Storage Replication: Difference between revisions

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without using shared storage. Replication uses snapshots to minimize traffic
without using shared storage. Replication uses snapshots to minimize traffic
sent over the network. Therefore, new data is sent only incrementally after
sent over the network. Therefore, new data is sent only incrementally after
an initial full sync. In the case of a node failure, your guest data is
the initial full sync. In the case of a node failure, your guest data is
still available on the replicated node.
still available on the replicated node.
The replication will be done automatically in configurable intervals.
The replication is done automatically in configurable intervals.
The minimum replication interval is one minute and the maximal interval is
The minimum replication interval is one minute, and the maximal interval
once a week. The format used to specify those intervals is a subset of
once a week. The format used to specify those intervals is a subset of
systemd calendar events, see
systemd calendar events, see
Schedule Format section:
Schedule Format section:
Every guest can be replicated to multiple target nodes, but a guest cannot
It is possible to replicate a guest to multiple target nodes,
get replicated twice to the same target node.
but not twice to the same target node.
Each replications bandwidth can be limited, to avoid overloading a storage
Each replications bandwidth can be limited, to avoid overloading a storage
or server.
or server.
Virtual guest with active replication cannot currently use online migration.
Guests with replication enabled can currently only be migrated offline.
Offline migration is supported in general. If you migrate to a node where
Only changes since the last replication (so-called deltas) need to be
the guests data is already replicated only the changes since the last
transferred if the guest is migrated to a node to which it already is
synchronisation (so called delta) must be sent, this reduces the required
replicated. This reduces the time needed significantly. The replication
time significantly. In this case the replication direction will also switch
direction automatically switches if you migrate a guest to the replication
nodes automatically after the migration finished.
target node.
For example: VM100 is currently on nodeA and gets replicated to nodeB.
For example: VM100 is currently on nodeA and gets replicated to nodeB.
You migrate it to nodeB, so now it gets automatically replicated back from
You migrate it to nodeB, so now it gets automatically replicated back from
nodeB to nodeA.
nodeB to nodeA.
If you migrate to a node where the guest is not replicated, the whole disk
If you migrate to a node where the guest is not replicated, the whole disk
data must send over. After the migration the replication job continues to
data must send over. After the migration, the replication job continues to
replicate this guest to the configured nodes.
replicate this guest to the configured nodes.
High-Availability is allowed in combination with storage replication, but it
High-Availability is allowed in combination with storage replication, but it
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Such a calendar event uses the following format:
Such a calendar event uses the following format:
[day(s)] [[start-time(s)][/repetition-time(s)]]
[day(s)] [[start-time(s)][/repetition-time(s)]]
This allows you to configure a set of days on which the job should run.
This format allows you to configure a set of days on which the job should run.
You can also set one or more start times, it tells the replication scheduler
You can also set one or more start times. It tells the replication scheduler
the moments in time when a job should start.
the moments in time when a job should start.
With this information we could create a job which runs every workday at 10
With this information we, can create a job which runs every workday at 10
PM: 'mon,tue,wed,thu,fri 22' which could be abbreviated to: 'mon..fri
PM: 'mon,tue,wed,thu,fri 22' which could be abbreviated to: 'mon..fri
22', most reasonable schedules can be written quite intuitive this way.
22', most reasonable schedules can be written quite intuitive this way.
Hours are set in 24h format.
Hours are formatted in 24-hour format.
To allow easier and shorter configuration one or more repetition times can
To allow a convenient and shorter configuration, one or more repeat times per
be set. They indicate that on the start-time(s) itself and the start-time(s)
guest can be set. They indicate that replications are done on the start-time(s)
plus all multiples of the repetition value replications will be done. If
itself and the start-time(s) plus all multiples of the repetition value. If
you want to start replication at 8 AM and repeat it every 15 minutes until
you want to start replication at 8 AM and repeat it every 15 minutes until
9 AM you would use: '8:00/15'
9 AM you would use: '8:00/15'
Here you see also that if no hour separation (:) is used the value gets
Here you see that if no hour separation (:), is used the value gets
interpreted as minute. If such a separation is used the value on the left
interpreted as minute. If such a separation is used, the value on the left
denotes the hour(s) and the value on the right denotes the minute(s).
denotes the hour(s), and the value on the right denotes the minute(s).
Further, you can use * to match all possible values.
Further, you can use * to match all possible values.
To get additional ideas look at
To get additional ideas look at
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tue, wed, thu, fri and sat. You may use multiple days as a comma-separated
tue, wed, thu, fri and sat. You may use multiple days as a comma-separated
list. A range of days can also be set by specifying the start and end day
list. A range of days can also be set by specifying the start and end day
separated by “..”, for example mon..fri. Those formats can be also
separated by “..”, for example mon..fri. These formats can be mixed.
mixed. If omitted '*' is assumed.
If omitted '*' is assumed.
time-format
time-format
A time format consists of hours and minutes interval lists.
A time format consists of hours and minutes interval lists.
Hours and minutes are separated by ':'. Both, hour and minute, can be list
Hours and minutes are separated by ':'. Both hour and minute can be list
and ranges of values, using the same format as days.
and ranges of values, using the same format as days.
First come hours then minutes, hours can be omitted if not needed, in this
