs from the source to the target instance, moves control to the target instance and then restarts the target instance.

  1. Stop the source instance (if it was running).
  2. Send the final filesystem delta to the target CN. For each filesystem dataset, this will create a zfs snapshot and then zfs send the difference from the previous snapshot to the zfs receive running on the target CN.
  3. Swap NICs - reserve NIC IP addresses in the source instance, delete the source NICs and then add same NICs to the target instance.
  4. Unregister the source instance from systems (set do_not_inventory on the source instance) and ensure it no longer auto-restarts.
  5. Register the target instance with systems (remove do_not_inventory from the target instance).
  6. Start the target instance (if it was previously running).
  7. Cleanup (remove source zone, or schedule for later removal).

Diagram

migration_diagram

User Interfaces

CloudAPI

The following endpoints will be available for customers:

  • ListMigrations (GET /:login/migrations Return a list of migrations (completed, running, scheduled, failed, ...).

    This should call VMAPI (GET /migrations?owner_uuid=:owner), which will return a JSON array of migration objects.

  • MigrateMachineEstimate (GET /:login/machines?action=migrate&migration_action=estimate) Estimates how long it will take to complete a migration of this instance.

    This should call VMAPI (POST /vms/:id?action=migrate&migration_action=estimate), which will return a migration estimate JSON object, e.g.

    { total_seconds: 123049372, downtime_seconds: 300, size_bytes: 43234995 }

  • MigrateMachine (POST /:login/machines/migrate?action=automatic&affinity=AFF) Starts a full "automatic" migration. Affinity rules can be added - see the provisioning section for details.

    This should call VMAPI (POST /vms/:id?action=migrate&migration_action=begin&migration_automatic=true&affinity=AFF), which on success will return a 202 code and a JSON object containing the job uuid.

  • AbortMigrateMachine (POST /:login/machines/migrate?action=abort) Aborts the current migration of this instance. See aborting a migration.

    This should call VMAPI (POST /vms/:id?action=migrate&migration_action=abort).

  • PauseMigrateMachine (POST /:login/machines/migrate?action=pause) Pauses (stops) the current migration of this instance. See pausing a migration. You can only pause a migration that is currently in the "sync" phase.

    This should call VMAPI (POST /vms/:id?action=migrate&migration_action=pause).

  • BeginMigrateMachine (POST /:login/machines/migrate?action=begin&affinity=AFF) Starts an on-demand migration (i.e. just the "begin" phase) for an instance. Affinity rules can be added - see the provisioning section for details.

    This should call VMAPI (POST /vms/:id?action=migrate&migration_action=begin&affinity=AFF), which on success will return a 202 code and a JSON object containing the job uuid.

  • SyncMigrateMachine (POST /:login/machines/migrate?action=sync) Runs the "sync" phase for an "on-demand" instance migration.

    This should call VMAPI (POST /vms/:id?action=migrate&migration_action=sync), which on success will return a 202 code and a JSON object containing the job uuid.

  • SwitchMigrateMachine (POST /:login/machines/migrate?action=switch) Runs the final "switch" phase for an "on-demand" instance migration.

    This should call VMAPI (POST /vms/:id?action=migrate&migration_action=switch), which on success will return a 202 code and a JSON object containing the job uuid.

  • WatchMigrateMachine (POST /:login/machines/migrate?action=watch) Will output events for a migration. There can be multiple watchers of a migration. See migration progress events for details.

    This should call VMAPI (POST /vms/:id?action=migrate&migration_action=watch), which on success will return a 200 code and a stream of JSON newline separated progress objects.

  • ScheduleMigrateMachine (POST /:login/machines/migrate?action=schedule&affinity=AFF) Schedule a migration of this instance. Affinity rules can be added - see the provisioning section for details.

    This should call VMAPI (POST /vms/:id?action=migrate&migration_action=schedule&affinity=AFF), which on success will return a 200 code and a JSON migration object.

Triton CLI

$ triton ls
SHORTID   NAME        IMG       STATE    FLAGS  AGE
4a364ec6  myinstance  46b4ceef  running  DF     3d

$ triton instance migrate --help
This will allow you to move an instance to a new server. A new
instance will be reserved and all of the instance filesystems will copied
across to the new instance. After the filesystems have been initially
synced, there will be a brief downtime of the instance and re-syncing of
the filesystems to the new instance, after that the new instance will be
started on the new server and the old instance will removed.