First are hours, then minutes. Hours can be omitted if not needed. In this
case '*' is assumed for the value of hours.
case '*' is assumed for the value of hours.
The valid range for values is 0-23 for hours and 0-59 for minutes.
The valid range for values is 0-23 for hours and 0-59 for minutes.
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Every minute (minimum interval)
Every minute (minimum interval)
Error Handling
Error Handling
If a replication job encounters problems it will be placed in error state.
If a replication job encounters problems, it is placed in an error state.
In this state the configured replication intervals get suspended
In this state, the configured replication intervals get suspended
temporarily. Then we retry the failed replication in a 30 minute interval,
temporarily. The failed replication is repeatedly tried again in a
once this succeeds the original schedule gets activated again.
30 minute interval.
Once this succeeds, the original schedule gets activated again.
Possible issues
Possible issues
This represents only the most common issues possible, depending on your
Some of the most common issues are in the following list. Depending on your
setup there may be also another cause.
setup there may be another cause.
Network is not working.
Network is not working.
No free space left on the replication target storage.
No free space left on the replication target storage.
Storage with same storage ID available on target node
Storage with same storage ID available on the target node
You can always use the replication log to get hints about a problems
You can always use the replication log to find out what is causing the problem.
cause.
Migrating a guest in case of Error
Migrating a guest in case of Error
In the case of a grave error a virtual guest may get stuck on a failed
In the case of a grave error, a virtual guest may get stuck on a failed
node. You then need to move it manually to a working node again.
node. You then need to move it manually to a working node again.
Example
Example
Lets assume that you have two guests (VM 100 and CT 200) running on node A
Let’s assume that you have two guests (VM 100 and CT 200) running on node A
and replicate to node B.
and replicate to node B.
Node A failed and can not get back online. Now you have to migrate the guest
Node A failed and can not get back online. Now you have to migrate the guest
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check if that the cluster is quorate
check if that the cluster is quorate
# pvecm status
# pvecm status
If you have no quorum we strongly advise to fix this first and make the
If you have no quorum, we strongly advise to fix this first and make the
   node operable again. Only if this is not possible at the moment you may
   node operable again. Only if this is not possible at the moment, you may
   use the following command to enforce quorum on the current node:
   use the following command to enforce quorum on the current node:
# pvecm expected 1
# pvecm expected 1
If expected votes are set avoid changes which affect the cluster
Avoid changes which affect the cluster if expected votes are set
(for example adding/removing nodes, storages, virtual guests) at all costs.
(for example adding/removing nodes, storages, virtual guests) at all costs.
Only use it to get vital guests up and running again or to resolve the quorum
Only use it to get vital guests up and running again or to resolve the quorum
issue itself.
issue itself.
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Remember to replace the VMIDs and node names with your respective values.
Remember to replace the VMIDs and node names with your respective values.
Managing Jobs
Managing Jobs
You can use the web GUI to create, modify and remove replication jobs
You can use the web GUI to create, modify, and remove replication jobs
easily. Additionally the command line interface (CLI) tool pvesr can be
easily. Additionally, the command line interface (CLI) tool pvesr can be
used to do this.
used to do this.
You can find the replication panel on all levels (datacenter, node, virtual
You can find the replication panel on all levels (datacenter, node, virtual
guest) in the web GUI. They differ in what jobs get shown: all, only node
guest) in the web GUI. They differ in which jobs get shown:
specific or only guest specific jobs.
all, node- or guest-specific jobs.
Once adding a new job you need to specify the virtual guest (if not already
When adding a new job, you need to specify the guest if not already selected
selected) and the target node. The replication
as well as the target node. The replication
schedule can be set if the default of all
schedule can be set if the default of all
15 minutes is not desired. You may also impose rate limiting on a
15 minutes is not desired. You may impose a rate-limit on a replication
replication job, this can help to keep the storage load acceptable.
job. The rate limit can help to keep the load on the storage acceptable.
A replication job is identified by an cluster-wide unique ID. This ID is
A replication job is identified by a cluster-wide unique ID. This ID is
composed of the VMID in addition to an job number.
composed of the VMID in addition to a job number.
This ID must only be specified manually if the CLI tool is used.
This ID must only be specified manually if the CLI tool is used.
Command Line Interface Examples
Command Line Interface Examples
Create a replication job which will run every 5 minutes with limited bandwidth of
Create a replication job which runs every 5 minutes with a limited bandwidth
10 mbps (megabytes per second) for the guest with guest ID 100.
of 10 Mbps (megabytes per second) for the guest with ID 100.
# pvesr create-local-job 100-0 pve1 --schedule "*/5" --rate 10
# pvesr create-local-job 100-0 pve1 --schedule "*/5" --rate 10
Disable an active job with ID 100-0
Disable an active job with ID 100-0.
# pvesr disable 100-0
# pvesr disable 100-0
Enable a deactivated job with ID 100-0
Enable a deactivated job with ID 100-0.
# pvesr enable 100-0
# pvesr enable 100-0
Change the schedule interval of the job with ID 100-0 to once a hour
Change the schedule interval of the job with ID 100-0 to once per hour.
# pvesr update 100-0 --schedule '*/00'
# pvesr update 100-0 --schedule '*/00'
</pvehide>
</pvehide>
<!--PVE_IMPORT_END_MARKER-->
<!--PVE_IMPORT_END_MARKER-->

Revision as of 08:51, 12 May 2020

The pvesr command-line tool manages the Proxmox VE storage replication framework. Storage replication brings redundancy for guests using local storage and reduces migration time.