$ triton instance migrate --list
INSTANCE   STATUS   PROGRESS

$ triton instance migrate [--automatic] 4a364ec6
This will migrate instance 4a364ec6 (myinstance).
The migration will take approximately 1h45m.
Do you wish to continue y/n? y
Migration of 4a364ec6 has started.
You can watch the migration using:
  triton instance migrate --watch 4a364ec6
or you can pause or abort the migration using:
  triton instance migrate --pause|--abort 4a364ec6

$ triton ls  # shows 'M' flag meaning the instance is migrating
SHORTID   NAME        IMG       STATE     FLAGS  AGE
4a364ec6  myinstance  46b4ceef  running   MDF    3d

$ triton instance migrate --watch 4a364ec6
Migration progress ($migrateUuid):
✓ making a reservation (3m18s)
☐ synchronizing filesystems (13% ETA 1h33m)
<Ctrl-C ... migration still continues>

$ triton instance migrate --watch 4a364ec6
Migration progress ($migrateUuid):
✓ making a reservation (3m18s)
✓ synchronizing filesystems (1h52m07s)
✓ synchronizing filesystems (1h52m07s)
✓ synchronizing filesystems (1h52m07s)
✓ stopping instance (19s)
✓ re-synchronizing filesystems (1m19s)
✓ switching instance information (0m30s)
✓ restarting instance (22s)
Migration was successful.

Begin phase example:

$ triton instance migrate --begin 4a364ec6
This will begin migrating instance 4a364ec6 (myinstance).
The begin phase will take approximately 3m10s, after which you will
can then perform the sync and/or switch phase.
Do you wish to continue y/n? y
Begin migration of 4a364ec6 has started.
You can watch the migration using:
  triton instance migrate --watch 4a364ec6
or you can pause or abort the migration using:
  triton instance migrate --pause|--abort 4a364ec6

$ triton instance migrate --watch 4a364ec6
Migration progress ($migrateUuid):
✓ making a reservation (3m18s)
✓ Begin migration phase was successful.

Sync phase example:

$ triton instance migrate --sync 4a364ec6
This will sync instance 4a364ec6 (myinstance).
The sync phase will take approximately 2h18m0s, after which you will
can then perform another sync or perform the final switch phase.
Do you wish to continue y/n? y
Sync migration of 4a364ec6 has started.
You can watch the migration using:
  triton instance migrate --watch 4a364ec6
or you can pause or abort the migration using:
  triton instance migrate --pause|--abort 4a364ec6

$ triton instance migrate --watch 4a364ec6
Migration progress ($migrateUuid):
✓ making a reservation (3m18s)
☐ synchronizing filesystems (39% ETA 1h33m)
...
✓ synchronizing filesystems (1h52m07s)
Sync migration was successful.

$ triton instance migrate --sync 4a364ec6
...
$ triton instance migrate --watch 4a364ec6
Migration progress ($migrateUuid):
✓ making a reservation (3m18s)
✓ synchronizing filesystems (1h52m07s)
✓ synchronizing filesystems (20m55s)
✓ synchronizing filesystems (5m01s)
☐ synchronizing filesystems (5% ETA 1m20s)
...
✓ synchronizing filesystems (1m13s)
Sync migration phase was successful.

Switch phase example:

$ triton instance migrate --switch 4a364ec6
This will switch instance 4a364ec6 (myinstance).
The switch phase will take approximately 5m0s, after which the migration
will be complete.
Do you wish to continue y/n? y
Switch migration of 4a364ec6 has started.
You can watch the migration using:
  triton instance migrate --watch 4a364ec6
or you can pause or abort the migration using:
  triton instance migrate --pause|--abort 4a364ec6

$ triton instance migrate --watch 4a364ec6
Migration progress ($migrateUuid):
✓ making a reservation (3m18s)
✓ synchronizing filesystems (1h52m07s)
✓ synchronizing filesystems (20m55s)
✓ synchronizing filesystems (5m01s)
✓ synchronizing filesystems (1m13s)
☐ switching instances (0% ETA 5m0s)
...
✓ switching instances (4m28s)
Migration was successful.