It replicates guest volumes to another node so that all data is available without using shared storage. Replication uses snapshots to minimize traffic sent over the network. Therefore, new data is sent only incrementally after the initial full sync. In the case of a node failure, your guest data is still available on the replicated node.

The replication is done automatically in configurable intervals. The minimum replication interval is one minute, and the maximal interval once a week. The format used to specify those intervals is a subset of systemd calendar events, see Schedule Format section:

It is possible to replicate a guest to multiple target nodes, but not twice to the same target node.

Each replications bandwidth can be limited, to avoid overloading a storage or server.

Only changes since the last replication (so-called deltas) need to be transferred if the guest is migrated to a node to which it already is replicated. This reduces the time needed significantly. The replication direction automatically switches if you migrate a guest to the replication target node.

For example: VM100 is currently on nodeA and gets replicated to nodeB. You migrate it to nodeB, so now it gets automatically replicated back from nodeB to nodeA.

If you migrate to a node where the guest is not replicated, the whole disk data must send over. After the migration, the replication job continues to replicate this guest to the configured nodes.

Important

High-Availability is allowed in combination with storage replication, but there may be some data loss between the last synced time and the time a node failed.

Supported Storage Types

Table 1. Storage Types
Description Plugin type Snapshots Stable

ZFS (local)

zfspool

yes

yes

Schedule Format

Replication uses calendar events for configuring the schedule.

Error Handling

If a replication job encounters problems, it is placed in an error state. In this state, the configured replication intervals get suspended temporarily. The failed replication is repeatedly tried again in a 30 minute interval. Once this succeeds, the original schedule gets activated again.

Possible issues

Some of the most common issues are in the following list. Depending on your setup there may be another cause.

  • Network is not working.

  • No free space left on the replication target storage.

  • Storage with same storage ID available on the target node

Note You can always use the replication log to find out what is causing the problem.

Migrating a guest in case of Error

In the case of a grave error, a virtual guest may get stuck on a failed node. You then need to move it manually to a working node again.

Example

Let’s assume that you have two guests (VM 100 and CT 200) running on node A and replicate to node B. Node A failed and can not get back online. Now you have to migrate the guest to Node B manually.

  • connect to node B over ssh or open its shell via the web UI

  • check if that the cluster is quorate 

    # pvecm status

  • If you have no quorum, we strongly advise to fix this first and make the node operable again. Only if this is not possible at the moment, you may use the following command to enforce quorum on the current node: 

    # pvecm expected 1
Warning Avoid changes which affect the cluster if expected votes are set (for example adding/removing nodes, storages, virtual guests) at all costs. Only use it to get vital guests up and running again or to resolve the quorum issue itself.

  • move both guest configuration files form the origin node A to node B: 

    # mv /etc/pve/nodes/A/qemu-server/100.conf /etc/pve/nodes/B/qemu-server/100.conf
# mv /etc/pve/nodes/A/lxc/200.conf /etc/pve/nodes/B/lxc/200.conf

  • Now you can start the guests again: 

    # qm start 100
# pct start 200

Remember to replace the VMIDs and node names with your respective values.

Managing Jobs

screenshot/gui-qemu-add-replication-job.png

You can use the web GUI to create, modify, and remove replication jobs easily. Additionally, the command-line interface (CLI) tool pvesr can be used to do this.

You can find the replication panel on all levels (datacenter, node, virtual guest) in the web GUI. They differ in which jobs get shown: all, node- or guest-specific jobs.

When adding a new job, you need to specify the guest if not already selected as well as the target node. The replication schedule can be set if the default of all 15 minutes is not desired. You may impose a rate-limit on a replication job. The rate limit can help to keep the load on the storage acceptable.

A replication job is identified by a cluster-wide unique ID. This ID is composed of the VMID in addition to a job number. This ID must only be specified manually if the CLI tool is used.

Command-line Interface Examples

Create a replication job which runs every 5 minutes with a limited bandwidth of 10 Mbps (megabytes per second) for the guest with ID 100.

# pvesr create-local-job 100-0 pve1 --schedule "*/5" --rate 10

Disable an active job with ID 100-0.

# pvesr disable 100-0

Enable a deactivated job with ID 100-0.

# pvesr enable 100-0

Change the schedule interval of the job with ID 100-0 to once per hour.

# pvesr update 100-0 --schedule '*/00'


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