Abort migration example:

$ triton instance migrate --abort 4a364ec6
This will abort migration of instance 4a364ec6 (myinstance).
You will need to start a new migration if you wish to migrate this instance.
Do you wish to continue y/n? y
Aborting migration of 4a364ec6.
You can watch the migration using:
  triton instance migrate --watch 4a364ec6

$ triton instance migrate --watch 4a364ec6
Migration progress ($migrateUuid):
✓ making a reservation (3m18s)
✓ synchronizing filesystems (1h52m07s)
☐ aborting
...
✓ aborted
Migration was aborted.

Pause migration example:

$ triton instance migrate --pause 4a364ec6
This will pause migration of instance 4a364ec6 (myinstance).
Once paused, you will be able to continue this migration using:
  triton instance migrate --sync|--switch 4a364ec6
Do you wish to continue y/n? y
Pausing migration of 4a364ec6.
You can watch the migration using:
  triton instance migrate --watch 4a364ec6

$ triton instance migrate --watch 4a364ec6
Migration progress ($migrateUuid):
✓ making a reservation (3m18s)
✓ synchronizing filesystems (1h52m07s)
☐ pausing
...
✓ paused
Migration was paused.

Scheduling example:

$ triton instance migrate --schedule "2 days from now" 4a364ec6
This will migrate instance $shortId. Do you wish to continue y/n?
Scheduling migration.
You can watch the migration using:
  triton instance migrate --watch 4a364ec6
or you can pause or abort the migration using:
  triton instance migrate --pause|--abort 4a364ec6
Initial scheduling complete - migration will commence at $date.

AdminUI

TODO: I'm not sure who looks after this?

Portal

TODO: I'm not sure who looks after this?

Persistent storage

A record of all migration operations will be stored in a reliable datastore (e.g. Moray). This will be used to report state to users and operators for past, ongoing and future migrations.

The following is an example Moray bucket schema to be used for migration operations:

migrations {

  uuid: "UUID"
    // The unique migration identifier.

  account_uuid: "UUID"
    // The account who invoked (and thus controls) this migration. The admin
    // user will always be able to control (schedule/abort) all migrations,
    // but non-admin accounts will only be allowed to control their own
    // migrations.

  vm_uuid: "UUID"
    // The uuid of the instance to be migrated.

  source_server_uuid: "UUID"
    // The uuid of the server where the source instance resides.

  target_server_uuid: "UUID"
    // The uuid of the server where the instance will be migrated to.

  automatic: "bool"
    // Whether this is a full automatic migration.

  state: "string"
    // The state the migration operation is currently in. It can be one of
    // the following states:
    //  "scheduled"  - the migration is scheduled, see "scheduled_timestamp"
    //  "running"    - migration running, see also "progress_history"
    //  "paused"     - the "begin" phase (and possibly "sync" phase) has
    //                 been run - now waiting for a call to "sync"
    //                 or the final call to "switch"
    //  "aborted"    - user or operator aborted the migration attempt
    //  "failed"     - migration operation could not complete, see "error"
    //  "successful" - migration was successfully completed, you can use the
    //                 workflow job_uuid to find when the migration started
    //                 and how long it took.

  created_timestamp: "string"
    // The ISO timestamp when the migration was first created. This will
    // also be used for queuing - migrations created first will be closer
    // to the start of the queue than those created later.
    // Example:
    //   2018-08-18T10:55:39.785Z

  scheduled_timestamp: "string"
    // The ISO timestamp when the migration should commence. The migration
    // service will be responsible to start this migration at the
    // appropriate time.
    // Example:
    //   2018-08-19T22:30:00.000Z

  started_timestamp: "string"
    // The ISO timestamp when the migration started. This is useful to see
    // how long a migration has taken.
    // Example:
    //   2018-08-19T22:30:05.142Z

  finished_timestamp: "string"
    // The ISO timestamp when the migration finished. This is useful to see
    // how long a migration took to complete.
    // Example:
    //   2018-08-19T23:49:45.880Z

  phase: "string"
    // The workflow stage that the migration is currently running, one of:
    //  "begin"            - just starting the migration operation
    //  "sync"             - updating filesystems using the "sync" phase
    //  "switch"           - sending final zfs datasets and switching
    //  "abort"            - aborting the migration

  num_sync_phases: "number"
    // The number of successful "sync" phases this migration has performed.

  last_sync_size: "number"
    // The size (in bytes) of the last successful sync.

  progress_history: "array of completed JSON progress events"
    // Each time a migration phase is started (or when a major migration
    // event has occurred) an entry will be added to this array. This will
    // be used to initially populate the migration watch event stream. See
    // the "Progress Events" section for details.
    // Example:
    //   [{begin}, {sync}, {sync}, {sync}, {switch}]

  error: "string"
    // If a migration fails - this is the error message of why it failed.

  errorDetail: "string"
    // Extra detailed information on why the migration failed, such as a
    // stacktrace, or the full error object/message from a failed API call.
}

Images

If the source image(s) still exist in IMGAPI, then the image(s) can be installed into the target server through the normal provisioning process.

When not available in IMGAPI, then the instance migration process must manually copy/install the image(s) into the target server itself. A "warning" progress event should be made in such a case, as some features (like CreateImageFromMachine) will depend on these images being available.

By having the source image(s) it will ensure that the space accounting is consistent on the source and target servers. This consistency will also help to avoid quota-related issues with snapshots and migrations.

Snapshots

ZFS snapshots will be used to synchronize the instance filesystems between the source and target servers.

Migration snapshots will be named @migrate-snap-N, where N is incremented by 1 for each incremental snapshot.

The initial snapshot will be a recursive snapshot created using zfs snapshot -r filesystem and will be sent to the target server using zfs send -R filesystem@migrate-snap-1 (the -R argument will ensure all snapshot names and all descendant filesystems are included) - this should also tie in well with Instance Snapshots, see RFD 148.

Incremental snapshots (used in the "sync" and "switch" phases) will use zfs send -I filesystem@migrate-snap-N-1 filesystem@migrate-snap-N, which will include any new snapshot names that were created between the snapshots.

Snapshot lifecycle

At least one snapshot will need to be used during migration. Two snapshots will need to be used during the zfs send/receive, after which snapshot N-1 can be destroyed on the source and target servers.

It should also be possible to reduce the number of source snapshots to at most one, by turning the source snapshot N-1 into a bookmark using zfs bookmark filesystem@migrate-snap-N-1. Unlike a snapshot, bookmarks do not consume any additional space and this will still allow zfs send to function correctly (albeit a little slower according to some web comments).

Snapshot quotas

In order to do a snapshot on the source (or receive a snapshot on the target), the quota may need to be increased or even be removed. The zfs refquota and/or volsize attributes should be sufficient for restricting growth of all but delegated datasets.

Provisioning

There are a lot of factors that need to be considered during the migration provisioning process.

Supporting services

Any Triton supporting services that the instance uses (e.g. NAT zones or Volumes zones) will need to be setup on the target server. If provisioning using DAPI, this should occur naturally.

Server next reboot date

Ideally, we want the migrated instance to land on a server that will not be rebooted soon. If DAPI happens to pick a machine that will be rebooted in another 2 weeks, that lessens the purpose of the current migration and will likely aggravate users. We want to make sure the migration provisioning process takes into account the next reboot attribute, see RFD 24, which if using DAPI should occur naturally.

Affinity constraints

Affinity hints are only applied at provisioning time. For docker containers, these hints are persisted as a triton label in the instance metadata. For other instance types, the hints are not persisted yet (TRITON-779 will address this - to store the affinity values). As such, DAPI will not be able to honor the original affinity constraints. We should allow the user/operator to input an optional affinity hint when they initiate/schedule migrations.

Traits and reserved CNs

DAPI can handle user specified traits, but there are cases where a trait has been specified by an operator, and as such, DAPI may not be able to handle that. In this case the operator will need to either remove that trait from the instance, or use the sdc-migrate tooling and specify the target server (the latter will not invoke the DAPI server selection algorithms).

Modified instances

DAPI uses the package size to determine the allocation. It's possible that an instance will have gone through an in-place update to increase their quota, ram, etc and no longer have sizes that match the original package. We will need to pass additional migration specific instance parameters to DAPI to specify the wanted size in such a case.

Network NICs

During the provisioning process, we do not want to allocate any NICs or assign IP addresses for the target instance. Instead, during the switch phase the NICs will need to be swapped from the source to the target instance. This NIC swap is accomplished by first reserving the IP, deleting the source instance NIC and then adding the NIC to the target instance. We used to have sporadic issues with duplicate IP addresses after migration when the IP of the instance being migrated was accidentally handed out for a different provisioning.

Rack Aware Networks

In the case of a CN being in a rack aware network - the DAPI provisioning process will need to take into account (if not already done) that the newly provisioned instance must reside in a CN in the same rack.

Provisioning Questions

  • if migration re-uses the existing cn-agent provisioning process, can some parts (e.g. imgadm image install and dataset creation) be skipped?

  • what other Triton components will need updating after a migration (i.e. are there any Triton services that expect the instance to be on a particular CN)?

Allowing user migrations

There will two settings controlling whether a user is allowed to migrate their instances. These settings can only be set by an operator.

  • sdc.migration.user_migration_allowed (boolean) value on the global SDC sapi object. When set to 'true' allows user migration for any of their instances on any CN, or 'false' to disallow allow user migrations. This setting can be overruled on a per-instance basis by the vm.internal_metadata.user_migration_allowed setting (see below).

    $ sdc-migrate settings
    $ sdc-migrate set allow_user_migrations true
    $ sdc-migrate set allow_user_migrations false
    
  • vm.internal_metadata.user_migration_allowed (boolean) property on the instance can be set to 'true' to allow user migration of this instance, or 'false' to disallow migration of this instance. This value overrides the sdc.migration.user_migration_allowed value. For a missing value (the default), the sdc.migration.user_migration_allowed setting is used.

    $ sdc-vmapi "/vms/$VM?action=update" -X POST -d '{ "internal_metadata": { "user_migration_enabled": true }}'
    $ sdc-migrate instance allow-user-migrations $VM
    $ sdc-migrate instance disallow-user-migrations $VM
    

Note that a previous version of the RFD also had a per-CN setting to control migrations off particular compute nodes - but this was judged to be ungainly (as it needed to modify the CNAPI server schema), and the same could be accomplished by using the "per-instance" setting on all instances on a given CN.

Re-sync algorithm

During the incremental filesystem update, the migration system will make a calculation to see if it needs to continue re-syncing the filesystem or perform the final sync and switch. The idea is that further re-syncing will reduce the time needed for the final sync (as the snapshots become smaller and the time needed to perform the sync becomes faster and faster), and thus reducing the downtime for the instance.

These are the criteria for when the migration can proceed to the final sync and switch:

  • the incremental snapshot size is less than a configured max size (e.g. 50MB)
  • the number of re-sync attempts exceeds a limit (e.g. 10)
  • the snapshot size has not reduced significantly in the last N re-sync attempts (e.g. last 3 re-syncs)

Limits

There are limits placed on migrations, to ensure that they do not affect (or limitedly affect) other tenants on the source.

The migration limit configuration will be stored in SAPI (initially set during the migration service installation, or falling back to defaults) as:

$ sdc-sapi /services?name=migration
{
  "allow_user_migrations": false,
  "max_running_migrations_per_cn": 5,
  "zfs_send_mbps_limit": 500 // megabits per second
}

$ sdc-migrate settings
$ sdc-migrate set max_running_migrations_per_cn 10
$ sdc-migrate set zfs_send_mbps_limit 10

The zfs send limits can be enforced by the application (e.g. Node.js), by adjusting the amount of data being sent/received (piped) to the target.

Open questions for limits

  • can these limits be set (overwritten) on a per CN basis too? Can changes to these limits be propagated to migrations that are currently running? Maybe a running migration task can check these settings every X minutes and apply updates when they change.

  • do we need a zfs_receive_bps_limit too - in case of different server hardware configurations?

  • what happens when we exceed the number of migrations allowed - is that a hard error back to the caller? Note that "pending" migrations would not be included in this count.

  • what happens when it cannot start a scheduled migration at the scheduled time, how much leeway is allowed there?

Error handling

Transient errors

Transient errors during a migration, like a network failure, or a triton component failure, will be handled and the migration process will retry to complete the migration step which failed. The migration workflow will have a hard coded number of retry operations per migration step with a backoff setting to allow a flexible delay between the retry operations.

Resumable zfs send/receive

The zfs send/receive operations (long/sustained network usage) will make use of the resumable zfs send/receive feature, so that the migration operation can resume in the case of an interruption, without needing to resend all of the zfs data.

In the case of a zfs send/recv failure, a receive_resume_token marker is set in the zfs filesystem, which can be used as an argument to zfs send, allowing the stream to continue from where it left off:

$ zfs receive -s  # Keep track of stream position
$ zfs send -s token  # Continue from this point in the send stream

Note that to use this flag, the storage pool must have the extensible_dataset feature enabled, which can be queried using:

$ zpool get -H -o value "feature@extensible_dataset" zones
enabled

Fatal errors

Fatal errors, such as:

  • source CN is no longer visible/responding

    A "partially" migrated instance will reside on the target CN. This will require operator intervention (to bring back the source CN). XXX: Can the instance owner delete that failed target instance?

  • target CN is no longer visible/responding

    The migration will be marked as "failed" and the source instance will be restored to it's prior state. A record of the failure is kept in the migration database.

  • when both the source and target CN are not visible/responding

    Not much can be done here - this will require operator intervention. If one or both of these CNs come back, then we move to above two items.

Aborting a migration

There are many factors to consider when aborting a migration, most of which will depend on the migration phase. There will even be times when it is not possible to abort at all (in the latter phases of the migration, when the instance migration is done and the process is now cleaning up).

It will be crucial that the source instance be returned to it's former state, including the re-instantiation of NICs (in the case they were deleted from the source instance - see the network provisioning notes) and returning to the source instance to it's previous running/stopped state.

An abort will cause VMAPI to issue stop/abort calls to any ongoing migration workflow jobs and cn-agent migration tasks, then VMAPI will rollback/cleanup the migration state and revert the source instance to it's former state (if it was changed).

Note that it will not be possible to resume an aborted migration, but one could start a new migration instead.

Pausing a migration

A migration can only be paused if it is in the "sync" phase.

It is possible to resume a paused migration, by running the "sync" and/or "switch" migration commands manually. If a migration was previously in "automatic" migration mode, then when it is resumed, it will continue in the "automatic" mode.

Progress

For each migration, we need the ability to show the current state and progress to the end user.

VMAPI will provide a migration progress endpoint that can be used to monitor the progress of the migration.

VMAPI will connect with the cn-agent task on the source instance (using a TCP/IP socket) to retrieve status, filesystem transfer rates and overall progress (e.g. from zfs send/receive).

TODO: Detail the events for this communication.

Progress events

The VMAPI migration watch endpoint will respond with a stream of JSON event objects. These JSON objects will be "progress" events that describe the migration phases and the progress/status for the migration phases and then the final event will be a special JSON "end" event.

Progress event

Progress events are sent when something important occurs during the migration. There are actually two styles of progress events - one for major events (and/or warnings) and one to show the sync progress (bandwidth and eta).

event {

  type: "string"
    // Type is "progress".

  phase: "string"
    // Current phase.

  state: "string"
    // State in "running".

  current_progress: "number"
    // This is how much progress has been made. For the sync phase, this is
    // the number of bytes that have been sent to the target.

  total_progress: "number"
    // This is total possible progress. For the sync phase, this is
    // the number of bytes that will need to be sent to the target.

  message: "string" (optional)
    // Additional description message for this task.

  error: "string" (optional)
    // Error occurred in this task - this is the description for that error.

  started_timestamp: "string" (optional)
    // The ISO timestamp when the phase was started.

  duration_ms: "number" (optional)
    // The number of milliseconds the phase has taken.

  eta_ms: "number" (optional)
    // Estimate of the number of milliseconds until the task is completed.

  transfer_bytes_second: "number" (optional)
    // The number of bytes being sent per second between the source and
    // target instances during the "sync" phase.
}

Progress event (sync started):

{
  "type": "progress"
  "phase": "sync",
  "state": "running",
  "message": "syncing data",
  "current_progress": 1,
  "total_progress": 100,
  "started_timestamp": "2019-02-04T22:13:49.947Z",
  "duration_ms": 377
}

Progress event (sync progress):

{
  "type": "progress"
  "phase": "sync",
  "state": "running",
  "current_progress": 45916,
  "total_progress": 45916,
  "eta_ms": 218,
  "transfer_bytes_second": 22943,
}

Progress event (switch removing snapshots):

{
  "type": "progress"
  "phase": "switch",
  "state": "running",
  "message": "removing sync snapshots",
  "current_progress": 90,
  "total_progress": 100,
}

End event

A final "end" event is sent when a migration task is completed. This can be used to see if the migration task was successful.

event {

  type: "string"
    // Type is "end"

  phase: "string"
    // The phase for this migration, one of "begin", "sync" or "switch".

  state: "string"
    // The status for the event, e.g. "paused", "successful" or "failed".

  message: "string" (optional)
    // Additional description message or error for this phase/state.
}

End event (sync successful, state now paused):

{
  "type": "end",
  "phase": "sync",
  "state": "paused"
}

End event (switch successful, migration finished):

{
  "type": "end",
  "phase": "switch",
  "state": "successful"
}

End event (switch failure):

{
  "type": "end",
  "phase": "switch",
  "state": "failed",
  "message": "Failed to switch instances - could not mount filesystem"
}

Scheduling

A migration can be scheduled to begin running at a configured time/date. In this case the migration will (immediately on receiving the migration schedule call) start the migration process which will provision the target instance and perform the initial zfs snapshot synchronization. At this point the migration process (if the scheduled time has not yet occurred) will pause, then at the scheduled time the final shutdown and re-synchronize will occur and will finish the migration process.

Migration service

A "triton-migrate-service" zone will be used to control scheduled migrations. It does this by loading the currently running/scheduled migrations from the migrations database (Moray bucket) and then periodically checks for updates.

Roles:

  • initiates the migration at the scheduled time

Needs to communicates with:

  • CNAPI
  • VMAPI

Note: This service could later be expanded to handle complete CN evacuations, but that is outside the scope of this RFD.

Operator Installation

Migration is separated into two main components:

Triton migration agent

The triton-migration-agent contains the executables (code) needed to perform an instance migration from one compute node to another. It must be installed in the GZ on all of the CNs which will support migration (i.e. all CNs). The triton-migration-agent can be installed and updated via sdcadm:

$ sdcadm experimental update-agents --latest --all

This agent can be installed anytime, but a migration from or to this CN cannot occur until this agent has been installed.

Triton migration scheduler

The scheduler's job is to start migration of instances at a predetermined time.

$ sdcadm post-setup migration-scheduler

This will create the triton-migration-scheduler zone on a headnode.

This zone is only necessary for migration scheduling, without this zone an instance migration cannot be scheduled (it will raise an error), but an on-demand instance migration can still be performed (if the triton-migration-agent is installed).

Milestones

M1: Initial setup

Setup the initial repository and testing infrastructure.

  • write brand specific tests for migration
    • create small test images (maybe Alpine based) instead of using Ubuntu?
  • initial handling for zfs snapshots and incremental zfs send/receive
  • verify zfs send/receive is working for all brands (SmartOS, Docker, LX, Kvm, Bhyve)

M2: Brand specific migration

Concentrate on getting Bhyve (and KVM?) working first. For datasets that are nearly full, the destination quota may need to be increased slightly, just enough for all necessary write operations. Ideally, the quota would be reduced to its original size after migration is complete.

  • modify zfs send/receive handling to support the brands needed
  • support multiple disks and different zfs filesystem layouts
  • add quota workaround for nearly full instances

M3: Update DAPI (CN provisioning/reservation)

DAPI will be updated to select the target CN and provision (reserve) the target instance.

  • provision (reserve) the target instance
    • choose CN
      • allow allocation to an operator specified server (via sdc-migrate)
      • pass through affinity rules, see the provisioning section
      • pass through instance size, see the provisioning section
    • allocate/reserve zone
    • install image(s)
    • setup support infrastructure (e.g. Fabric NAT)

M4: Update VMAPI

VMAPI will trigger and respond to migration calls, using workflow and cn-agent tasks to control and monitor the migration.

  • add VMAPI API migration endpoint
  • add workflow tasks for control of the migration states
  • add CNAPI APIs to feed into cn-agent tasks
  • create cn-agent tasks to initiate sending/receiving
  • other actions on source/target instance cannot occur during a migration
  • instance cleanup
  • update sdc-clients to add VMAPI migration API wrappers

M5: Create sdc-migrate tooling

Operators will need a tool to be able to perform a migration from inside of a CN/Headnode:

$ sdc-migrate settings [$CN]
{
    "allow_user_migrations": false,
    "max_migrations_per_cn": 5,
    "zfs_send_mbps_limit": 500
}
# Allow updating too?

$ sdc-migrate list [$CN1 $CN2 $CN3]
...<Show ongoing migrations, state, percentage>

$ sdc-migrate set allow_user_migrations true
Success - datacenter allows user migrations.
$ sdc-migrate disallow-cn $CN
Success - datacenter disallows user migrations.

$ sdc-migrate instance $VM allow_user_migrations true
Success - instance $VM can be user migrated.
$ sdc-migrate instance $VM allow_user_migrations false
Success - instance $VM will not allow user migration.

$ sdc-migrate migrate $INSTANCE [$CN]
Migrating $INSTANCE
...<progress events>
...<progress events>
<Ctrl-C ... workflow job still continues>

$ sdc-migrate watch $INSTANCE
...<progress events>
Success - instance $INSTANCE has been migrated to server $CN.

$ sdc-migrate abort $INSTANCE
...<progress events>
Success - migration was aborted, instance $INSTANCE is now $STATE.

Once an instance has been flagged as allowing migrations then the operator can send an announcement to that user, allowing them to migrate their own instances via CloudAPI or Triton command line. The portal (AdminUI) may make use of these flags to notify that a CN is going down and that they can migrate their instances.

M6: End user migration

Allow users to perform or schedule their own instance migrations.

  • add CloudAPI migration APIs
    • this may want to be a workflow job
    • add support for polling migration job
  • add Triton cli instance migration support

M7: Migration estimate

Add APIs that will return an estimate of the time taken to perform a migration. It should also be possible to get an estimate for a currently running migration.

M8: Migration scheduling

Add ability to schedule a migration.

  • create the migration scheduling zone
  • write queue handling for migrations
  • connect with VMAPI for performing migrations
  • update tooling (sdc-migrate, node-triton and portal) to support migration scheduling

Open Questions and TODOs

Backups

  • do we want to create a permanent (or for a limited time) snapshot/backup of the migrated instance (e.g. stored in Manta) in case something goes amiss, i.e. it looks like migration was successful and the source instance was deleted, but in actual fact the target instance is not functioning and now we are SOL? An alternative to this is that we mark the original source instance as migrated and leave it around for later possible restoration, but this would not be good in the case of a CN being decommissioned.

Networking

  • do the source and destination CNs need to be in the same subnet?

  • are there any other restrictions imposed by instance and/or CN networking (e.g. arp, DNS, routing, firewall)?

  • how to control the creation of networks such that two instances have the same IP address/MAC etc... does it initially provision the target instance without networks and then later remove these networks from the source and then re-add to the target instance (this is what the legacy-migrate script does, but it also reserves the IP addresses so that they are not lost)?

Caveats

  • CPU must to be the same on both source and target CN (e.g. 64-bit Intel with Vt/Vx support).
  • Destination platform version needs to be equal to or greater than the source platform version.
  • The image(s) from which the instance was created must still exist in IMGAPI
  • Source/Dest time must be in sync.
  • PCI-passthrough (and similar) devices will not be supported.
  • Guest within a guest is not supported?
  • Cannot migrate core Triton services?

Tests

This is mostly a dev notes section to run tests on these items:

  • delegated datasets
  • multiple disks (e.g. KVM disk-0, disk-1, disk-2)
  • SSH daemons (and their keys) check they work correctly in a migrated instance
  • ensure cannot migrate the same instance at the same time (double migration)
  • ensure other actions to the instance (start, stop, delete, etc...) cannot be performed while an instance is migrating
  • ensure changes made to the instance (like alias change, adding a tag) during the sync phase are propagated to the target instance
  • ensure an owner cannot stop (abort) an operator controlled migration
  • for incremental, ensure the instance has not been reprovisioned in the time between the initial snapshot send and the incremental snapshot send.
  • failure cases, by the truckload

References

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